Ccnp Ospf Final

RJP Infotek (P) Ltd, Chennai

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OSPF- HANDS ON LAB: Like EIGRP, OSPF also contains three tables as mentioned below: 1) Routing table: - Commonly named a forwarding database - Contains list of best paths to destinations Command to verify this is: R#show ip route 2) Neighbor table: - Also known as the adjacency database - Contains list of recognized neighbors Command to verify this is: R#show ip ospf neighbor 3) Database table: - Typically referred to as Link State Database(LSDB) - Contains all routers and their attached links in the area or network - Identical LSDB for all routers within an area Command to verify this is: R#show ip ospf database OSPF-SINGLE AREA CONCEPTS: The behavior of OSPF in a single area varies for different network environments, which are discussed as follows: I) OSPF ON POINT-TO-POINT WAN LINK: Consider the below diagram. IP addresses are configured as shown.

OSPF CONFIGURATION: To configure the OSPF routing protocol, we use the syntax:

R(config)#router ospf<process-id> A point to be noted is that the process-id can be different in different routers; this does not refer to the Autonomous System number like other protocols. The syntax to publish Networks is shown below: R(config-router)#network <net-id> <wcm> area <area-id> Here, net-id is the major network, Wcm is the wild-card mask and the Area-id is the area which the network belongs. The configurations are shown below: Router1(config)#router ospf 1


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Router1(config-router)#network 1.0.0.0 0.255.255.255 area 0 Router1(config-router)#network 25.0.0.0 0.255.255.255 area 0 Router3(config)#router ospf 3 Router3(config-router)#network 3.0.0.0 0.255.255.255 area 0 Router3(config-router)#network 25.0.0.0 0.255.255.255 area 0 NOTE : If contiguous subnets are to be published, it’s possible to reduce the no. of network commands by using an ‘inclusive’ WCM with the least no. of network commands. VERIFICATION: First, we have to see the running configuration of the two routers:

1) Routing table: Now, the routing table of the two routers are seen & OSPF learnt routes are indicated with "O" as shown below: Router1#sh ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is not set C 1.0.0.0/8 is directly connected, Loopback0 3.0.0.0/32 is subnetted, 1 subnets O 3.3.3.3 [110/65] via 25.0.0.2, 00:04:41, Serial0 C 25.0.0.0/8 is directly connected, Serial0


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Router3#sh ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is not set 1.0.0.0/32 is subnetted, 1 subnets O 1.1.1.1 [110/65] via 25.0.0.1, 00:05:26, Serial0 C 3.0.0.0/8 is directly connected, Loopback0 C 25.0.0.0/8 is directly connected, Serial0 NOTE: One can observe that Loopbacks are learnt as /32 networks by OSPF even though a different Mask is specified. Also note that no auto-summarization is supported by OSPF. 2) Neighbor Table: Router1#sh ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 3.3.3.3 1 FULL/ - 00:00:33 25.0.0.2 Serial0 Router3#sh ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 1 FULL/ - 00:00:39 25.0.0.1 Serial0 In the above output, Neighbor ID - represents the neighbor router's id Pri - represents the priority of the neighbor. State - represents the state of neighbor router's adjacency formation. Address - represents the neighbor's interface address through which it is connected to the neighbor. Interface - represents the interface from which it is connected to the neighbor. 3) Database table: Now we can see the output for the above point-to-point WAN link: Router1#sh ip ospf database OSPF Router with ID (1.1.1.1) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 158 0x80000003 0x9BAA 3 3.3.3.3 3.3.3.3 159 0x80000003 0xD163 3 Router3#sh ip ospf database


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OSPF Router with ID (3.3.3.3) (Process ID 3) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 362 0x80000003 0x9BAA 3 3.3.3.3 3.3.3.3 362 0x80000003 0xD163 3 The database table will give us information about various types of link state advertisements. In the above output, we see "router link state" (LSA Type 1) within a single area-area 0.Each router in the area advertises details of links connected to it. This advertisement is seen only in the router’s connected area. The details in the table are: Link ID - represents the advertising router's Router id ADV Router - same as above Link count - represents the number of the links of that router connected to this area.

The loopback has a link count of 1 and point-to-point link has a count of 2. Seq# - represents the sequence number. The other important commands to verify the OSPF operation are: #show ip ospf interface the highlighted part in the output gives us some important information. Router1#sh ip ospf interface Serial0 is up, line protocol is up Internet Address 25.0.0.1/8, Area 0 Process ID 1, Router ID 1.1.1.1,Network Type POINT_TO_POINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:05 Index 2/2, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 1, maximum is 1 Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 3.3.3.3 Suppress hello for 0 neighbor(s) Loopback0 is up, line protocol is up Internet Address 1.1.1.1/8, Area 0 Process ID 1, Router ID 1.1.1.1, Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host From the above output, we can see the Process ID, Router ID, cost of that link (metric) Network Type - which shows POINT_TO_POINT for WAN links and LOOPBACK for loopbacks, Hello and Dead time intervals, neighbor count and neighbor’s router-id.


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INITIAL NEIGHBOR FORMATION - DEBUG OUTPUT : OSPF packet types and their description are given below:

Type Packet Name Description

1 Hello Discovers neighbors, builds &

maintains adjacencies between them

2 DBD Checks for database synchronization between routers

3 LSR Requests specific link-state records from router to router

4 LSU Sends specifically requested link-state records

5 LSAck

Acknowledges the other packet types (except hellos)

And also, we have to see the various states and its description as seen below in the table:

States Description

Down Not exchanged any information with other routers

Init Sends Hello packet to all directly connected routers

Two-way After hello received, it adds all routers to its neighbor table and sends a Hello

to all connected routers

Exstart MASTER/SLAVE is decided based on highest Router-Id

Exchange Exchanges DBD packets

Loading Link state request is sent and LSU is received(i.e., actual LSU exchange)

Full After all LSReqs are satisfied, forms FULL adjacency with the neighbor

From the debug output, we can see the different states of communication Router1#debug ip ospf adj OSPF adjacency events debugging is on 00:18:36: OSPF: Send DBD to 3.3.3.3 on Serial0 seq 0x1517 opt 0x42 flag 0x7 len 32 state INIT 00:18:36: OSPF: 2 Way Communication to 3.3.3.3 on Serial0, state 2WAY 00:18:36: OSPF: Send DBD to 3.3.3.3 on Serial0 seq 0x1518 opt 0x42 flag 0x7 len 32 00:18:36: OSPF: Rcv DBD from 3.3.3.3 on Serial0 seq 0x1518 opt 0x42 flag 0x7 len 32 mtu 1500 state EXSTART 00:18:36: OSPF: NBR Negotiation Done. We are the SLAVE


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00:18:36: OSPF: Send DBD to 3.3.3.3 on Serial0 seq 0x1518 opt 0x42 flag 0x2 len 72 00:18:36: OSPF: Rcv DBD from 3.3.3.3 on Serial0 seq 0x1519 opt 0x42 flag 0x3 len 72 mtu 1500 state EXCHANGE 00:18:36: OSPF: Send DBD to 3.3.3.3 on Serial0 seq 0x1519 opt 0x42 flag 0x0 len 32 00:18:36: OSPF: Database request to 3.3.3.3 00:18:36: OSPF: sent LS REQ packet to 25.0.0.2, length 12 00:18:36: OSPF: Rcv DBD from 3.3.3.3 on Serial0 seq 0x151A opt 0x42 flag 0x1 len 32 mtu 1500 state EXCHANGE 00:18:36: OSPF: Exchange Done with 3.3.3.3 on Serial0 00:18:36: OSPF: Send DBD to 3.3.3.3 on Serial0 seq 0x151A opt 0x42 flag 0x0 len 32 00:18:36: OSPF: Synchronized with 3.3.3.3 on Serial0, state FULL 00:18:36: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on Serial0 from LOADING to FULL, Loading Done II) OSPF IN BROADCAST MULTIACCESS NETWORK (LAN) :

DR and BDR are elected in OSPF - LAN. The important aspects to know are: • All (DROther) routers form full adjacencies with the DR and BDR only. • Packets to the DR and the BDR use 224.0.0.6. • Packets from DR to all other routers use 224.0.0.5. ROUTER-ID: Any router is known to OSPF by the OSPF router ID.

• LSDBs use the OSPF router ID to differentiate one router from the next. • By default, the router ID is the highest IP address on a logical interface

(loopback). If no loopbacks are configured, it takes the highest physical interface address at the moment of OSPF process startup.

• The OSPF router-id command can be used to override the OSPF router ID (recommended). For this command to take effect, it’s necessary to clear the OSPF process.

The command for router-id is configured under the router ospf config as shown: Router(config-router)#router-id <ip-address> For the above scenario, the interface and OSPF configurations with Router-id are done as shown in the running configuration below:


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VERIFICATION: The configurations are to be verified first in the running configuration of the three routers as shown below:

The three tables are to be verified as follows (output similar on all routers): Router1#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route Gateway of last resort is not set C 1.0.0.0/8 is directly connected, Loopback0 2.0.0.0/32 is subnetted, 1 subnets O 2.0.0.1 [110/11] via 25.0.0.2, 00:25:03, Ethernet0 3.0.0.0/32 is subnetted, 1 subnets O 3.0.0.1 [110/11] via 25.0.0.3, 00:25:03, Ethernet0 C 25.0.0.0/8 is directly connected, Ethernet0 Observe, the router with highest Router-Id is elected as DR & next highest as BDR. Also note that the election is non-preemptive; i.e. once a DR (& BDR) is elected, it will never be replaced by any other Router with a higher Priority / Router-id. Only when the DR goes down will the BDR become DR & a new BDR is elected from the DROTHER routers. Router1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 1 FULL/BDR 00:00:35 25.0.0.2 Ethernet0 3.3.3.3 1 FULL/DR 00:00:33 25.0.0.3 Ethernet0 Router2# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 1 FULL/DROTHER 00:00:35 25.0.0.1 Ethernet0 3.3.3.3 1 FULL/DR 00:00:31 25.0.0.3 Ethernet0 Router3#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface


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1.1.1.1 1 FULL/DROTHER 00:00:36 25.0.0.1 Ethernet0 2.2.2.2 1 FULL/BDR 00:00:33 25.0.0.2 Ethernet0 In the database table, we can observe 2 types of LSAs now., TYPE1 LSA - Router link states – lists the routers & connected links in that area. TYPE2 LSA - Net link states - Displays the DR Advt. in the LAN within the area. Router1#show ip ospf database OSPF Router with ID (1.1.1.1) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 153 0x80000003 0xE4F1 2 2.2.2.2 2.2.2.2 156 0x80000003 0xB913 2 3.3.3.3 3.3.3.3 155 0x80000003 0x8E34 2 Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 25.0.0.3 3.3.3.3 151 0x80000002 0x4F0 The output says that DR WITH ROUTER- ID 3.3.3.3 connected (to network 25.0.0.0) through its interface 25.0.0.3. The database table should be the same on all routers within the same area. Router2#show ip ospf database OSPF Router with ID (2.2.2.2) (Process ID 2) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 896 0x80000003 0xE4F1 2 2.2.2.2 2.2.2.2 898 0x80000003 0xB913 2 3.3.3.3 3.3.3.3 898 0x80000003 0x8E34 2 Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 25.0.0.3 3.3.3.3 894 0x80000002 0x4F0 Some more useful command outputs are shown below: Router1#sh ip ospf interface e0 – displays output only for e0 interface Ethernet0 is up, line protocol is up Internet Address 25.0.0.1/8, Area 0 Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10 Transmit Delay is 1 sec, State DROTHER, Priority 1 Designated Router (ID) 3.3.3.3, Interface address 25.0.0.3 Backup Designated router (ID) 2.2.2.2, Interface address 25.0.0.2 Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:02 Index 2/2, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 0, maximum is 1


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Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 2, Adjacent neighbor count is 2 Adjacent with neighbor 2.2.2.2 (Backup Designated Router) Adjacent with neighbor 3.3.3.3 (Designated Router) Suppress hello for 0 neighbor(s) Loopback0 is up, line protocol is up Internet Address 1.0.0.1/8, Area 0 Process ID 1, Router ID 1.1.1.1, Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host The information highlighted in RED are specific to BMA (Broadcast Multi Access) interface. Router1#sh ip ospf Routing Process "ospf 1" with ID 1.1.1.1 and Domain ID 0.0.0.1 Supports only single TOS(TOS0) routes Supports opaque LSA SPF schedule delay 5 secs, Hold time between two SPFs 10 secs Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs Number of external LSA 0. Checksum Sum 0x0 Number of opaque AS LSA 0. Checksum Sum 0x0 Number of DCbitless external and opaque AS LSA 0 Number of DoNotAge external and opaque AS LSA 0 Number of areas in this router is 1. 1 normal 0 stub 0 nssa External flood list length 0 Area BACKBONE(0) Number of interfaces in this area is 2 Area has no authentication SPF algorithm executed 4 times Area ranges are Number of LSA 4. Checksum Sum 0x23128 Number of opaque link LSA 0. Checksum Sum 0x0 Number of DCbitless LSA 0 Number of indication LSA 0 Number of DoNotAge LSA 0 Flood list length 0 Significant information are Highlighted (& self explanatory). Router1#show ip protocols Routing Protocol is "ospf 1" Invalid after 0 seconds, hold down 0, flushed after 0 Outgoing update filter list for all interfaces is Incoming update filter list for all interfaces is Routing for Networks: 1.0.0.0 25.0.0.0 Routing Information Sources:


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Gateway Distance Last Update 2.2.2.2 110 00:11:52 3.3.3.3 110 00:11:52 Distance: (default is 110) INFLUENCING DR ELECTION BY CHANGING THE PRIORITY: We can manually change the interface PRIORITY (values – 0 to 255. Default priority is 1, highest priority would be 255 & 0 indicates ineligibility to participate in election) from which we can have the control over the election process to make our specified router as DR and BDR. The interface with the highest priority becomes the DR, and the interface with the second-highest priority becomes the BDR. The important points about priority are given below: • This interface configuration command assigns the OSPF priority to an interface. • Different interfaces on a router may be assigned different values. • The default priority is 1(0 for non-BMA interfaces in latest IOS versions). The range is from 0 to 255. • 0 means the router cannot be the DR or BDR. • A router that is not the DR or BDR is DROTHER. The syntax of the command used to configure the priority is shown: Router(config-if)#ip ospf priority <number> In the above Scenario, Router3 was the DR and Router2 BDR. Now, We’ll make Router1 - DR and Router3 - BDR by changing the priority of the three routers as displayed below:

After changing the priority, we have to clear the ospf process on all routers using the below command. Router#clear ip ospf process Now, the neighbor table should reflect the change: Router1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 3.3.3.3 20 FULL/BDR 00:00:38 25.0.0.3 Ethernet0 2.2.2.2 10 FULL/DROTHER 00:00:35 25.0.0.2 Ethernet0 Router2#show ip sop neighbor


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Neighbor ID Pri State Dead Time Address Interface 3.3.3.3 20 FULL/BDR 00:00:33 25.0.0.3 Ethernet0 1.1.1.1 30 FULL/DR 00:00:36 25.0.0.1 Ethernet0 Router3#show ip sop neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 10 FULL/DROTHER 00:00:30 25.0.0.2 Ethernet0 1.1.1.1 30 FULL/DR 00:00:36 25.0.0.1 Ethernet0 This can also be seen in the output of the command show ip ospf interface: Router1#show ip ospf interface e0 Ethernet0 is up, line protocol is up Internet Address 25.0.0.1/8, Area 0 Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10 Transmit Delay is 1 sec, State DR, Priority 30 Designated Router (ID) 1.1.1.1, Interface address 25.0.0.1 Backup Designated router (ID) 3.3.3.3, Interface address 25.0.0.3 Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:07 Index 2/2, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 1, maximum is 2 Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 2, Adjacent neighbor count is 2 Adjacent with neighbor 3.3.3.3 (Backup Designated Router) Adjacent with neighbor 2.2.2.2 Suppress hello for 0 neighbor(s) III) OSPF OVER DIFFERENT NBMA MODES: As described in RFC 2328, OSPF runs in one of the following two official modes in NBMA topologies: Non BROADCAST: The nonbroadcast (NBMA) mode simulates the operation of OSPF in broadcast networks. Neighbors must be manually configured, and DR and BDR election is required. This configuration is typically used with fully meshed SVC networks. Point-To-Multipoint: In this environment, the routers automatically identify their neighboring routers but do not elect a DR and BDR. This configuration is typically used with partially meshed / HUB & Spoke PVC networks. Both the above environments use single subnet. Cisco has defined the following additional modes:

Point-to-multipoint nonbroadcast Broadcast Point-to-point


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i) NONBROADCAST MODE (NBMA): Before we start the configuration, we have to know the characteristics of this mode.

This mode is the default mode. This is a RFC-compliant mode. It should be configured with one IP subnet. Neighbors must be manually configured. DR and BDR election takes place (automatically) in this mode. DR and BDR need to have full connectivity with all other routers. This mode is typically used in a full-mesh SVC topology.

For the FULL MESH scenario shown below, the configuration and output verification of this mode is to be done as follows:

The basic interface configuration and OSPF configuration are done which was shown in the running configuration as below:


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Now, we have to configure the network type in the interface mode with the ip ospf network command. Router(config)#interface <interface name> Router(config-if)#ip ospf network <one of NBMA mode> Specifying the network type for the NBMA network explicitly is not required because the default is NON_BROADCAST, ie NBMA And also in this mode, the neighbor has to be manually configured in the OSPF process with the command as given below: Router(config)#router ospf <process-id> Router(config-router)#neighbor <neighbor-address> In a full-mesh NBMA topology, you may need neighbor statements on all routers unless you have statically configured the DR and BDR using the priority command, in which case it is sufficient to configure neighbor statements on DR & BDR alone. Configuration: The “OSPF Network ” configuration has to be done in the serial interface of all the routers to select the non-broadcast mode and in the OSPF process. We shall configure


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this command (for our understanding) even though this is the default mode. The neighbor configuration is also demonstrated in the below example. R1(config)#interface serial 0 R1(config-if)#ip ospf network non-broadcast R1(config-if)#exit R1(config)#router ospf 1 R1(config-router)#neighbor 11.0.0.2 R1(config-router)#neighbor 11.0.0.3 R1(config-router)#neighbor 11.0.0.4 R2(config)#interface serial 0 R2(config-if)#ip ospf network non-broadcast R2(config-if)#exit R2(config-router)#neighbor 11.0.0.1 R2(config-router)#neighbor 11.0.0.3 R2(config-router)#neighbor 11.0.0.4 R3(config)#interface serial 0 R3(config-if)#ip ospf network non-broadcast R3(config-if)#exit R3(config-router)#neighbor 11.0.0.1 R3(config-router)#neighbor 11.0.0.2 R3(config-router)#neighbor 11.0.0.4 R4(config)#interface serial 0 R4(config-if)#ip ospf network non-broadcast R4(config-if)#exit R4(config-router)#neighbor 11.0.0.1 R4(config-router)#neighbor 11.0.0.2 R4(config-router)#neighbor 11.0.0.3 OUTPUT VERIFICATION: The highlighted part of the output reveals BMA equivalent behavior of OSPF in this mode. R1#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is not set C 1.0.0.0/8 is directly connected, Loopback0 2.0.0.0/32 is subnetted, 1 subnets O 2.0.0.1 [110/65] via 11.0.0.2, 00:01:11, Serial0 3.0.0.0/32 is subnetted, 1 subnets


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O 3.0.0.1 [110/65] via 11.0.0.3, 00:01:11, Serial0 4.0.0.0/32 is subnetted, 1 subnets O 4.0.0.1 [110/65] via 11.0.0.4, 00:01:11, Serial0 C 11.0.0.0/8 is directly connected, Serial0 R1#show ip ospf database OSPF Router with ID (1.0.0.1) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.0.0.1 1.0.0.1 160 0x8000000D 0x00D3E1 2 2.0.0.1 2.0.0.1 193 0x80000007 0x0003B4 2 3.0.0.1 3.0.0.1 73 0x80000006 0x00F7BC 2 4.0.0.1 4.0.0.1 534 0x80000004 0x0007AB 2 Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 11.0.0.4 4.0.0.1 258 0x80000004 0x007696 R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.0.0.1 1 FULL/BDR 00:01:52 11.0.0.2 Serial0 3.0.0.1 1 2WAY/DROTHER 00:01:31 11.0.0.3 Serial0 4.0.0.1 1 FULL/DR 00:01:55 11.0.0.4 Serial0 The above also reveals that DROTHER routers stop at the 2-WAY state and don’t form full adjacency (same would have been the case in LAN environment). R1#show ip ospf interface Serial0 is up, line protocol is up Internet Address 11.0.0.1/8, Area 0 Process ID 1, Router ID 1.0.0.1, Network Type NON_BROADCAST, Cost: 64 Transmit Delay is 1 sec, State DROTHER, Priority 1 Designated Router (ID) 4.0.0.1, Interface address 11.0.0.4 Backup Designated router (ID) 2.0.0.1, Interface address 11.0.0.2 Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5 oob-resync timeout 120 Hello due in 00:00:00 Index 2/2, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 0, maximum is 1 Last flood scan time is 0 msec, maximum is 8 msec Neighbor Count is 3, Adjacent neighbor count is 2 Adjacent with neighbor 2.0.0.1 (Backup Designated Router) Adjacent with neighbor 4.0.0.1 (Designated Router) Suppress hello for 0 neighbor(s) Loopback0 is up, line protocol is up Internet Address 1.0.0.1/8, Area 0 Process ID 1, Router ID 1.0.0.1, Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host


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Let’s try viewing the details of Router Link States using the below command. R1#show ip ospf database router OSPF Router with ID (1.0.0.1) (Process ID 1) Router Link States (Area 0) LS age: 310 Options: (No TOS-capability, DC) LS Type: Router Links Link State ID: 1.0.0.1 Advertising Router: 1.0.0.1 LS Seq Number: 8000000D Checksum: 0xD3E1 Length: 48 Number of Links: 2 Link connected to: a Transit Network (Link ID) Designated Router address: 11.0.0.4 (Link Data) Router Interface address: 11.0.0.1 Number of TOS metrics: 0 TOS 0 Metrics: 64 Link connected to: a Stub Network (Link ID) Network/subnet number: 1.0.0.1 (Link Data) Network Mask: 255.255.255.255 Number of TOS metrics: 0 TOS 0 Metrics: 1 LS age: 344 Options: (No TOS-capability, DC) LS Type: Router Links Link State ID: 2.0.0.1 Advertising Router: 2.0.0.1 LS Seq Number: 80000007 Checksum: 0x3B4 Length: 48 Number of Links: 2 Link connected to: a Stub Network (Link ID) Network/subnet number: 2.0.0.1 (Link Data) Network Mask: 255.255.255.255 Number of TOS metrics: 0 TOS 0 Metrics: 1 Link connected to: a Transit Network (Link ID) Designated Router address: 11.0.0.4 (Link Data) Router Interface address: 11.0.0.2 Number of TOS metrics: 0 TOS 0 Metrics: 64 LS age: 226 Options: (No TOS-capability, DC)


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LS Type: Router Links Link State ID: 3.0.0.1 Advertising Router: 3.0.0.1 LS Seq Number: 80000006 Checksum: 0xF7BC Length: 48 Number of Links: 2 Link connected to: a Transit Network (Link ID) Designated Router address: 11.0.0.4 (Link Data) Router Interface address: 11.0.0.3 Number of TOS metrics: 0 TOS 0 Metrics: 64 Link connected to: a Stub Network (Link ID) Network/subnet number: 3.0.0.1 (Link Data) Network Mask: 255.255.255.255 Number of TOS metrics: 0 TOS 0 Metrics: 1 LS age: 688 Options: (No TOS-capability, DC) LS Type: Router Links Link State ID: 4.0.0.1 Advertising Router: 4.0.0.1 LS Seq Number: 80000004 Checksum: 0x7AB Length: 48 Number of Links: 2 Link connected to: a Transit Network (Link ID) Designated Router address: 11.0.0.4 (Link Data) Router Interface address: 11.0.0.4 Number of TOS metrics: 0 TOS 0 Metrics: 64 Link connected to: a Stub Network (Link ID) Network/subnet number: 4.0.0.1 (Link Data) Network Mask: 255.255.255.255 Number of TOS metrics: 0 TOS 0 Metrics: 1 The above reveals details of the Link count details of links in the area. Note that link count is taken by OSPF as below.

1. Link count is 1 for LAN 2. Link count is 2 for point-to-point WAN 3. Link count is 1 + no. of neighbors in Frame Relay.

Similar outputs can be observed in other routers too. A few outputs on R2 is also shown below.


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R2#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set 1.0.0.0/32 is subnetted, 1 subnets O 1.0.0.1 [110/65] via 11.0.0.1, 00:03:51, Serial0 C 2.0.0.0/8 is directly connected, Loopback0 3.0.0.0/32 is subnetted, 1 subnets O 3.0.0.1 [110/65] via 11.0.0.3, 00:03:51, Serial0 4.0.0.0/32 is subnetted, 1 subnets O 4.0.0.1 [110/65] via 11.0.0.4, 00:03:51, Serial0 C 11.0.0.0/8 is directly connected, Serial0 R2#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 4.0.0.1 1 FULL/DR 00:01:43 11.0.0.4 Serial0 3.0.0.1 1 FULL/DROTHER 00:01:49 11.0.0.3 Serial0 1.0.0.1 1 FULL/DROTHER 00:01:51 11.0.0.1 Serial0 R2#show ip ospf database OSPF Router with ID (2.0.0.1) (Process ID 2) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.0.0.1 1.0.0.1 344 0x8000000D 0xD3E1 2 2.0.0.1 2.0.0.1 375 0x80000007 0x3B4 2 3.0.0.1 3.0.0.1 256 0x80000006 0xF7BC 2 4.0.0.1 4.0.0.1 718 0x80000004 0x7AB 2 Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 11.0.0.4 4.0.0.1 441 0x80000004 0x7696 R2#show ip ospf interface Loopback0 is up, line protocol is up Internet Address 2.0.0.1/8, Area 0 Process ID 2, Router ID 2.0.0.1, Network Type LOOPBACK, Cost: 1 Serial0 is up, line protocol is up Internet Address 11.0.0.2/8, Area 0 Process ID 2, Router ID 2.0.0.1, Network Type NON_BROADCAST, Cost: 64 Transmit Delay is 1 sec, State BDR, Priority 1 Designated Router (ID) 4.0.0.1, Interface address 11.0.0.4 Backup Designated router (ID) 2.0.0.1, Interface address 11.0.0.2 Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5


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Hello due in 00:00:19 Index 1/1, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 0, maximum is 1 Last flood scan time is 0 msec, maximum is 4 msec Neighbor Count is 3, Adjacent neighbor count is 3 Adjacent with neighbor 4.0.0.1 (Designated Router) Adjacent with neighbor 3.0.0.1 Adjacent with neighbor 1.0.0.1 Suppress hello for 0 neighbor(s) R3#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 4.0.0.1 1 FULL/DR 00:01:31 11.0.0.4 Serial0 2.0.0.1 1 FULL/BDR 00:01:57 11.0.0.2 Serial0 1.0.0.1 1 2WAY/DROTHER 00:01:39 11.0.0.1 Serial0 ii) BROADCAST MODE: Before we start the configuration, we have to know the characteristics of this mode:

This mode is a Cisco extension. This makes the WAN interface appear to be a LAN. It must be configured with one IP subnet. It uses multicast OSPF hello packet to automatically discover the neighbors. DR and BDR election takes place in this mode. This mode is suggested for a full-mesh PVC topology.

The difference in configuration compared to the earlier case is that we would not be issuing the neighbor command and we would specify the Network Type as “BROADCAST”.


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Note that these configurations are done in Frame-relay FULL-MESH topology. Here goes the configuration: Router(config)#interface serial 0 Router(config-if)#ip ospf network broadcast OUTPUT VERIFICATION on R1: We would observe similar outputs as the previous case but for the fact that neighbors are automatically discovered! R1#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is not set C 1.0.0.0/8 is directly connected, Loopback0 2.0.0.0/32 is subnetted, 1 subnets O 2.0.0.1 [110/65] via 11.0.0.2, 00:38:54, Serial0 3.0.0.0/32 is subnetted, 1 subnets O 3.0.0.1 [110/65] via 11.0.0.3, 00:38:54, Serial0 4.0.0.0/32 is subnetted, 1 subnets O 4.0.0.1 [110/65] via 11.0.0.4, 00:38:54, Serial0 C 11.0.0.0/8 is directly connected, Serial0 R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.0.0.1 1 FULL/DROTHER 00:00:30 11.0.0.2 Serial0 3.0.0.1 1 FULL/DR 00:00:35 11.0.0.3 Serial0 4.0.0.1 1 FULL/DROTHER 00:00:37 11.0.0.4 Serial0 R1#show ip ospf database OSPF Router with ID (1.0.0.1) (Process ID 1) Router Link States (Area 0)


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Link ID ADV Router Age Seq# Checksum Link count 1.0.0.1 1.0.0.1 604 0x80000004 0x00DBE3 2 2.0.0.1 2.0.0.1 473 0x80000003 0x00E8D3 2 3.0.0.1 3.0.0.1 507 0x80000003 0x00F3C4 2 4.0.0.1 4.0.0.1 391 0x80000003 0x00FEB5 2 Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 11.0.0.3 3.0.0.1 507 0x80000004 0x008D81 The below command helps verify the interface Network Type configured. R1#show ip ospf interface Serial0 is up, line protocol is up Internet Address 11.0.0.1/8, Area 0 Process ID 1, Router ID 1.0.0.1, Network Type BROADCAST, Cost: 64 Transmit Delay is 1 sec, State BDR, Priority 1 Designated Router (ID) 3.0.0.1, Interface address 11.0.0.3 Backup Designated router (ID) 1.0.0.1, Interface address 11.0.0.1 Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 oob-resync timeout 40 Hello due in 00:00:01 Index 2/2, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 0, maximum is 1 Last flood scan time is 0 msec, maximum is 4 msec Neighbor Count is 3, Adjacent neighbor count is 3 Adjacent with neighbor 2.0.0.1 Adjacent with neighbor 3.0.0.1 (Designated Router) Adjacent with neighbor 4.0.0.1 Suppress hello for 0 neighbor(s) Loopback0 is up, line protocol is up Internet Address 1.0.0.1/8, Area 0 Process ID 1, Router ID 1.0.0.1, Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host iii) POINT-TO-MULTIPOINT MODE: The characteristics of this mode are:

RFC-compliant mode. Single IP subnet. Automatically discovery of neighbors. No DR and BDR election. Typically used in partial-mesh or star topology.


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Note that these configurations are done in Frame-relay PARTIAL-MESH topology. Now, we have to configure this network mode (point-to-multipoint) in the interface of all the routers with the below command: Router(config)#interface serial 0 Router(config-if)#ip ospf network point-to-multipoint OUTPUT VERIFICATION ON R1: R1#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is not set C 1.0.0.0/8 is directly connected, Loopback0 2.0.0.0/32 is subnetted, 1 subnets O 2.0.0.1 [110/65] via 11.0.0.2, 00:00:59, Serial0 3.0.0.0/32 is subnetted, 1 subnets O 3.0.0.1 [110/65] via 11.0.0.3, 00:00:59, Serial0 4.0.0.0/32 is subnetted, 1 subnets O 4.0.0.1 [110/65] via 11.0.0.4, 00:00:59, Serial0 11.0.0.0/8 is variably subnetted, 4 subnets, 2 masks O 11.0.0.3/32 [110/64] via 11.0.0.3, 00:00:59, Serial0 O 11.0.0.2/32 [110/64] via 11.0.0.2, 00:01:00, Serial0


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O 11.0.0.4/32 [110/64] via 11.0.0.4, 00:01:01, Serial0 C 11.0.0.0/8 is directly connected, Serial0 Note the neighbor table reveals only connected Routers form full adjacency while database remains same on all routers R1#show ip ospf neighbors Neighbor ID Pri State Dead Time Address Interface 4.0.0.1 0 FULL/ - 00:01:30 11.0.0.4 Serial0 3.0.0.1 0 FULL/ - 00:01:54 11.0.0.3 Serial0 2.0.0.1 0 FULL/ - 00:01:41 11.0.0.2 Serial0 R2#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.0.0.1 1 FULL/ - 00:01:34 11.0.0.1 Serial0 R3#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 4.0.0.1 1 FULL/ - 00:01:49 11.0.0.4 Serial0 1.0.0.1 1 FULL/ - 00:01:55 11.0.0.1 Serial0 R4#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 3.0.0.1 1 FULL/ - 00:01:37 11.0.0.3 Serial0 1.0.0.1 1 FULL/ - 00:01:49 11.0.0.1 Serial0 R1#show ip ospf database OSPF Router with ID (1.0.0.1) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.0.0.1 1.0.0.1 120 0x8000000A 0x0016D5 5 2.0.0.1 2.0.0.1 211 0x80000007 0x00396F 3 3.0.0.1 3.0.0.1 103 0x80000008 0x00DC64 4 4.0.0.1 4.0.0.1 103 0x80000008 0x003507 4 R2#show ip ospf database OSPF Router with ID (2.0.0.1) (Process ID 2) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.0.0.1 1.0.0.1 304 0x8000000A 0x16D5 5 2.0.0.1 2.0.0.1 394 0x80000007 0x396F 3 3.0.0.1 3.0.0.1 286 0x80000008 0xDC64 4 4.0.0.1 4.0.0.1 286 0x80000008 0x3507 4 R3#show ip ospf database OSPF Router with ID (3.0.0.1) (Process ID 3) Router Link States (Area 0)


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Link ID ADV Router Age Seq# Checksum Link count 1.0.0.1 1.0.0.1 402 0x8000000A 0x16D5 5 2.0.0.1 2.0.0.1 492 0x80000007 0x396F 3 3.0.0.1 3.0.0.1 383 0x80000008 0xDC64 4 4.0.0.1 4.0.0.1 384 0x80000008 0x3507 4 R4#show ip ospf database OSPF Router with ID (4.0.0.1) (Process ID 4) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.0.0.1 1.0.0.1 478 0x8000000A 0x16D5 5 2.0.0.1 2.0.0.1 569 0x80000007 0x396F 3 3.0.0.1 3.0.0.1 461 0x80000008 0xDC64 4 4.0.0.1 4.0.0.1 460 0x80000008 0x3507 4 The Network type can be verified using the show ip ospf interface command. Also note the hello/dead intervals. R1#show ip ospf interface Serial0 is up, line protocol is up Internet Address 11.0.0.1/8, Area 0 Process ID 1, Router ID 1.0.0.1, Network Type POINT_TO_MULTIPOINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT, Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5 oob-resync timeout 120 Hello due in 00:00:14 Index 2/2, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 1, maximum is 1 Last flood scan time is 8 msec, maximum is 8 msec Neighbor Count is 3, Adjacent neighbor count is 3 Adjacent with neighbor 4.0.0.1 Adjacent with neighbor 3.0.0.1 Adjacent with neighbor 2.0.0.1 Suppress hello for 0 neighbor(s) Loopback0 is up, line protocol is up Internet Address 1.0.0.1/8, Area 0 Process ID 1, Router ID 1.0.0.1, Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host iv) POINT-TO-MULTIPOINT NONBROADCAST MODE: Characteristics of this mode:

This mode is a Cisco extension. If multicast and broadcast are not enabled on the virtual circuits, the RFC-compliant

point-to-multipoint mode cannot be used because the router cannot dynamically discover its neighboring routers using hello multicast packets; this Cisco mode should be used instead.


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Neighbors must be manually configured in this mode using neighbor command. DR and BDR election is not required in this mode. This mode used in a partial-mesh or a star topology. Single subnet configured.

PARTIAL-MESH topology is considered in this experiment. The OSPF interface configuration must be “point-to-multipoint non-broadcast” and is configured as shown below: Router(config)#interface serial 0 Router(config-if)#ip ospf network point-to-multipoint non-broadcast Neighbors are configured as discussed earlier: Router(config)#router ospf <process-id> Router(config-router)#neighbor <neighbor-address> Output Verification: As the outputs are very similar to the above scenario, only a sample output on R1 is displayed below. R1#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route


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o - ODR, P - periodic downloaded static route Gateway of last resort is not set C 1.0.0.0/8 is directly connected, Loopback0 2.0.0.0/32 is subnetted, 1 subnets O 2.0.0.1 [110/65] via 11.0.0.2, 00:01:35, Serial0 3.0.0.0/32 is subnetted, 1 subnets O 3.0.0.1 [110/65] via 11.0.0.3, 00:01:35, Serial0 4.0.0.0/32 is subnetted, 1 subnets O 4.0.0.1 [110/65] via 11.0.0.4, 00:01:35, Serial0 11.0.0.0/8 is variably subnetted, 4 subnets, 2 masks O 11.0.0.3/32 [110/64] via 11.0.0.3, 00:01:35, Serial0 O 11.0.0.2/32 [110/64] via 11.0.0.2, 00:01:36, Serial0 C 11.0.0.0/8 is directly connected, Serial0 O 11.0.0.4/32 [110/64] via 11.0.0.4, 00:01:36, Serial0 R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.0.0.1 0 FULL/ - 00:01:59 11.0.0.2 Serial0 3.0.0.1 0 FULL/ - 00:01:42 11.0.0.3 Serial0 4.0.0.1 0 FULL/ - 00:01:57 11.0.0.4 Serial0 R1#show ip ospf database OSPF Router with ID (1.0.0.1) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.0.0.1 1.0.0.1 125 0x80000004 0x00529F 5 2.0.0.1 2.0.0.1 125 0x80000002 0x00436A 3 3.0.0.1 3.0.0.1 144 0x80000003 0x00E65F 4 4.0.0.1 4.0.0.1 158 0x80000002 0x004101 4 Note the network type in the below doesn’t reveal any different as compared to the previous case study output! R1#show ip ospf interface Serial0 is up, line protocol is up Internet Address 11.0.0.1/8, Area 0 Process ID 1, Router ID 1.0.0.1, Network Type POINT_TO_MULTIPOINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT, Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5 oob-resync timeout 120 Hello due in 00:00:22 Index 2/2, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 1, maximum is 1 Last flood scan time is 4 msec, maximum is 8 msec Neighbor Count is 3, Adjacent neighbor count is 3 Adjacent with neighbor 2.0.0.1 Adjacent with neighbor 3.0.0.1


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Adjacent with neighbor 4.0.0.1 Suppress hello for 0 neighbor(s) Loopback0 is up, line protocol is up Internet Address 1.0.0.1/8, Area 0 Process ID 1, Router ID 1.0.0.1, Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host v) POINT-TO-POINT: Characteristics of this mode:

This mode is a Cisco extension. It must be configured with different IP subnet on each sub-interface. No DR or BDR election. This can be used when only two routers need to form an adjacency on a pair of

interfaces. This mode can be used in a partial-mesh or a star topology using sub-interface

configuration on the HUB Router.

Let’s try experimenting on a Star (HUB and SPOKE) topology using sub-interface on the HUB router. We’ll have to configure each spoke as a point-to-point network & separate subnet.


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Configuration of Hub: Let’s first configure the OSPF network type : R1(config-if)#interface serial 0 R1(config-if)#encapsulation frame-relay R1(config-if)#interface serial 0.1 point-to-point R1(config-subif)#ip ospf network point-to-point R1(config-subif)#interface serial 0.2 point-to-point R1(config-subif)#ip ospf network point-to-point R1(config-subif)#interface serial 0.2 point-to-point R1(config-subif)#ip ospf network point-to-point Configuration of Spokes: Spoke1: R2(config)#interface serial 0 R1(config-if)# encapsulation frame-relay R2(config-if)#ip ospf network point-to-point Spoke2: R3(config-if)#interface serial 0 R3(config-if)#encapsulation frame-relay R3(config-if)#ip ospf network point-to-point Spoke3: R4(config-if)#interface serial 0 R4(config-if)#encapsulation frame-relay R4(config-if)#ip ospf network point-to-point Output of Hub: R1#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set C 1.0.0.0/8 is directly connected, Loopback0 2.0.0.0/32 is subnetted, 1 subnets O 2.0.0.1 [110/65] via 11.0.0.2, 00:01:09, Serial0.1 3.0.0.0/32 is subnetted, 1 subnets O 3.0.0.1 [110/65] via 12.0.0.2, 00:01:09, Serial0.2 4.0.0.0/32 is subnetted, 1 subnets O 4.0.0.1 [110/65] via 13.0.0.2, 00:01:09, Serial0.3 C 11.0.0.0/8 is directly connected, Serial0.1


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C 12.0.0.0/8 is directly connected, Serial0.2 C 13.0.0.0/8 is directly connected, Serial0.3 R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 4.0.0.1 1 FULL/ - 00:00:32 13.0.0.2 Serial0.3 3.0.0.1 1 FULL/ - 00:00:35 12.0.0.2 Serial0.2 2.0.0.1 1 FULL/ - 00:00:33 11.0.0.2 Serial0.1 R1#show ip ospf database OSPF Router with ID (1.0.0.1) (Process ID 5) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.0.0.1 1.0.0.1 210 0x80000009 0x2AC6 7 2.0.0.1 2.0.0.1 933 0x80000004 0x9AD2 3 3.0.0.1 3.0.0.1 210 0x80000003 0xC3A5 3 4.0.0.1 4.0.0.1 394 0x80000004 0xE87A 3 R1#show ip ospf interface Serial0.3 is up, line protocol is up Internet Address 13.0.0.1/8, Area 0 Process ID 5, Router ID 1.0.0.1, Network Type POINT_TO_POINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:00 Index 4/4, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 1, maximum is 1 Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 4.0.0.1 Suppress hello for 0 neighbor(s) Serial0.2 is up, line protocol is up Internet Address 12.0.0.1/8, Area 0 Process ID 5, Router ID 1.0.0.1, Network Type POINT_TO_POINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:00 Index 3/3, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 1, maximum is 1 Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 3.0.0.1 Suppress hello for 0 neighbor(s) Serial0.1 is up, line protocol is up Internet Address 11.0.0.1/8, Area 0 Process ID 5, Router ID 1.0.0.1, Network Type POINT_TO_POINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT,


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Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:04 Index 2/2, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 1, maximum is 1 Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 2.0.0.1 Suppress hello for 0 neighbor(s) Loopback0 is up, line protocol is up Internet Address 1.0.0.1/8, Area 0 Process ID 5, Router ID 1.0.0.1, Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host OUTPUT OF SPOKES: We’ll observe on spoke 1 & all the others are similar Spoke1: R2#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area Gateway of last resort is not set 1.0.0.0/32 is subnetted, 1 subnets O 1.0.0.1 [110/65] via 11.0.0.1, 00:07:29, Serial0 C 2.0.0.0/8 is directly connected, Loopback0 3.0.0.0/32 is subnetted, 1 subnets O 3.0.0.1 [110/129] via 11.0.0.1, 00:07:29, Serial0 4.0.0.0/32 is subnetted, 1 subnets O 4.0.0.1 [110/129] via 11.0.0.1, 00:07:29, Serial0 C 11.0.0.0/8 is directly connected, Serial0 O 12.0.0.0/8 [110/128] via 11.0.0.1, 00:07:29, Serial0 O 13.0.0.0/8 [110/128] via 11.0.0.1, 00:07:29, Serial0 R2#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.0.0.1 1 FULL/ - 00:00:32 11.0.0.1 Serial0 Note : Adjacency only with R1 R2#show ip ospf database OSPF Router with ID (2.0.0.1) (Process ID 6) Router Link States (Area 0) Link ID ADV Router Age eq# Checksum Link count 1.0.0.1 1.0.0.1 483 0x80000009 0x2AC6 7 2.0.0.1 2.0.0.1 1205 0x80000004 0x9AD2 3 3.0.0.1 3.0.0.1 483 0x80000003 0xC3A5 3


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4.0.0.1 4.0.0.1 667 0x80000004 0xE87A 3 R2#show ip ospf interface Serial0 is up, line protocol is up Internet Address 11.0.0.2/8, Area 0 Process ID 6, Router ID 2.0.0.1, Network Type POINT_TO_POINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:03 Index 1/1, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 1, maximum is 1 Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 1.0.0.1 <==========Adjacency only with R1 Suppress hello for 0 neighbor(s) Loopback0 is up, line protocol is up Internet Address 2.0.0.1/8, Area 0 Process ID 6, Router ID 2.0.0.1, Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host


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OSPF MULTI-AREA CONCEPTS: In this topic, we mainly have to know about the Types of LSA's that OSPF uses to maintain information about various networks. Let’s briefly discuss these LSAs to begin with. Type 1: Router Link Every router generates router link advertisements for each area to which it belongs. Router link advertisements describe the state of the links of the router to the area and are flooded only within the area. Type 2: Net Link DRs generate network link advertisements for multi-access networks that describe the set of routers attached to a particular multi-access network. Network link advertisements are flooded in the area that contains the network. The link-state ID of the type 2 LSA is the IP interface address of the DR. Types 3 and 4: ABRs generate summary link advertisements. Summary link advertisements describe the following interarea routes: • Type 3 (Summary Net Link) describes routes to networks and aggregate routes. • Type 4 (ASB Summary Link) describes routes to ASBRs.

The link-state ID is the destination network number for type 3 LSAs and the router ID of ASBR for type 4 LSAs. These LSAs are flooded throughout the backbone area to the other ABRs. These link entries are not flooded into totally stubby areas or not-so-stubby areas (NSSAs). Type 5: External AS Link ASBRs generate AS external link advertisements. External link advertisements describe routes to destinations external to the AS and are flooded everywhere with the exception of stub areas, totally stubby areas, and NSSAs. The link-state ID of the type 5 LSA is the external network number. Type 6: Type 6 LSAs are specialized LSAs that are used in multicast OSPF applications. Type 7: Type 7 is an LSA type that is used in NSSAs – generated by ASBRs of NSSA.


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Let’s now configure OSPF with three areas (i.e., Area0, 1&2) and an external AS i.e, Protocol other than OSPF (Here it is EIGRP 100) and observe the LSAs used.

The ip addresses to be configured on the interfaces of the routers as seen in the above scenario are given below: For R1: For R2: SERIAL0 - 11.0.0.1/8 SERIAL0 - 11.0.0.2/8 LOOPBACK0 - 1.0.0.1/16 SERIAL1 - 12.0.0.1/8 LOOPBACK1 - 1.1.0.1/16 LOOPBACK0 - 3.0.0.1/16 LOOPBACK1 - 3.1.0.1/16 For R3: For R4: SERIAL0 - 13.0.0.1/8 SERIAL0 - 13.0.0.2/8 SERIAL1 - 12.0.0.2/8 LOOPBACK0 - 4.0.0.1/16 LOOPBACK0 - 100.0.0.1/16 LOOPBACK1 - 4.1.0.1/16 LOOPBACK1 - 100.1.0.1/16 LOOPBACK2 - 2.0.0.1/16 LOOPBACK3 - 2.1.0.1/16 OSPF with Router-id configurations are done in all routers as shown below: ROUTER1: R1(config)#router ospf 1 R1(config-router)#router-id 1.1.1.1 R1(config-router)#network 11.0.0.0 0.255.255.255 area 1 R1(config-router)#network 1.0.0.0 0.0.255.255 area 1 R1(config-router)#network 1.1.0.0 0.0.255.255 area 1 ROUTER2:


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R2(config)#router ospf 2 R2(config-router)#router-id 2.2.2.2 R2(config-router)#network 11.0.0.0 0.255.255.255 area 1 R2(config-router)#network 3.0.0.0 0.0.255.255 area 1 R2(config-router)#network 3.1.0.0 0.0.255.255 area 1 R2(config-router)#network 12.0.0.0 0.255.255.255 area 0 ROUTER3: R3(config)#router ospf 3 R3(config-router)#router-id 3.3.3.3 R3(config-router)#network 12.0.0.0 0.255.255.255 area 0 R3(config-router)#network 100.0.0.0 0.0.255.255 area 0 R3(config-router)#network 100.1.0.0 0.0.255.255 area 0 R4(config-router)#network 13.0.0.0 0.255.255.255 area 2 R3(config-router)#network 2.0.0.0 0.0.255.255 area 2 R3(config-router)#network 2.1.0.0 0.0.255.255 area 2 ROUTER4: R4(config)#router ospf 4 R4(config-router)#router-id 4.4.4.4 R4(config-router)#network 4.0.0.0 0.0.255.255 area 2 R4(config-router)#network 4.1.0.0 0.0.255.255 area 2 R4(config-router)#exit OUTPUT VERIFICATION: The Routing table differentiates Inter-area routes with the “O IA” indication. R1#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is not set 1.0.0.0/16 is subnetted, 2 subnets C 1.1.0.0 is directly connected, Loopback1 C 1.0.0.0 is directly connected, Loopback0 2.0.0.0/32 is subnetted, 2 subnets O IA 2.1.0.1 [110/129] via 11.0.0.2, 00:21:00, Serial0 O IA 2.0.0.1 [110/129] via 11.0.0.2, 00:21:00, Serial0 100.0.0.0/32 is subnetted, 2 subnets O IA 100.1.0.1 [110/129] via 11.0.0.2, 00:21:00, Serial0


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O IA 100.0.0.1 [110/129] via 11.0.0.2, 00:21:00, Serial0 3.0.0.0/32 is subnetted, 2 subnets O 3.0.0.1 [110/65] via 11.0.0.2, 00:27:27, Serial0 O 3.1.0.1 [110/65] via 11.0.0.2, 00:27:27, Serial0 4.0.0.0/32 is subnetted, 2 subnets O IA 4.1.0.1 [110/193] via 11.0.0.2, 00:02:16, Serial0 O IA 4.0.0.1 [110/193] via 11.0.0.2, 00:02:16, Serial0 C 11.0.0.0/8 is directly connected, Serial0 O IA 12.0.0.0/8 [110/192] via 11.0.0.2, 00:21:03, Serial0 O IA 13.0.0.0/8 [110/256] via 11.0.0.2, 00:02:16, Serial0 Neighbor table doesn’t display any difference R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 0 FULL/ - 00:00:33 11.0.0.2 Serial0 All significant differences can be observed only in the database table. Database table displays the database of connected areas – in this case area 1 for Router R1. Inter-area routes (Summary Net Link States) for area 1 i.e. routes from aea 0 & other areas, are displayed under the Link Id column & ABR info (through which these networks are learnt) under the ADV Router column. R1#show ip ospf database OSPF Router with ID (1.1.1.1) (Process ID 1) Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 642 0x80000004 0x00163D 4 2.2.2.2 2.2.2.2 1846 0x80000007 0x009EA7 4 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 2.0.0.1 2.2.2.2 1455 0x80000001 0x00B53B 2.1.0.1 2.2.2.2 1455 0x80000001 0x00A946 4.0.0.1 2.2.2.2 328 0x80000001 0x001E90 4.1.0.1 2.2.2.2 328 0x80000001 0x00129B 12.0.0.0 2.2.2.2 1455 0x80000004 0x00AFF5 13.0.0.0 2.2.2.2 328 0x80000002 0x00293D 100.0.0.1 2.2.2.2 1455 0x80000001 0x00B6D7 100.1.0.1 2.2.2.2 1455 0x80000001 0x00AAE2 R2 borders Area 1 & Area 0 and so we’ll be able to view both area databases. Observe that area 1 database of R2 is no different from that of R1. R2#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP


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i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set 1.0.0.0/32 is subnetted, 2 subnets O 1.0.0.1 [110/65] via 11.0.0.1, 00:35:30, Serial0 O 1.1.0.1 [110/65] via 11.0.0.1, 00:35:30, Serial0 2.0.0.0/32 is subnetted, 2 subnets O IA 2.1.0.1 [110/65] via 12.0.0.2, 00:29:09, Serial1 O IA 2.0.0.1 [110/65] via 12.0.0.2, 00:29:09, Serial1 100.0.0.0/32 is subnetted, 2 subnets O 100.1.0.1 [110/65] via 12.0.0.2, 00:29:09, Serial1 O 100.0.0.1 [110/65] via 12.0.0.2, 00:29:09, Serial1 3.0.0.0/16 is subnetted, 2 subnets C 3.1.0.0 is directly connected, Loopback1 C 3.0.0.0 is directly connected, Loopback0 4.0.0.0/32 is subnetted, 2 subnets O IA 4.1.0.1 [110/129] via 12.0.0.2, 00:10:26, Serial1 O IA 4.0.0.1 [110/129] via 12.0.0.2, 00:10:26, Serial1 C 11.0.0.0/8 is directly connected, Serial0 C 12.0.0.0/8 is directly connected, Serial1 O IA 13.0.0.0/8 [110/192] via 12.0.0.2, 00:10:26, Serial1 The inter-area routes shown above are those learnt from R3 (area 2) R2#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 3.3.3.3 1 FULL/ - 00:00:33 12.0.0.2 Serial1 1.1.1.1 1 FULL/ - 00:00:32 11.0.0.1 Serial0 R2#show ip ospf database OSPF Router with ID (2.2.2.2) (Process ID 2) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 2.2.2.2 2.2.2.2 1778 0x80000005 0x86E3 2 3.3.3.3 3.3.3.3 1777 0x80000002 0x1169 4 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 1.0.0.1 2.2.2.2 238 0x80000002 0xC030 1.1.0.1 2.2.2.2 239 0x80000002 0xB43B 2.0.0.1 3.3.3.3 1773 0x80000001 0x1518 2.1.0.1 3.3.3.3 1773 0x80000001 0x923 3.0.0.1 2.2.2.2 239 0x80000002 0x240B 3.1.0.1 2.2.2.2 239 0x80000002 0x1816 4.0.0.1 3.3.3.3 642 0x80000001 0x7D6D 4.1.0.1 3.3.3.3 642 0x80000001 0x7178 11.0.0.0 2.2.2.2 240 0x80000002 0x3EAA 13.0.0.0 3.3.3.3 643 0x80000002 0x881A


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Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 959 0x80000004 0x163D 4 2.2.2.2 2.2.2.2 242 0x80000008 0x9CA8 4 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 2.0.0.1 2.2.2.2 1770 0x80000001 0xB53B 2.1.0.1 2.2.2.2 1770 0x80000001 0xA946 4.0.0.1 2.2.2.2 644 0x80000001 0x1E90 4.1.0.1 2.2.2.2 644 0x80000001 0x129B 12.0.0.0 2.2.2.2 1770 0x80000004 0xAFF5 13.0.0.0 2.2.2.2 644 0x80000002 0x293D 100.0.0.1 2.2.2.2 1771 0x80000001 0xB6D7 100.1.0.1 2.2.2.2 1771 0x80000001 0xAAE2 In the above database table, observe that area 0 database has router-ids of R2 & R3 under the ADV Router column. These being the ABRs of areas 1 & 2, advertise routes into area 0, the networks of areas 1 & 2. We must be able to view similar information on R3 as well. Only that it’ll reveal information reg. area 0 & area 2. R3#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/32 is subnetted, 2 subnets O IA 1.0.0.1 [110/129] via 12.0.0.1, 00:11:52, Serial1 O IA 1.1.0.1 [110/129] via 12.0.0.1, 00:11:52, Serial1 2.0.0.0/16 is subnetted, 2 subnets C 2.0.0.0 is directly connected, Loopback0 C 2.1.0.0 is directly connected, Loopback1 100.0.0.0/16 is subnetted, 2 subnets C 100.0.0.0 is directly connected, Loopback2 C 100.1.0.0 is directly connected, Loopback3 3.0.0.0/32 is subnetted, 2 subnets O IA 3.0.0.1 [110/65] via 12.0.0.1, 00:11:53, Serial1 O IA 3.1.0.1 [110/65] via 12.0.0.1, 00:11:53, Serial1 4.0.0.0/32 is subnetted, 2 subnets O 4.1.0.1 [110/65] via 13.0.0.2, 00:11:55, Serial0 O 4.0.0.1 [110/65] via 13.0.0.2, 00:11:55, Serial0 O IA 11.0.0.0/8 [110/128] via 12.0.0.1, 00:11:55, Serial1 C 12.0.0.0/8 is directly connected, Serial1 C 13.0.0.0/8 is directly connected, Serial0


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R3#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 1 FULL/ - 00:00:39 12.0.0.1 Serial1 4.4.4.4 1 FULL/ - 00:00:39 13.0.0.2 Serial0 R3#show ip ospf database OSPF Router with ID (3.3.3.3) (Process ID 3) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 2.2.2.2 2.2.2.2 1866 0x80000005 0x86E3 2 3.3.3.3 3.3.3.3 1864 0x80000002 0x1169 4 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 1.0.0.1 2.2.2.2 327 0x80000002 0xC030 1.1.0.1 2.2.2.2 327 0x80000002 0xB43B 2.0.0.1 3.3.3.3 1860 0x80000001 0x1518 2.1.0.1 3.3.3.3 1860 0x80000001 0x923 3.0.0.1 2.2.2.2 327 0x80000002 0x240B 3.1.0.1 2.2.2.2 327 0x80000002 0x1816 4.0.0.1 3.3.3.3 728 0x80000001 0x7D6D 4.1.0.1 3.3.3.3 728 0x80000001 0x7178 11.0.0.0 2.2.2.2 328 0x80000002 0x3EAA 13.0.0.0 3.3.3.3 729 0x80000002 0x881A Router Link States (Area 2) Link ID ADV Router Age Seq# Checksum Link count 3.3.3.3 3.3.3.3 737 0x80000003 0x2F06 4 4.4.4.4 4.4.4.4 737 0x80000007 0xBE6A 4 Summary Net Link States (Area 2) Link ID ADV Router Age Seq# Checksum 1.0.0.1 3.3.3.3 1862 0x80000001 0x2786 1.1.0.1 3.3.3.3 1862 0x80000001 0x1B91 3.0.0.1 3.3.3.3 1862 0x80000001 0x8A61 3.1.0.1 3.3.3.3 1862 0x80000001 0x7E6C 11.0.0.0 3.3.3.3 1862 0x80000001 0xA401 12.0.0.0 3.3.3.3 741 0x80000002 0x950E 100.0.0.1 3.3.3.3 1862 0x80000001 0x16B4 100.1.0.1 3.3.3.3 1862 0x80000001 0xABF The output on R4 will appear much similar to that on R1 R4#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area


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N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/32 is subnetted, 2 subnets O IA 1.0.0.1 [110/193] via 13.0.0.1, 00:12:44, Serial0 O IA 1.1.0.1 [110/193] via 13.0.0.1, 00:12:44, Serial0 2.0.0.0/32 is subnetted, 2 subnets O 2.1.0.1 [110/65] via 13.0.0.1, 00:12:44, Serial0 O 2.0.0.1 [110/65] via 13.0.0.1, 00:12:44, Serial0 100.0.0.0/32 is subnetted, 2 subnets O IA 100.1.0.1 [110/65] via 13.0.0.1, 00:12:44, Serial0 O IA 100.0.0.1 [110/65] via 13.0.0.1, 00:12:44, Serial0 3.0.0.0/32 is subnetted, 2 subnets O IA 3.0.0.1 [110/129] via 13.0.0.1, 00:12:45, Serial0 O IA 3.1.0.1 [110/129] via 13.0.0.1, 00:12:45, Serial0 4.0.0.0/16 is subnetted, 2 subnets C 4.0.0.0 is directly connected, Loopback0 C 4.1.0.0 is directly connected, Loopback1 5.0.0.0/16 is subnetted, 2 subnets C 5.1.0.0 is directly connected, Loopback3 C 5.0.0.0 is directly connected, Loopback2 O IA 11.0.0.0/8 [110/192] via 13.0.0.1, 00:12:46, Serial0 O IA 12.0.0.0/8 [110/192] via 13.0.0.1, 00:12:46, Serial0 C 13.0.0.0/8 is directly connected, Serial0 R4#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 3.3.3.3 1 FULL/ - 00:00:39 13.0.0.1 Serial0 R4#show ip ospf database OSPF Router with ID (4.4.4.4) (Process ID 4) Router Link States (Area 2) Link ID ADV Router Age Seq# Checksum Link count 3.3.3.3 3.3.3.3 797 0x80000003 0x2F06 4 4.4.4.4 4.4.4.4 797 0x80000007 0xBE6A 4 Summary Net Link States (Area 2) Link ID ADV Router Age Seq# Checksum 1.0.0.1 3.3.3.3 1922 0x80000001 0x2786 1.1.0.1 3.3.3.3 1922 0x80000001 0x1B91 3.0.0.1 3.3.3.3 1922 0x80000001 0x8A61 3.1.0.1 3.3.3.3 1922 0x80000001 0x7E6C 11.0.0.0 3.3.3.3 1922 0x80000001 0xA401 12.0.0.0 3.3.3.3 801 0x80000002 0x950E 100.0.0.1 3.3.3.3 1922 0x80000001 0x16B4 100.1.0.1 3.3.3.3 1922 0x80000001 0xABF


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EXTERNAL LSA – USING REDISTRIBUTION: Let’s publish 5.0.0.0 network in EIGRP & redistribute it into OSPF to emulate External AS connectivity.

The ip addresses to be configured on the interfaces of the routers as seen in the above scenario are given below: For R1: For R2: SERIAL0 - 11.0.0.1/8 SERIAL0 - 11.0.0.2/8 LOOPBACK0 - 1.0.0.1/16 SERIAL1 - 12.0.0.1/8 LOOPBACK1 - 1.1.0.1/16 LOOPBACK0 - 3.0.0.1/16 LOOPBACK1 - 3.1.0.1/16 For R3: For R4: SERIAL0 - 13.0.0.1/8 SERIAL0 - 13.0.0.2/8 SERIAL1 - 12.0.0.2/8 LOOPBACK0 - 4.0.0.1/16 LOOPBACK0 - 100.0.0.1/16 LOOPBACK1 - 4.1.0.1/16 LOOPBACK1 - 100.1.0.1/16 LOOPBACK2 – 5.0.0.1/16 LOOPBACK2 - 2.0.0.1/16 LOOPBACK3 – 5.1.0.1/16 LOOPBACK3 - 2.1.0.1/16 R4(config)#router eigrp 100 R4(config-router)#network 5.0.0.0 R4(config-router)#no auto-summary EIGRP is redistributed in to OSPF on R4. To redistribute, the command is : R(config)#router ospf <process-id> R(config-router)#redistribute eigrp <AS number> subnets This command is to be configured in the router R4 as shown in the Scenario:


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R4(config)#router ospf 4 R4(config-router)#redistribute eigrp 100 subnets We’ll now be able to observe LSAs 4 & 5 in the database table & external routes are indicated as “O E2” in the routing table. R1#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is not set 1.0.0.0/16 is subnetted, 2 subnets C 1.1.0.0 is directly connected, Loopback1 C 1.0.0.0 is directly connected, Loopback0 2.0.0.0/32 is subnetted, 2 subnets O IA 2.1.0.1 [110/129] via 11.0.0.2, 01:09:38, Serial0 O IA 2.0.0.1 [110/129] via 11.0.0.2, 01:09:38, Serial0 100.0.0.0/32 is subnetted, 2 subnets O IA 100.1.0.1 [110/129] via 11.0.0.2, 01:09:38, Serial0 O IA 100.0.0.1 [110/129] via 11.0.0.2, 01:09:38, Serial0 3.0.0.0/32 is subnetted, 2 subnets O 3.0.0.1 [110/65] via 11.0.0.2, 01:16:04, Serial0 O 3.1.0.1 [110/65] via 11.0.0.2, 01:16:04, Serial0 4.0.0.0/32 is subnetted, 2 subnets O IA 4.1.0.1 [110/193] via 11.0.0.2, 00:50:59, Serial0 O IA 4.0.0.1 [110/193] via 11.0.0.2, 00:50:59, Serial0 5.0.0.0/16 is subnetted, 2 subnets O E2 5.1.0.0 [110/20] via 11.0.0.2, 00:01:21, Serial0 O E2 5.0.0.0 [110/20] via 11.0.0.2, 00:01:21, Serial0 C 11.0.0.0/8 is directly connected, Serial0 O IA 12.0.0.0/8 [110/192] via 11.0.0.2, 01:09:46, Serial0 O IA 13.0.0.0/8 [110/192] via 11.0.0.2, 00:01:27, Serial0 Observe that E2 being the default “type” of LSA 5, only external redistributed metric of 20 is seen on all the routers’ E2 routes. (If they had been modified as E1, the internal metric would have been added). R1#show ip ospf database OSPF Router with ID (1.1.1.1) (Process ID 1) Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 1445 0x80000005 0x00143E 4 2.2.2.2 2.2.2.2 730 0x80000009 0x009AA9 4


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Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 2.0.0.1 2.2.2.2 228 0x80000003 0x00B13D 2.1.0.1 2.2.2.2 228 0x80000003 0x00A548 4.0.0.1 2.2.2.2 1228 0x80000002 0x001C91 4.1.0.1 2.2.2.2 1228 0x80000002 0x00109C 12.0.0.0 2.2.2.2 228 0x80000006 0x00ABF7 13.0.0.0 2.2.2.2 170 0x80000004 0x00A202 100.0.0.1 2.2.2.2 228 0x80000003 0x00B2D9 100.1.0.1 2.2.2.2 228 0x80000003 0x00A6E4 Summary ASB Link States (Area 1) Link ID ADV Router Age Seq# Checksum 4.4.4.4 2.2.2.2 170 0x80000001 0x008B18 LSA 4 is observed in this area. While the Link Id refers to the ASBR’s (R4) Router id, ADV Router is its own ABR’s (R2) router id from which ASBR information is learnt. Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 4.4.4.4 180 0x80000001 0x002E5D 0 5.1.0.0 4.4.4.4 180 0x80000001 0x002268 0 Type -5 LSAs are shown separately – not under any specific area. The ADV Router represents the ASBR (Router-id of R4) Moving to the next router, let’s observe the difference. The routing table is similar to R1. R2#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/32 is subnetted, 2 subnets O 1.0.0.1 [110/65] via 11.0.0.1, 01:18:49, Serial0 O 1.1.0.1 [110/65] via 11.0.0.1, 01:18:49, Serial0 2.0.0.0/32 is subnetted, 2 subnets O IA 2.1.0.1 [110/65] via 12.0.0.2, 00:04:04, Serial1 O IA 2.0.0.1 [110/65] via 12.0.0.2, 00:04:04, Serial1 100.0.0.0/32 is subnetted, 2 subnets O 100.1.0.1 [110/65] via 12.0.0.2, 01:12:28, Serial1 O 100.0.0.1 [110/65] via 12.0.0.2, 01:12:28, Serial1 3.0.0.0/16 is subnetted, 2 subnets C 3.1.0.0 is directly connected, Loopback1 C 3.0.0.0 is directly connected, Loopback0 4.0.0.0/32 is subnetted, 2 subnets


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O IA 4.1.0.1 [110/129] via 12.0.0.2, 00:04:06, Serial1 O IA 4.0.0.1 [110/129] via 12.0.0.2, 00:04:06, Serial1 5.0.0.0/16 is subnetted, 2 subnets O E2 5.1.0.0 [110/20] via 12.0.0.2, 00:04:06, Serial1 O E2 5.0.0.0 [110/20] via 12.0.0.2, 00:04:06, Serial1 C 11.0.0.0/8 is directly connected, Serial0 C 12.0.0.0/8 is directly connected, Serial1 O IA 13.0.0.0/8 [110/128] via 12.0.0.2, 00:04:06, Serial1 The difference can be observed in the Summary ASB Link state. While the ASBR information remains same, the ADV Router is R3 (Router-id 3.3.3.3) for Area 0 & R2 (Router-id 2.2.2.2) for Area 1. This also reveals that the database for a particular area is same irrespective of the router on which we see the detail. R2#show ip ospf database OSPF Router with ID (2.2.2.2) (Process ID 2) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 2.2.2.2 2.2.2.2 316 0x80000007 0x82E5 2 3.3.3.3 3.3.3.3 441 0x80000004 0xD6B 4 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 1.0.0.1 2.2.2.2 819 0x80000003 0xBE31 1.1.0.1 2.2.2.2 819 0x80000003 0xB23C 2.0.0.1 3.3.3.3 441 0x80000003 0x111A 2.1.0.1 3.3.3.3 441 0x80000003 0x525 3.0.0.1 2.2.2.2 819 0x80000003 0x220C 3.1.0.1 2.2.2.2 819 0x80000003 0x1617 4.0.0.1 3.3.3.3 1190 0x80000002 0x7B6E 4.1.0.1 3.3.3.3 1190 0x80000002 0x6F79 11.0.0.0 2.2.2.2 819 0x80000003 0x3CAB 13.0.0.0 3.3.3.3 260 0x80000004 0x2DE Summary ASB Link States (Area 0) Link ID ADV Router Age Seq# Checksum 4.4.4.4 3.3.3.3 261 0x80000001 0xEAF4 Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 1538 0x80000005 0x143E 4 2.2.2.2 2.2.2.2 822 0x80000009 0x9AA9 4 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 2.0.0.1 2.2.2.2 319 0x80000003 0xB13D 2.1.0.1 2.2.2.2 319 0x80000003 0xA548 4.0.0.1 2.2.2.2 1320 0x80000002 0x1C91 4.1.0.1 2.2.2.2 1320 0x80000002 0x109C 12.0.0.0 2.2.2.2 319 0x80000006 0xABF7 13.0.0.0 2.2.2.2 262 0x80000004 0xA202 100.0.0.1 2.2.2.2 319 0x80000003 0xB2D9


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100.1.0.1 2.2.2.2 319 0x80000003 0xA6E4 Summary ASB Link States (Area 1) Link ID ADV Router Age Seq# Checksum 4.4.4.4 2.2.2.2 263 0x80000001 0x8B18 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 4.4.4.4 271 0x80000001 0x2E5D 0 5.1.0.0 4.4.4.4 271 0x80000001 0x2268 0 Finally, if you observe the output of the routers(below) of area 2 (where ASBR exists), LSA type 4 is not present! As the ASBR is in the same area, every router knows about the ASBR from the Router Link state & so no separate LSA is required. R3#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/32 is subnetted, 2 subnets O IA 1.0.0.1 [110/129] via 12.0.0.1, 00:05:38, Serial1 O IA 1.1.0.1 [110/129] via 12.0.0.1, 00:05:38, Serial1 2.0.0.0/16 is subnetted, 2 subnets C 2.0.0.0 is directly connected, Loopback0 C 2.1.0.0 is directly connected, Loopback1 100.0.0.0/16 is subnetted, 2 subnets C 100.0.0.0 is directly connected, Loopback2 C 100.1.0.0 is directly connected, Loopback3 3.0.0.0/32 is subnetted, 2 subnets O IA 3.0.0.1 [110/65] via 12.0.0.1, 00:05:39, Serial1 O IA 3.1.0.1 [110/65] via 12.0.0.1, 00:05:39, Serial1 4.0.0.0/32 is subnetted, 2 subnets O 4.1.0.1 [110/65] via 13.0.0.2, 00:05:41, Serial0 O 4.0.0.1 [110/65] via 13.0.0.2, 00:05:41, Serial0 5.0.0.0/16 is subnetted, 2 subnets O E2 5.1.0.0 [110/20] via 13.0.0.2, 00:05:41, Serial0 O E2 5.0.0.0 [110/20] via 13.0.0.2, 00:05:41, Serial0 O IA 11.0.0.0/8 [110/128] via 12.0.0.1, 00:05:41, Serial1 C 12.0.0.0/8 is directly connected, Serial1 C 13.0.0.0/8 is directly connected, Serial0 R3#show ip ospf database OSPF Router with ID (3.3.3.3) (Process ID 3) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 2.2.2.2 2.2.2.2 407 0x80000007 0x82E5 2 3.3.3.3 3.3.3.3 530 0x80000004 0xD6B 4


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Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 1.0.0.1 2.2.2.2 910 0x80000003 0xBE31 1.1.0.1 2.2.2.2 910 0x80000003 0xB23C 2.0.0.1 3.3.3.3 530 0x80000003 0x111A 2.1.0.1 3.3.3.3 530 0x80000003 0x525 3.0.0.1 2.2.2.2 910 0x80000003 0x220C 3.1.0.1 2.2.2.2 910 0x80000003 0x1617 4.0.0.1 3.3.3.3 1279 0x80000002 0x7B6E 4.1.0.1 3.3.3.3 1279 0x80000002 0x6F79 11.0.0.0 2.2.2.2 910 0x80000003 0x3CAB 13.0.0.0 3.3.3.3 349 0x80000004 0x2DE Summary ASB Link States (Area 0) Link ID ADV Router Age Seq# Checksum 4.4.4.4 3.3.3.3 350 0x80000001 0xEAF4 Router Link States (Area 2) Link ID ADV Router Age Seq# Checksum Link count 3.3.3.3 3.3.3.3 1528 0x80000004 0x2D07 4 4.4.4.4 4.4.4.4 357 0x80000009 0xC064 4 Summary Net Link States (Area 2) Link ID ADV Router Age Seq# Checksum 1.0.0.1 3.3.3.3 532 0x80000003 0x2388 1.1.0.1 3.3.3.3 532 0x80000003 0x1793 3.0.0.1 3.3.3.3 532 0x80000003 0x8663 3.1.0.1 3.3.3.3 532 0x80000003 0x7A6E 11.0.0.0 3.3.3.3 532 0x80000003 0xA003 12.0.0.0 3.3.3.3 1528 0x80000003 0x930F 100.0.0.1 3.3.3.3 532 0x80000003 0x12B6 100.1.0.1 3.3.3.3 532 0x80000003 0x6C1 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 4.4.4.4 358 0x80000001 0x2E5D 0 5.1.0.0 4.4.4.4 359 0x80000001 0x2268 0 R4#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/32 is subnetted, 2 subnets O IA 1.0.0.1 [110/193] via 13.0.0.1, 00:06:52, Serial0 O IA 1.1.0.1 [110/193] via 13.0.0.1, 00:06:52, Serial0 2.0.0.0/32 is subnetted, 2 subnets O 2.1.0.1 [110/65] via 13.0.0.1, 00:06:52, Serial0 O 2.0.0.1 [110/65] via 13.0.0.1, 00:06:52, Serial0


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100.0.0.0/32 is subnetted, 2 subnets O IA 100.1.0.1 [110/65] via 13.0.0.1, 00:06:52, Serial0 O IA 100.0.0.1 [110/65] via 13.0.0.1, 00:06:52, Serial0 3.0.0.0/32 is subnetted, 2 subnets O IA 3.0.0.1 [110/129] via 13.0.0.1, 00:06:54, Serial0 O IA 3.1.0.1 [110/129] via 13.0.0.1, 00:06:54, Serial0 4.0.0.0/16 is subnetted, 2 subnets C 4.0.0.0 is directly connected, Loopback0 C 4.1.0.0 is directly connected, Loopback1 5.0.0.0/16 is subnetted, 2 subnets C 5.1.0.0 is directly connected, Loopback3 C 5.0.0.0 is directly connected, Loopback2 O IA 11.0.0.0/8 [110/192] via 13.0.0.1, 00:06:55, Serial0 O IA 12.0.0.0/8 [110/192] via 13.0.0.1, 00:06:55, Serial0 C 13.0.0.0/8 is directly connected, Serial0 R4#show ip ospf database OSPF Router with ID (4.4.4.4) (Process ID 4) Router Link States (Area 2) Link ID ADV Router Age Seq# Checksum Link count 3.3.3.3 3.3.3.3 1602 0x80000004 0x2D07 4 4.4.4.4 4.4.4.4 29 0x80000009 0xC064 4 Summary Net Link States (Area 2) Link ID ADV Router Age Seq# Checksum 1.0.0.1 3.3.3.3 606 0x80000003 0x2388 1.1.0.1 3.3.3.3 606 0x80000003 0x1793 3.0.0.1 3.3.3.3 606 0x80000003 0x8663 3.1.0.1 3.3.3.3 606 0x80000003 0x7A6E 11.0.0.0 3.3.3.3 606 0x80000003 0xA003 12.0.0.0 3.3.3.3 1602 0x80000003 0x930F 100.0.0.1 3.3.3.3 606 0x80000003 0x12B6 100.1.0.1 3.3.3.3 606 0x80000003 0x6C1 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 4.4.4.4 429 0x80000001 0x2E5D 0 5.1.0.0 4.4.4.4 431 0x80000001 0x2268 0 In the above output, we see all the links. So, summarization has to be done in all ABRs to have only the summary entries. ROUTE SUMMARIZATION: Route summarization is a key to scalability in OSPF. Route summarization helps solve the below problems: • Minimizes routing table entries • Restricts updates of a topology change • Reduces LSA type 3 and 5 flooding and saves CPU resources The two types of summarization are described as follows:


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1) Interarea route summarization: Interarea route summarization is done on area border routers (ABRs) and applies to routes from within each area. To perform effective interarea route summarization, contiguous network numbers within areas should be assigned. The command to configure summarization is:

Router(config-router)#area <area-id> range <address> <mask> 2) External route summarization: External route summarization is specific to external routes that are injected into OSPF via route redistribution. Only autonomous system boundary routers (ASBRs) summarize external routes. The command to configure this summarization is: Router(config-router)#summary-address <ip-address> <mask> If we consider the earlier discussed Scenario, inter-area summarization is done on the Routers R2 and R3 which are ABRs and the external summarization is to done in the router R4 which is an ASBR. The Configuration details are shown as below: R2(config)#router ospf 2 R2(config-router)#area 1 range 1.0.0.0 255.254.0.0 R2(config-router)#area 1 range 3.0.0.0 255.254.0.0 R2(config-router)#area 0 range 100.0.0.0 255.254.0.0 R3(config)#router ospf 3 R3(config-router)#area 0 range 100.0.0.0 255.254.0.0 R3(config-router)#area 2 range 2.0.0.0 255.254.0.0 R3(config-router)#area 2 range 4.0.0.0 255.254.0.0 R4(config)#router ospf 4 R4(config-router)#summary-address 5.0.0.0 255.254.0.0 In the output on the 4 routers, summarised LSAs are seen in the database table and the summary route entries in the routing table (highlighted) as follows: R1#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route Gateway of last resort is not set 1.0.0.0/16 is subnetted, 2 subnets C 1.1.0.0 is directly connected, Loopback1 C 1.0.0.0 is directly connected, Loopback0


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2.0.0.0/15 is subnetted, 1 subnets O IA 2.0.0.0 [110/129] via 11.0.0.2, 00:21:42, Serial0 100.0.0.0/15 is subnetted, 1 subnets O IA 100.0.0.0 [110/129] via 11.0.0.2, 00:13:12, Serial0 3.0.0.0/32 is subnetted, 2 subnets O 3.0.0.1 [110/65] via 11.0.0.2, 02:05:44, Serial0 O 3.1.0.1 [110/65] via 11.0.0.2, 02:05:44, Serial0 4.0.0.0/15 is subnetted, 1 subnets O IA 4.0.0.0 [110/193] via 11.0.0.2, 00:15:02, Serial0 5.0.0.0/15 is subnetted, 1 subnets O E2 5.0.0.0 [110/20] via 11.0.0.2, 00:18:49, Serial0 C 11.0.0.0/8 is directly connected, Serial0 O IA 12.0.0.0/8 [110/128] via 11.0.0.2, 00:25:32, Serial0 O IA 13.0.0.0/8 [110/192] via 11.0.0.2, 00:51:02, Serial0 R1#show ip ospf database OSPF Router with ID (1.1.1.1) (Process ID 1) Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 318 0x80000007 0x001040 4 2.2.2.2 2.2.2.2 1601 0x8000000A 0x0098AA 4 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 2.0.0.0 2.2.2.2 1314 0x80000001 0x00BB37 4.0.0.0 2.2.2.2 912 0x80000001 0x00248C 12.0.0.0 2.2.2.2 1115 0x80000008 0x0025BC 13.0.0.0 2.2.2.2 1115 0x80000005 0x00A003 100.0.0.0 2.2.2.2 803 0x80000001 0x00BCD3 Summary ASB Link States (Area 1) Link ID ADV Router Age Seq# Checksum 4.4.4.4 2.2.2.2 1115 0x80000002 0x008919 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 4.4.4.4 1140 0x80000003 0x002664 0 R2#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 1.0.0.1/32 [110/65] via 11.0.0.1, 00:14:25, Serial0 O 1.0.0.0/15 is a summary, 00:14:25, Null0 O 1.1.0.1/32 [110/65] via 11.0.0.1, 00:14:25, Serial0 2.0.0.0/15 is subnetted, 1 subnets O IA 2.0.0.0 [110/65] via 12.0.0.2, 00:14:25, Serial1 100.0.0.0/8 is variably subnetted, 3 subnets, 2 masks


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O 100.0.0.0/15 is a summary, 00:14:25, Null0 O 100.1.0.1/32 [110/65] via 12.0.0.2, 00:14:25, Serial1 O 100.0.0.1/32 [110/65] via 12.0.0.2, 00:14:26, Serial1 3.0.0.0/8 is variably subnetted, 3 subnets, 2 masks C 3.1.0.0/16 is directly connected, Loopback1 O 3.0.0.0/15 is a summary, 00:14:26, Null0 C 3.0.0.0/16 is directly connected, Loopback0 4.0.0.0/15 is subnetted, 1 subnets O IA 4.0.0.0 [110/129] via 12.0.0.2, 00:14:27, Serial1 5.0.0.0/15 is subnetted, 1 subnets O E2 5.0.0.0 [110/20] via 12.0.0.2, 00:14:27, Serial1 C 11.0.0.0/8 is directly connected, Serial0 C 12.0.0.0/8 is directly connected, Serial1 O IA 13.0.0.0/8 [110/128] via 12.0.0.2, 00:14:28, Serial1 R2#show ip ospf database OSPF Router with ID (2.2.2.2) (Process ID 2) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 2.2.2.2 2.2.2.2 1191 0x80000008 0x80E6 2 3.3.3.3 3.3.3.3 1341 0x80000005 0xB6C 4 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 1.0.0.0 2.2.2.2 1616 0x80000001 0xC82B 2.0.0.0 3.3.3.3 1391 0x80000001 0x1B14 3.0.0.0 2.2.2.2 1608 0x80000001 0x2C06 4.0.0.0 3.3.3.3 989 0x80000001 0x8369 11.0.0.0 2.2.2.2 1677 0x80000004 0x3AAC 13.0.0.0 3.3.3.3 1341 0x80000005 0xFFDF Summary ASB Link States (Area 0) Link ID ADV Router Age Seq# Checksum 4.4.4.4 3.3.3.3 1341 0x80000002 0xE8F5 Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 396 0x80000007 0x1040 4 2.2.2.2 2.2.2.2 1679 0x8000000A 0x98AA 4 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 2.0.0.0 2.2.2.2 1392 0x80000001 0xBB37 4.0.0.0 2.2.2.2 990 0x80000001 0x248C 12.0.0.0 2.2.2.2 1193 0x80000008 0x25BC 13.0.0.0 2.2.2.2 1193 0x80000005 0xA003 100.0.0.0 2.2.2.2 881 0x80000001 0xBCD3 Summary ASB Link States (Area 1) Link ID ADV Router Age Seq# Checksum 4.4.4.4 2.2.2.2 1193 0x80000002 0x8919 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag


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5.0.0.0 4.4.4.4 1218 0x80000003 0x2664 0 R3#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/15 is subnetted, 1 subnets O IA 1.0.0.0 [110/129] via 12.0.0.1, 00:16:46, Serial1 2.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 2.0.0.0/15 is a summary, 00:16:46, Null0 C 2.0.0.0/16 is directly connected, Loopback0 C 2.1.0.0/16 is directly connected, Loopback1 100.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 100.0.0.0/15 is a summary, 00:16:46, Null0 C 100.0.0.0/16 is directly connected, Loopback2 C 100.1.0.0/16 is directly connected, Loopback3 3.0.0.0/15 is subnetted, 1 subnets O IA 3.0.0.0 [110/65] via 12.0.0.1, 00:16:47, Serial1 4.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 4.0.0.0/15 is a summary, 00:16:48, Null0 O 4.1.0.1/32 [110/65] via 13.0.0.2, 00:16:48, Serial0 O 4.0.0.1/32 [110/65] via 13.0.0.2, 00:16:48, Serial0 5.0.0.0/15 is subnetted, 1 subnets O E2 5.0.0.0 [110/20] via 13.0.0.2, 00:16:48, Serial0 O IA 11.0.0.0/8 [110/128] via 12.0.0.1, 00:16:48, Serial1 C 12.0.0.0/8 is directly connected, Serial1 C 13.0.0.0/8 is directly connected, Serial0 R3#show ip ospf database OSPF Router with ID (3.3.3.3) (Process ID 3) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 2.2.2.2 2.2.2.2 1219 0x80000008 0x80E6 2 3.3.3.3 3.3.3.3 1367 0x80000005 0xB6C 4 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 1.0.0.0 2.2.2.2 1645 0x80000001 0xC82B 2.0.0.0 3.3.3.3 1417 0x80000001 0x1B14 3.0.0.0 2.2.2.2 1637 0x80000001 0x2C06 4.0.0.0 3.3.3.3 1015 0x80000001 0x8369 11.0.0.0 2.2.2.2 1706 0x80000004 0x3AAC 13.0.0.0 3.3.3.3 1367 0x80000005 0xFFDF Summary ASB Link States (Area 0) Link ID ADV Router Age Seq# Checksum 4.4.4.4 3.3.3.3 1367 0x80000002 0xE8F5


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Router Link States (Area 2) Link ID ADV Router Age Seq# Checksum Link count 3.3.3.3 3.3.3.3 327 0x80000006 0x2909 4 4.4.4.4 4.4.4.4 1297 0x8000000A 0xBE65 4 Summary Net Link States (Area 2) Link ID ADV Router Age Seq# Checksum 1.0.0.0 3.3.3.3 1645 0x80000001 0x2D82 3.0.0.0 3.3.3.3 1637 0x80000001 0x905D 11.0.0.0 3.3.3.3 1369 0x80000004 0x9E04 12.0.0.0 3.3.3.3 1444 0x80000005 0xDD3 100.0.0.0 3.3.3.3 1444 0x80000001 0x1CB0 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 4.4.4.4 1244 0x80000003 0x2664 0 R4#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/15 is subnetted, 1 subnets O IA 1.0.0.0 [110/193] via 13.0.0.1, 00:06:05, Serial0 2.0.0.0/32 is subnetted, 2 subnets O 2.1.0.1 [110/65] via 13.0.0.1, 00:06:05, Serial0 O 2.0.0.1 [110/65] via 13.0.0.1, 00:06:05, Serial0 100.0.0.0/15 is subnetted, 1 subnets O IA 100.0.0.0 [110/65] via 13.0.0.1, 00:06:05, Serial0 3.0.0.0/15 is subnetted, 1 subnets O IA 3.0.0.0 [110/129] via 13.0.0.1, 00:06:05, Serial0 4.0.0.0/16 is subnetted, 2 subnets C 4.0.0.0 is directly connected, Loopback0 C 4.1.0.0 is directly connected, Loopback1 5.0.0.0/8 is variably subnetted, 3 subnets, 2 masks C 5.1.0.0/16 is directly connected, Loopback3 O 5.0.0.0/15 is a summary, 00:21:06, Null0 C 5.0.0.0/16 is directly connected, Loopback2 O IA 11.0.0.0/8 [110/192] via 13.0.0.1, 00:06:07, Serial0 O IA 12.0.0.0/8 [110/128] via 13.0.0.1, 00:06:07, Serial0 C 13.0.0.0/8 is directly connected, Serial0 R4#show ip ospf database OSPF Router with ID (4.4.4.4) (Process ID 4) Router Link States (Area 2) Link ID ADV Router Age Seq# Checksum Link count 3.3.3.3 3.3.3.3 359 0x80000006 0x2909 4 4.4.4.4 4.4.4.4 1325 0x8000000A 0xBE65 4


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Summary Net Link States (Area 2) Link ID ADV Router Age Seq# Checksum 1.0.0.0 3.3.3.3 1676 0x80000001 0x2D82 3.0.0.0 3.3.3.3 1667 0x80000001 0x905D 11.0.0.0 3.3.3.3 1399 0x80000004 0x9E04 12.0.0.0 3.3.3.3 1474 0x80000005 0xDD3 100.0.0.0 3.3.3.3 1474 0x80000001 0x1CB0 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 4.4.4.4 1273 0x80000003 0x2664 0 AREA types: This topic describes the different OSPF area types: • Standard area • Backbone area (Transit area) • Stub area • Totally stubby area(TSA) • Not so stubby area(NSSA) The characteristics assigned to an area control the type of route information that it receives. Standard area: This is the default area type & it accepts all LSAs. Backbone area (Transit area): The backbone area is the central area to which all other areas connect to exchange information. The backbone area is always area 0. The OSPF backbone allows all LSAs into it. Stub area: This area does not accept information about routes external (LSA 4 & 5) to the autonomous system (AS) - non-OSPF routes. They use a default route instead. Stub areas cannot contain autonomous system boundary routers (ASBRs) (except that the ABRs may also be ASBRs). Totally stubby area: This area does not accept external AS routes or summary routes from other areas internal to the AS (LSAs 3,4 & 5 not allowed). A default route used instead. Totally stubby areas cannot contain ASBRs (except that the ABRs may also be ASBRs). NSSA: NSSA is an addendum to the OSPF RFC. This area defines a special LSA type 7.An NSSA offers benefits that are similar to those of a stub or totally stubby area. However, NSSAs allow ASBRs, which is contrary to stub area. 1) Configuration of STUB Area: An area can be stub if:

• There is a single ABR (typically) • All routers in the area are configured as stub routers. • There is no ASBR in the area. • The area is not area 0. • No virtual links go through the area. The command to configure the stub area under OSPF process is given below:


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Router(config-router)#area <area-id> stub Where, all routers in the stub area must use the stub command. we can now observe the following: • External LSAs are stopped from entering the stub area. • Default route is advertised into stub area by the ABR. For the Scenario discussed above,area1 is to be configured as stub and the outputs verified for the STUB area as follows:

The interface configurations and OSPF-MULTIAREA configurations are done as before. Now, the stub area configuration has to be done as follows: CONFIGURATION: The configuration are done in the Routers of the area1(i.e,R1 and R2): R1(config)#router ospf 1 R1(config-router)#area 1 stub R2(config)#router ospf 2 R2(config-router)#area 1 stub OUTPUT VERIFICATION: R1#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is 11.0.0.2 to network 0.0.0.0 1.0.0.0/16 is subnetted, 2 subnets C 1.1.0.0 is directly connected, Loopback1 C 1.0.0.0 is directly connected, Loopback0 2.0.0.0/15 is subnetted, 1 subnets O IA 2.0.0.0 [110/129] via 11.0.0.2, 00:01:17, Serial0 100.0.0.0/15 is subnetted, 1 subnets


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O IA 100.0.0.0 [110/129] via 11.0.0.2, 00:01:17, Serial0 3.0.0.0/32 is subnetted, 2 subnets O 3.0.0.1 [110/65] via 11.0.0.2, 00:01:17, Serial0 O 3.1.0.1 [110/65] via 11.0.0.2, 00:01:17, Serial0 4.0.0.0/15 is subnetted, 1 subnets O IA 4.0.0.0 [110/193] via 11.0.0.2, 00:01:18, Serial0 C 11.0.0.0/8 is directly connected, Serial0 O IA 12.0.0.0/8 [110/128] via 11.0.0.2, 00:01:20, Serial0 O IA 13.0.0.0/8 [110/192] via 11.0.0.2, 00:01:20, Serial0 O*IA 0.0.0.0/0 [110/65] via 11.0.0.2, 00:01:20, Serial0 In the above output, No external routes are seen in the Routing Table – only a default route points to the ABR of the area. The same will be reflected in the database table Summary Net Link States (as seen below). R1#show ip ospf database OSPF Router with ID (1.1.1.1) (Process ID 1) Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 138 0x80000009 0x002A26 4 2.2.2.2 2.2.2.2 138 0x8000000D 0x00B091 4 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 0.0.0.0 2.2.2.2 144 0x80000001 0x0075C0 2.0.0.0 2.2.2.2 144 0x80000003 0x00D51D 4.0.0.0 2.2.2.2 13 0x80000003 0x003E72 12.0.0.0 2.2.2.2 13 0x8000000A 0x003FA2 13.0.0.0 2.2.2.2 1 3 0x80000007 0x00BAE8 100.0.0.0 2.2.2.2 144 0x80000002 0x00D8B8 But R2 is connected to area 0 as well. We’ll be able to observe all details in area 0 This is seen in the below outputs: R2#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 1.0.0.1/32 [110/65] via 11.0.0.1, 00:42:01, Serial0 O 1.0.0.0/15 is a summary, 00:42:01, Null0 O 1.1.0.1/32 [110/65] via 11.0.0.1, 00:42:01, Serial0 2.0.0.0/15 is subnetted, 1 subnets O IA 2.0.0.0 [110/65] via 12.0.0.2, 00:40:36, Serial1 100.0.0.0/8 is variably subnetted, 3 subnets, 2 masks


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O 100.0.0.0/15 is a summary, 00:41:51, Null0 O 100.1.0.1/32 [110/65] via 12.0.0.2, 00:41:51, Serial1 O 100.0.0.1/32 [110/65] via 12.0.0.2, 00:41:52, Serial1 3.0.0.0/8 is variably subnetted, 3 subnets, 2 masks C 3.1.0.0/16 is directly connected, Loopback1 O 3.0.0.0/15 is a summary, 00:42:02, Null0 C 3.0.0.0/16 is directly connected, Loopback0 4.0.0.0/15 is subnetted, 1 subnets O IA 4.0.0.0 [110/129] via 12.0.0.2, 00:40:38, Serial1 5.0.0.0/15 is subnetted, 1 subnets O E2 5.0.0.0 [110/20] via 12.0.0.2, 00:40:38, Serial1 C 11.0.0.0/8 is directly connected, Serial0 C 12.0.0.0/8 is directly connected, Serial1 O IA 13.0.0.0/8 [110/192] via 12.0.0.2, 00:40:38, Serial1 R2#show ip ospf database OSPF Router with ID (2.2.2.2) (Process ID 2) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 2.2.2.2 2.2.2.2 518 0x80000003 0x8AE1 2 3.3.3.3 3.3.3.3 615 0x80000004 0xD6B 4 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 1.0.0.0 2.2.2.2 518 0x80000002 0xC62C 2.0.0.0 3.3.3.3 615 0x80000002 0x1915 3.0.0.0 2.2.2.2 518 0x80000002 0x2A07 4.0.0.0 3.3.3.3 615 0x80000002 0x816A 11.0.0.0 2.2.2.2 518 0x80000002 0x3EAA 13.0.0.0 3.3.3.3 615 0x80000003 0x861B Summary ASB Link States (Area 0) Link ID ADV Router Age Seq# Checksum 4.4.4.4 3.3.3.3 615 0x80000002 0xE8F5 Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 692 0x80000006 0x1241 4 2.2.2.2 2.2.2.2 520 0x80000003 0x4FFC 4 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 0.0.0.0 2.2.2.2 520 0x80000002 0x73C1 2.0.0.0 2.2.2.2 520 0x80000002 0xD71C 4.0.0.0 2.2.2.2 520 0x80000002 0x4071 12.0.0.0 2.2.2.2 520 0x80000003 0xCFD8 13.0.0.0 2.2.2.2 520 0x80000003 0x4522 100.0.0.0 2.2.2.2 520 0x80000002 0xD8B8 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 4.4.4.4 437 0x80000002 0x2863 0


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R3#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/15 is subnetted, 1 subnets O IA 1.0.0.0 [110/129] via 12.0.0.1, 00:44:16, Serial1 2.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 2.0.0.0/15 is a summary, 00:44:16, Null0 C 2.0.0.0/16 is directly connected, Loopback0 C 2.1.0.0/16 is directly connected, Loopback1 100.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 100.0.0.0/15 is a summary, 00:45:23, Null0 C 100.0.0.0/16 is directly connected, Loopback2 C 100.1.0.0/16 is directly connected, Loopback3 3.0.0.0/15 is subnetted, 1 subnets O IA 3.0.0.0 [110/65] via 12.0.0.1, 00:44:17, Serial1 4.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 4.0.0.0/15 is a summary, 00:44:18, Null0 O 4.1.0.1/32 [110/65] via 13.0.0.2, 00:44:18, Serial0 O 4.0.0.1/32 [110/65] via 13.0.0.2, 00:44:18, Serial0 5.0.0.0/15 is subnetted, 1 subnets O E2 5.0.0.0 [110/20] via 13.0.0.2, 00:44:18, Serial0 O IA 11.0.0.0/8 [110/128] via 12.0.0.1, 00:44:18, Serial1 C 12.0.0.0/8 is directly connected, Serial1 C 13.0.0.0/8 is directly connected, Serial0 R3#show ip ospf database OSPF Router with ID (3.3.3.3) (Process ID 3) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 2.2.2.2 2.2.2.2 731 0x80000003 0x8AE1 2 3.3.3.3 3.3.3.3 827 0x80000004 0xD6B 4 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 1.0.0.0 2.2.2.2 731 0x80000002 0xC62C 2.0.0.0 3.3.3.3 827 0x80000002 0x1915 3.0.0.0 2.2.2.2 731 0x80000002 0x2A07 4.0.0.0 3.3.3.3 827 0x80000002 0x816A 11.0.0.0 2.2.2.2 731 0x80000002 0x3EAA 13.0.0.0 3.3.3.3 827 0x80000003 0x861B Summary ASB Link States (Area 0) Link ID ADV Router Age Seq# Checksum 4.4.4.4 3.3.3.3 827 0x80000002 0xE8F5 Router Link States (Area 2)


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Link ID ADV Router Age Seq# Checksum Link count 3.3.3.3 3.3.3.3 827 0x80000005 0x2B08 4 4.4.4.4 4.4.4.4 648 0x80000003 0x66C4 4 Summary Net Link States (Area 2) Link ID ADV Router Age Seq# Checksum 1.0.0.0 3.3.3.3 828 0x80000002 0x2B83 3.0.0.0 3.3.3.3 828 0x80000002 0x8E5E 11.0.0.0 3.3.3.3 828 0x80000002 0xA202 12.0.0.0 3.3.3.3 828 0x80000003 0x930F 100.0.0.0 3.3.3.3 828 0x80000002 0x1AB1 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 4.4.4.4 648 0x80000002 0x2863 0 R4# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set 1.0.0.0/15 is subnetted, 1 subnets O IA 1.0.0.0 [110/193] via 13.0.0.1, 00:45:35, Serial0 2.0.0.0/32 is subnetted, 2 subnets O 2.1.0.1 [110/65] via 13.0.0.1, 00:45:35, Serial0 O 2.0.0.1 [110/65] via 13.0.0.1, 00:45:35, Serial0 100.0.0.0/15 is subnetted, 1 subnets O IA 100.0.0.0 [110/65] via 13.0.0.1, 00:45:35, Serial0 3.0.0.0/15 is subnetted, 1 subnets O IA 3.0.0.0 [110/129] via 13.0.0.1, 00:45:35, Serial0 4.0.0.0/16 is subnetted, 2 subnets C 4.0.0.0 is directly connected, Loopback0 C 4.1.0.0 is directly connected, Loopback1 5.0.0.0/8 is variably subnetted, 3 subnets, 2 masks C 5.1.0.0/16 is directly connected, Loopback3 O 5.0.0.0/15 is a summary, 00:45:53, Null0 C 5.0.0.0/16 is directly connected, Loopback2 O IA 11.0.0.0/8 [110/192] via 13.0.0.1, 00:45:37, Serial0 O IA 12.0.0.0/8 [110/192] via 13.0.0.1, 00:45:37, Serial0 C 13.0.0.0/8 is directly connected, Serial0 R4# show ip ospf database OSPF Router with ID (4.4.4.4) (Process ID 4) Router Link States (Area 2) Link ID ADV Router Age Seq# Checksum Link count 3.3.3.3 3.3.3.3 916 0x80000005 0x2B08 4 4.4.4.4 4.4.4.4 734 0x80000003 0x66C4 4


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Summary Net Link States (Area 2) Link ID ADV Router Age Seq# Checksum 1.0.0.0 3.3.3.3 916 0x80000002 0x2B83 3.0.0.0 3.3.3.3 916 0x80000002 0x8E5E 11.0.0.0 3.3.3.3 916 0x80000002 0xA202 12.0.0.0 3.3.3.3 916 0x80000003 0x930F 100.0.0.0 3.3.3.3 916 0x80000002 0x1AB1 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 4.4.4.4 734 0x80000002 0x2863 0 2) Configuring TOTALLY STUBBY AREA(TSA): An area can be totally stub with the following characteristics: • External LSAs are stopped. • Summary LSAs are stopped. • Routing table is reduced to a minimum. • All routers must be configured as stub. • ABR must be configured as totally stubby. • This is a Cisco proprietary feature. The command to configure totally stub area under OSPF process is same as in stub area for all routers inside the TSA: Router(config-router)#area <area-id> stub But the ABR has to be configured as shown below: Router(config-router)#area <area-id> stub no-summary In the below scenario, area1 is to be configured as totally-stub :

The interface configurations and OSPF-MULTIAREA configurations are done as in MULTIAREA scenario. TSA specific configuration has to be done as follows:


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CONFIGURATION: The configuration changes are only done in the Routers of area1(R1 and R2): In the internal router of area1 (R1): R1(config)#router ospf 1 R1(config-router)#area 1 stub In the ABR router of area1 (R2): R2(config)#router ospf 2 R2(config-router)#area 1 stub no-summary OUTPUT VERIFICATION: We will see that router R1, which is completely inside area1, which is configured as TSA does not receive any inter-area and external routes (routing table) and also it doesn't receive any summary LSAs- type 3,type4 and type5 LSAs which we’ll see in R1’s database table: R1#show ip route Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route Gateway of last resort is 11.0.0.2 to network 0.0.0.0 1.0.0.0/16 is subnetted, 2 subnets C 1.1.0.0 is directly connected, Loopback1 C 1.0.0.0 is directly connected, Loopback0 3.0.0.0/32 is subnetted, 2 subnets O 3.0.0.1 [110/65] via 11.0.0.2, 00:04:46, Serial0 O 3.1.0.1 [110/65] via 11.0.0.2, 00:04:46, Serial0 C 11.0.0.0/8 is directly connected, Serial0 O*IA 0.0.0.0/0 [110/65] via 11.0.0.2, 00:04:46, Serial0 R1#show ip ospf database OSPF Router with ID (1.1.1.1) (Process ID 1) Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 306 0x80000009 0x000C44 4 2.2.2.2 2.2.2.2 307 0x80000006 0x0049FF 4 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 0.0.0.0 2.2.2.2 308 0x80000004 0x006FC3 However, R2 being an ABR, will receive all LSAs by virtue of its area 0 connectivity – as seen in the below outputs.


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R2#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set 1.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 1.0.0.1/32 [110/65] via 11.0.0.1, 00:02:53, Serial0 O 1.0.0.0/15 is a summary, 00:02:53, Null0 O 1.1.0.1/32 [110/65] via 11.0.0.1, 00:02:53, Serial0 2.0.0.0/15 is subnetted, 1 subnets O IA 2.0.0.0 [110/65] via 12.0.0.2, 00:02:53, Serial1 100.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 100.0.0.0/15 is a summary, 00:03:13, Null0 O 100.1.0.1/32 [110/65] via 12.0.0.2, 00:03:13, Serial1 O 100.0.0.1/32 [110/65] via 12.0.0.2, 00:03:14, Serial1 3.0.0.0/8 is variably subnetted, 3 subnets, 2 masks C 3.1.0.0/16 is directly connected, Loopback1 O 3.0.0.0/15 is a summary, 00:02:54, Null0 C 3.0.0.0/16 is directly connected, Loopback0 4.0.0.0/15 is subnetted, 1 subnets O IA 4.0.0.0 [110/129] via 12.0.0.2, 00:02:55, Serial1 5.0.0.0/15 is subnetted, 1 subnets O E2 5.0.0.0 [110/20] via 12.0.0.2, 00:02:55, Serial1 C 11.0.0.0/8 is directly connected, Serial0 C 12.0.0.0/8 is directly connected, Serial1 O IA 13.0.0.0/8 [110/192] via 12.0.0.2, 00:02:55, Serial1 R2#show ip ospf database OSPF Router with ID (2.2.2.2) (Process ID 2) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 2.2.2.2 2.2.2.2 1356 0x80000004 0x88E2 2 3.3.3.3 3.3.3.3 1426 0x80000005 0xB6C 4 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 1.0.0.0 2.2.2.2 195 0x80000001 0xC82B 2.0.0.0 3.3.3.3 1426 0x80000003 0x1716 3.0.0.0 2.2.2.2 1356 0x80000003 0x2808 4.0.0.0 3.3.3.3 1426 0x80000003 0x7F6B 11.0.0.0 2.2.2.2 195 0x80000004 0xBCE9 13.0.0.0 3.3.3.3 1426 0x80000004 0x841C Summary ASB Link States (Area 0) Link ID ADV Router Age Seq# Checksum 4.4.4.4 3.3.3.3 1426 0x80000003 0xE6F6


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Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 208 0x80000009 0xC44 4 2.2.2.2 2.2.2.2 209 0x80000006 0x49FF 4 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 0.0.0.0 2.2.2.2 210 0x80000004 0x6FC3 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 4.4.4.4 1271 0x80000003 0x2664 0 As R3 & R4 belong to normal areas, they will not restrict any LSAs & their tables would remain as in a normal multi-area scenario – hence not discussed. 3) Configuration of NOT-SO-STUBBY AREA(NSSA): The characteristics of Not-so-stubby area are as follows: • NSSA is an RFC addendum. • NSSA breaks the stub area rules. • ASBR is allowed in NSSA. • Special LSA type 7 defined, sent by NSSA ASBR into this area. • ABR converts LSA type 7 to LSA type 5 & sends it into area 0.

• ABR sends default route into NSSA instead of external routes from other ASBRs.

The command to configure NSSA area is direct & simple: Router(config-router)#area <area-id> nssa (Earlier IOS versions of) NSSA ABR must be configured to generate a default route into the NSSA. Automated in Latest IOS (12.4). Router(config-router)#area <area-id> nssa default-information-originate Here, this keyword default-information-originate is used to generate and inject a type 7 default route into the NSSA area. Let’s configure area 2 as NSSA:


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The interface configurations and OSPF-MULTIAREA configurations are done as before The Not-so-stubby area configuration has to be done as follows: CONFIGURATION: The configuration are done in the Routers of the area 2 (R3 and R4): In the ASBR router of area2 (R4): R4(config)#router ospf 4 R4(config-router)#area 2 nssa In the ABR router of area2 (R3): R3(config)#router ospf 3 R3(config-router)#area 2 nssa OUTPUT VERIFICATION: All the routers in AREA2 i.e,R3 and R4 are configured as NSSA and the outputs of the router R4 in that area shows the TYPE-7 LSA instead of TYPE-5 LSA and the ABR in that area2 shows TYPE-7 LSA for AREA2 and it has the converted this into TYPE-5 LSA to be sent into the BACKBONE and other areas. These are seen in the database table. In the routing table, We can see the indication of external route as O N2 which was seen before as O E2 where, O N2 denotes the OSPF NSSA external type 2. The outputs are shown below: R3#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/15 is subnetted, 1 subnets O IA 1.0.0.0 [110/129] via 12.0.0.1, 00:00:29, Serial1 2.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 2.0.0.0/15 is a summary, 00:00:29, Null0 C 2.0.0.0/16 is directly connected, Loopback0 C 2.1.0.0/16 is directly connected, Loopback1 100.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 100.0.0.0/15 is a summary, 00:00:29, Null0 C 100.0.0.0/16 is directly connected, Loopback2 C 100.1.0.0/16 is directly connected, Loopback3 3.0.0.0/15 is subnetted, 1 subnets O IA 3.0.0.0 [110/65] via 12.0.0.1, 00:00:30, Serial1 4.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 4.0.0.0/15 is a summary, 00:00:31, Null0 O 4.1.0.1/32 [110/65] via 13.0.0.2, 00:00:31, Serial0


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O 4.0.0.1/32 [110/65] via 13.0.0.2, 00:00:31, Serial0 5.0.0.0/15 is subnetted, 1 subnets O N2 5.0.0.0 [110/20] via 13.0.0.2, 00:00:31, Serial0 O IA 11.0.0.0/8 [110/128] via 12.0.0.1, 00:00:31, Serial1 C 12.0.0.0/8 is directly connected, Serial1 C 13.0.0.0/8 is directly connected, Serial0 R3#show ip ospf database OSPF Router with ID (3.3.3.3) (Process ID 3) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 2.2.2.2 2.2.2.2 719 0x80000006 0x84E4 2 3.3.3.3 3.3.3.3 63 0x80000008 0xB67 4 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 1.0.0.0 2.2.2.2 284 0x80000001 0xC82B 2.0.0.0 3.3.3.3 802 0x80000005 0x1318 3.0.0.0 2.2.2.2 719 0x80000005 0x240A 4.0.0.0 3.3.3.3 53 0x80000001 0x8369 11.0.0.0 2.2.2.2 310 0x80000006 0x36AE 13.0.0.0 3.3.3.3 73 0x80000007 0xFBE1 Router Link States (Area 2) Link ID ADV Router Age Seq# Checksum Link count 3.3.3.3 3.3.3.3 57 0x8000000B 0xCA5A 4 4.4.4.4 4.4.4.4 58 0x80000008 0x21E 4 Summary Net Link States (Area 2) Link ID ADV Router Age Seq# Checksum 1.0.0.0 3.3.3.3 65 0x80000002 0xD0D7 3.0.0.0 3.3.3.3 65 0x80000006 0x2CB6 11.0.0.0 3.3.3.3 65 0x80000007 0x3E5B 12.0.0.0 3.3.3.3 65 0x80000007 0xAE2A 100.0.0.0 3.3.3.3 65 0x80000006 0xB70A Type-7 AS External Link States (Area 2) Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 4.4.4.4 118 0x80000001 0xDBA0 0 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 3.3.3.3 54 0x80000001 0x8EFB 0 R4#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/15 is subnetted, 1 subnets O IA 1.0.0.0 [110/193] via 13.0.0.1, 00:02:12, Serial0


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2.0.0.0/32 is subnetted, 2 subnets O 2.1.0.1 [110/65] via 13.0.0.1, 00:02:12, Serial0 O 2.0.0.1 [110/65] via 13.0.0.1, 00:02:12, Serial0 100.0.0.0/15 is subnetted, 1 subnets O IA 100.0.0.0 [110/65] via 13.0.0.1, 00:02:12, Serial0 3.0.0.0/15 is subnetted, 1 subnets O IA 3.0.0.0 [110/129] via 13.0.0.1, 00:02:12, Serial0 4.0.0.0/16 is subnetted, 2 subnets C 4.0.0.0 is directly connected, Loopback0 C 4.1.0.0 is directly connected, Loopback1 5.0.0.0/8 is variably subnetted, 3 subnets, 2 masks C 5.1.0.0/16 is directly connected, Loopback3 O 5.0.0.0/15 is a summary, 02:26:41, Null0 C 5.0.0.0/16 is directly connected, Loopback2 O IA 11.0.0.0/8 [110/192] via 13.0.0.1, 00:02:14, Serial0 O IA 12.0.0.0/8 [110/128] via 13.0.0.1, 00:02:14, Serial0 C 13.0.0.0/8 is directly connected, Serial0 R4#show ip ospf database OSPF Router with ID (4.4.4.4) (Process ID 4) Router Link States (Area 2) Link ID ADV Router Age Seq# Checksum Link count 3.3.3.3 3.3.3.3 151 0x8000000B 0xCA5A 4 4.4.4.4 4.4.4.4 151 0x80000008 0x21E 4 Summary Net Link States (Area 2) Link ID ADV Router Age Seq# Checksum 1.0.0.0 3.3.3.3 157 0x80000002 0xD0D7 3.0.0.0 3.3.3.3 157 0x80000006 0x2CB6 11.0.0.0 3.3.3.3 157 0x80000007 0x3E5B 12.0.0.0 3.3.3.3 157 0x80000007 0xAE2A 100.0.0.0 3.3.3.3 157 0x80000006 0xB70A Type-7 AS External Link States (Area 2) Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 4.4.4.4 209 0x80000001 0xDBA0 0 There is no default route seen in the TYPE-7 LSA. Because, the default route isn’t generated automatically, we have to manually generate and inject the default route into the NSSA(area2).The command is configured on R3 (ABR) as below: CONFIGURATION: In the ABR router of area2 (R3): R3(config)#router ospf 3 R3(config-router)#area 2 nssa default-information-originate OUTPUT VERIFICATION: R3#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area


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N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/15 is subnetted, 1 subnets O IA 1.0.0.0 [110/129] via 12.0.0.1, 00:22:25, Serial1 2.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 2.0.0.0/15 is a summary, 00:22:26, Null0 C 2.0.0.0/16 is directly connected, Loopback0 C 2.1.0.0/16 is directly connected, Loopback1 100.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 100.0.0.0/15 is a summary, 00:22:36, Null0 C 100.0.0.0/16 is directly connected, Loopback2 C 100.1.0.0/16 is directly connected, Loopback3 3.0.0.0/15 is subnetted, 1 subnets O IA 3.0.0.0 [110/65] via 12.0.0.1, 00:22:27, Serial1 4.0.0.0/8 is variably subnetted, 3 subnets, 2 masks O 4.0.0.0/15 is a summary, 00:22:28, Null0 O 4.1.0.1/32 [110/65] via 13.0.0.2, 00:22:28, Serial0 O 4.0.0.1/32 [110/65] via 13.0.0.2, 00:22:28, Serial0 5.0.0.0/15 is subnetted, 1 subnets O N2 5.0.0.0 [110/20] via 13.0.0.2, 00:22:28, Serial0 O IA 11.0.0.0/8 [110/128] via 12.0.0.1, 00:22:28, Serial1 C 12.0.0.0/8 is directly connected, Serial1 C 13.0.0.0/8 is directly connected, Serial0 R3#show ip ospf database OSPF Router with ID (3.3.3.3) (Process ID 3) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 2.2.2.2 2.2.2.2 555 0x80000007 0x82E5 2 3.3.3.3 3.3.3.3 1380 0x8000000A 0x769 4 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 1.0.0.0 2.2.2.2 294 0x80000002 0xC62C 2.0.0.0 3.3.3.3 600 0x80000006 0x1119 3.0.0.0 2.2.2.2 555 0x80000006 0x220B 4.0.0.0 3.3.3.3 1360 0x80000001 0x8369 11.0.0.0 2.2.2.2 294 0x80000007 0x34AF 13.0.0.0 3.3.3.3 79 0x80000008 0xF9E2 Router Link States (Area 2) Link ID ADV Router Age Seq# Checksum Link count 3.3.3.3 3.3.3.3 1371 0x8000000E 0xC45D 4 4.4.4.4 4.4.4.4 1368 0x8000000A 0xFD20 4 Summary Net Link States (Area 2) Link ID ADV Router Age Seq# Checksum 1.0.0.0 3.3.3.3 1384 0x80000004 0xCCD9


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3.0.0.0 3.3.3.3 1384 0x80000008 0x28B8 11.0.0.0 3.3.3.3 1384 0x8000000A 0x385E 12.0.0.0 3.3.3.3 1384 0x80000009 0xAA2C 100.0.0.0 3.3.3.3 1384 0x80000008 0xB30C Type-7 AS External Link States (Area 2) Link ID ADV Router Age Seq# Checksum Tag 0.0.0.0 3.3.3.3 1384 0x80000001 0xE2BE 0 5.0.0.0 4.4.4.4 2 0x80000002 0xD9A1 0 Type-5 AS External Link States Link ID ADV Router Age Seq# Checksum Tag 5.0.0.0 3.3.3.3 1362 0x80000001 0x8EFB 0 R4#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is 13.0.0.1 to network 0.0.0.0 1.0.0.0/15 is subnetted, 1 subnets O IA 1.0.0.0 [110/193] via 13.0.0.1, 00:33:12, Serial0 2.0.0.0/32 is subnetted, 2 subnets O 2.1.0.1 [110/65] via 13.0.0.1, 00:33:12, Serial0 O 2.0.0.1 [110/65] via 13.0.0.1, 00:33:12, Serial0 100.0.0.0/15 is subnetted, 1 subnets O IA 100.0.0.0 [110/65] via 13.0.0.1, 00:33:12, Serial0 3.0.0.0/15 is subnetted, 1 subnets O IA 3.0.0.0 [110/129] via 13.0.0.1, 00:33:12, Serial0 4.0.0.0/16 is subnetted, 2 subnets C 4.0.0.0 is directly connected, Loopback0 C 4.1.0.0 is directly connected, Loopback1 5.0.0.0/8 is variably subnetted, 3 subnets, 2 masks C 5.1.0.0/16 is directly connected, Loopback3 O 5.0.0.0/15 is a summary, 03:06:31, Null0 C 5.0.0.0/16 is directly connected, Loopback2 O IA 11.0.0.0/8 [110/192] via 13.0.0.1, 00:33:15, Serial0 O IA 12.0.0.0/8 [110/128] via 13.0.0.1, 00:33:15, Serial0 C 13.0.0.0/8 is directly connected, Serial0 O*N2 0.0.0.0/0 [110/1] via 13.0.0.1, 00:33:15, Serial0 R4#show ip ospf database OSPF Router with ID (4.4.4.4) (Process ID 4) Router Link States (Area 2) Link ID ADV Router Age Seq# Checksum Link count 3.3.3.3 3.3.3.3 2019 0x8000000E 0xC45D 4 4.4.4.4 4.4.4.4 134 0x8000000B 0xFB21 4 Summary Net Link States (Area 2)


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Link ID ADV Router Age Seq# Checksum 1.0.0.0 3.3.3.3 225 0x80000005 0xCADA 3.0.0.0 3.3.3.3 225 0x80000009 0x26B9 11.0.0.0 3.3.3.3 226 0x8000000B 0x365F 12.0.0.0 3.3.3.3 226 0x8000000A 0xA82D 100.0.0.0 3.3.3.3 226 0x80000009 0xB10D Type-7 AS External Link States (Area 2) Link ID ADV Router Age Seq# Checksum Tag 0.0.0.0 3.3.3.3 226 0x80000002 0xE0BF 0 5.0.0.0 4.4.4.4 646 0x80000002 0xD9A1 0 The other Routers’ Routing Table will show 5.0.0.0 network as O E2 as they belong to other areas & R3 has converted LSA type 7 to LSA type 5. OSPF - VIRTUAL LINK: A virtual link is a link that allows discontiguous area 0s to be connected, or that allows a disconnected area to be connected to area 0, via a transit area. The OSPF virtual link feature should be used only in very specific cases, for temporary connections or backup after a failure. Virtual links should not be used as a primary backbone design feature. The configuration is done on the 2 ABRs of the below diagram. The Command for the configuration of virtual-link - done in the OSPF config mode as given below: Router(config)#router ospf <process-id> Router(config-router)#area <area-id> virtual-link <router-id> Where, area-id represents an area ID to the transit area for the virtual link. router-id represents the router ID of the virtual link neighbor (other ABR). NOTE: Transit area cannot be a stub area. In the Scenario given below, area 0 is discontiguous because of a network failure. Now, we have to configure a virtual link as a backup strategy to temporarily reconnect area 0; area 1 is used as the transit area.

The basic interface and OSPF configurations are done as seen below:


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When we see the routing table, it shows the inter-area routes as below: Router1#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set C 1.0.0.0/8 is directly connected, Loopback0 3.0.0.0/32 is subnetted, 1 subnets O IA 3.3.3.3 [110/65] via 25.0.0.2, 00:13:53, Serial0 C 25.0.0.0/8 is directly connected, Serial0 Router3#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR Gateway of last resort is not set 1.0.0.0/32 is subnetted, 1 subnets O IA 1.1.1.1 [110/65] via 25.0.0.1, 00:11:22, Serial0 C 3.0.0.0/8 is directly connected, Loopback0 C 25.0.0.0/8 is directly connected, Serial0 The Router-id is seen using show ip ospf & we can observe area0 being inactive : Router1#sh ip ospf Routing Process "ospf 1" with ID 1.1.1.1 and Domain ID 0.0.0.1 Supports only single TOS(TOS0) routes


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Supports opaque LSA It is an area border router SPF schedule delay 5 secs, Hold time between two SPFs 10 secs Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs Number of external LSA 0. Checksum Sum 0x0 Number of opaque AS LSA 0. Checksum Sum 0x0 Number of DCbitless external and opaque AS LSA 0 Number of DoNotAge external and opaque AS LSA 0 Number of areas in this router is 2. 2 normal 0 stub 0 nssa External flood list length 0 Area BACKBONE(0) (Inactive) Number of interfaces in this area is 1 Area has no authentication SPF algorithm executed 22 times Area ranges are Number of LSA 3. Checksum Sum 0x19347 Number of opaque link LSA 0. Checksum Sum 0x0 Number of DCbitless LSA 0 Number of indication LSA 0 Number of DoNotAge LSA 0 Flood list length 0 Area 1 Number of interfaces in this area is 1 Area has no authentication SPF algorithm executed 3 times Area ranges are Number of LSA 4. Checksum Sum 0x1E91B Number of opaque link LSA 0. Checksum Sum 0x0 Number of DCbitless LSA 0 Number of indication LSA 0 Number of DoNotAge LSA 0 Flood list length 0 Router3#show ip ospf Routing Process "ospf 1" with ID 3.3.3.3 and Domain ID 0.0.0.1 Supports only single TOS(TOS0) routes Supports opaque LSA It is an area border router SPF schedule delay 5 secs, Hold time between two SPFs 10 secs Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs Number of external LSA 0. Checksum Sum 0x0 Number of opaque AS LSA 0. Checksum Sum 0x0 Number of DCbitless external and opaque AS LSA 0 Number of DoNotAge external and opaque AS LSA 0 Number of areas in this router is 2. 2 normal 0 stub 0 nssa External flood list length 0 Area BACKBONE(0) (Inactive) Number of interfaces in this area is 1


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Area has no authentication SPF algorithm executed 22 times Area ranges are Number of LSA 3. Checksum Sum 0x1B655 Number of opaque link LSA 0. Checksum Sum 0x0 Number of DCbitless LSA 0 Number of indication LSA 0 Number of DoNotAge LSA 0 Flood list length 0 Area 1 Number of interfaces in this area is 1 Area has no authentication SPF algorithm executed 2 times Area ranges are Number of LSA 4. Checksum Sum 0x1E91B Number of opaque link LSA 0. Checksum Sum 0x0 Number of DCbitless LSA 0 Number of indication LSA 0 Number of DoNotAge LSA 0 Flood list length 0 CONFIGURATION: Now, the Virtual-link configuration has to be done in the two ABRs as shown: Router1(config)#router ospf 1 Router1(config-router)#area 1 virtual-link 3.3.3.3 Router3(config)#router ospf 1 Router3(config-router)#area 1 virtual-link 1.1.1.1 OUTPUT: Now, the area0 becomes ACTIVE : Router1#show ip ospf Routing Process "ospf 1" with ID 1.1.1.1 and Domain ID 0.0.0.1 Supports only single TOS(TOS0) routes Supports opaque LSA It is an area border router SPF schedule delay 5 secs, Hold time between two SPFs 10 secs Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs Number of external LSA 0. Checksum Sum 0x0 Number of opaque AS LSA 0. Checksum Sum 0x0 Number of DCbitless external and opaque AS LSA 0 Number of DoNotAge external and opaque AS LSA 0 Number of areas in this router is 2. 2 normal 0 stub 0 nssa External flood list length 0 Area BACKBONE(0) Number of interfaces in this area is 2 Area has no authentication SPF algorithm executed 24 times


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Area ranges are Number of LSA 4. Checksum Sum 0x15644 Number of opaque link LSA 0. Checksum Sum 0x0 Number of DCbitless LSA 0 Number of indication LSA 0 Number of DoNotAge LSA 2 Flood list length 0 Area 1 Number of interfaces in this area is 1 Area has no authentication SPF algorithm executed 3 times Area ranges are Number of LSA 6. Checksum Sum 0x2E3F7 Number of opaque link LSA 0. Checksum Sum 0x0 Number of DCbitless LSA 0 Number of indication LSA 0 Number of DoNotAge LSA 0 Flood list length 0 Router3#show ip ospf Routing Process "ospf 1" with ID 3.3.3.3 and Domain ID 0.0.0.1 Supports only single TOS(TOS0) routes Supports opaque LSA It is an area border router SPF schedule delay 5 secs, Hold time between two SPFs 10 secs Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs Number of external LSA 0. Checksum Sum 0x0 Number of opaque AS LSA 0. Checksum Sum 0x0 Number of DCbitless external and opaque AS LSA 0 Number of DoNotAge external and opaque AS LSA 0 Number of areas in this router is 2. 2 normal 0 stub 0 nssa External flood list length 0 Area BACKBONE(0) Number of interfaces in this area is 2 Area has no authentication SPF algorithm executed 24 times Area ranges are Number of LSA 4. Checksum Sum 0x15644 Number of opaque link LSA 0. Checksum Sum 0x0 Number of DCbitless LSA 0 Number of indication LSA 0 Number of DoNotAge LSA 2 Flood list length 0 Area 1 Number of interfaces in this area is 1 Area has no authentication SPF algorithm executed 2 times Area ranges are


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Number of LSA 6. Checksum Sum 0x2E3F7 Number of opaque link LSA 0. Checksum Sum 0x0 Number of DCbitless LSA 0 Number of indication LSA 0 Number of DoNotAge LSA 0 Flood list length 0 The command to verify the existence of virtual-link is show ip ospf virtual-links : Router1#show ip ospf virtual-links Virtual Link OSPF_VL0 to router 3.3.3.3 is up Run as demand circuit DoNotAge LSA allowed. Transit area 1, via interface Serial0, Cost of using 64 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:04 Adjacency State FULL (Hello suppressed) Index 1/2, retransmission queue length 0, number of retransmission 1 First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0) Last retransmission scan length is 1, maximum is 1 Last retransmission scan time is 0 msec, maximum is 0 msec Router3#show ip ospf virtual-links Virtual Link OSPF_VL0 to router 1.1.1.1 is up Run as demand circuit DoNotAge LSA allowed. Transit area 1, via interface Serial0, Cost of using 64 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:06 Adjacency State FULL (Hello suppressed) Index 1/2, retransmission queue length 0, number of retransmission 1 First 0x0(0)/0x0(0) Next 0x0(0)/0x0(0) Last retransmission scan length is 1, maximum is 1 Last retransmission scan time is 0 msec, maximum is 0 msec In the database table, we can DNA (which means DoNotAge) for those networks learnt via Virtual-Links. This is seen below in the output: Router1#show ip ospf database OSPF Router with ID (1.1.1.1) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 478 0x80000013 0x7724 2 3.3.3.3 3.3.3.3 5 (DNA) 0x80000014 0x4B3E 2 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 25.0.0.0 1.1.1.1 1764 0x80000002 0x2876


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25.0.0.0 3.3.3.3 1282 (DNA) 0x80000001 0x6B6C Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 1768 0x80000002 0x60FA 2 3.3.3.3 3.3.3.3 1765 0x80000002 0x91C0 2 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 1.1.1.1 1.1.1.1 1859 0x80000001 0x47EC 1.1.1.1 3.3.3.3 473 0x80000001 0x8D5E 3.3.3.3 1.1.1.1 472 0x80000001 0x6D7E 3.3.3.3 3.3.3.3 1780 0x80000001 0xAE75 Router3#show ip ospf database OSPF Router with ID (3.3.3.3) (Process ID 1) Router Link States (Area 0) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 5 (DNA) 0x80000013 0x7724 2 3.3.3.3 3.3.3.3 522 0x80000014 0x4B3E 2 Summary Net Link States (Area 0) Link ID ADV Router Age Seq# Checksum 25.0.0.0 1.1.1.1 1286 (DNA) 0x80000002 0x2876 25.0.0.0 3.3.3.3 1804 0x80000001 0x6B6C Router Link States (Area 1) Link ID ADV Router Age Seq# Checksum Link count 1.1.1.1 1.1.1.1 1812 0x80000002 0x60FA 2 3.3.3.3 3.3.3.3 1808 0x80000002 0x91C0 2 Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum 1.1.1.1 1.1.1.1 1904 0x80000001 0x47EC 1.1.1.1 3.3.3.3 509 0x80000001 0x8D5E 3.3.3.3 1.1.1.1 510 0x80000001 0x6D7E 3.3.3.3 3.3.3.3 1816 0x80000001 0xAE75 To see the detailed database of the neighbor, the below command is used: Router1#show ip ospf database router 3.3.3.3 OSPF Router with ID (1.1.1.1) (Process ID 1) Router Link States (Area 0) Routing Bit Set on this LSA LS age: 5 (DoNotAge) Options: (No TOS-capability, DC) LS Type: Router Links Link State ID: 3.3.3.3 Advertising Router: 3.3.3.3 LS Seq Number: 80000014 Checksum: 0x4B3E Length: 48 Area Border Router


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Number of Links: 2 Link connected to: a Virtual Link (Link ID) Neighboring Router ID: 1.1.1.1 (Link Data) Router Interface address: 25.0.0.2 Number of TOS metrics: 0 TOS 0 Metrics: 64 Link connected to: a Stub Network (Link ID) Network/subnet number: 3.3.3.3 (Link Data) Network Mask: 255.255.255.255 Number of TOS metrics: 0 TOS 0 Metrics: 1 Router Link States (Area 1) Routing Bit Set on this LSA LS age: 534 Options: (No TOS-capability, DC) LS Type: Router Links Link State ID: 3.3.3.3 Advertising Router: 3.3.3.3 LS Seq Number: 80000003 Checksum: 0x8FC1 Length: 48 Area Border Router Number of Links: 2 Link connected to: another Router (point-to-point) (Link ID) Neighboring Router ID: 1.1.1.1 (Link Data) Router Interface address: 25.0.0.2 Number of TOS metrics: 0 TOS 0 Metrics: 64 Link connected to: a Stub Network (Link ID) Network/subnet number: 25.0.0.0 (Link Data) Network Mask: 255.0.0.0 Number of TOS metrics: 0 TOS 0 Metrics: 64 Router3#show ip ospf database router 1.1.1.1 OSPF Router with ID (3.3.3.3) (Process ID 1) Router Link States (Area 0) Routing Bit Set on this LSA LS age: 5 (DoNotAge) Options: (No TOS-capability, DC) LS Type: Router Links Link State ID: 1.1.1.1 Advertising Router: 1.1.1.1 LS Seq Number: 80000013 Checksum: 0x7724 Length: 48 Area Border Router Number of Links: 2


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Link connected to: a Virtual Link (Link ID) Neighboring Router ID: 3.3.3.3 (Link Data) Router Interface address: 25.0.0.1 Number of TOS metrics: 0 TOS 0 Metrics: 64 Link connected to: a Stub Network (Link ID) Network/subnet number: 1.1.1.1 (Link Data) Network Mask: 255.255.255.255 Number of TOS metrics: 0 TOS 0 Metrics: 1 Router Link States (Area 1) Routing Bit Set on this LSA LS age: 681 Options: (No TOS-capability, DC) LS Type: Router Links Link State ID: 1.1.1.1 Advertising Router: 1.1.1.1 LS Seq Number: 80000003 Checksum: 0x5EFB Length: 48 Area Border Router Number of Links: 2 Link connected to: another Router (point-to-point) (Link ID) Neighboring Router ID: 3.3.3.3 (Link Data) Router Interface address: 25.0.0.1 Number of TOS metrics: 0 TOS 0 Metrics: 64 Link connected to: a Stub Network (Link ID) Network/subnet number: 25.0.0.0 (Link Data) Network Mask: 255.0.0.0 Number of TOS metrics: 0 TOS 0 Metrics: 64 OSPF AUTHENTICATION: OSPF neighbor authentication (also called neighbor router authentication or route authentication) can be configured such that routers can participate in routing based on predefined passwords. When neighbor authentication has been configured on a router, the router authenticates the source of each routing update packet that it receives. This is accomplished by the exchange of an authenticating key (sometimes referred to as a password) that is known to both the sending and the receiving router. By default, OSPF uses null authentication, which means that routing exchanges over a network are not authenticated. OSPF supports two other authentication methods:

• simple password authentication (also called plain-text authentication), • MD5 authentication.


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Configuration of SIMPLE (TEXT) PASSWORD AUTHENTICATION: The configuration of simple TEXT password authentication is given in 2 simple steps – both in the interface configuration mode: Step1: Enable TEXT Authentication in the appropriate interface Router(config-if)#ip ospf authentication Step2: Assign a common password to be used with both the neighboring routers Router(config-if)#ip ospf authentication-key <password> Note : We can enable the authentication for a particular area in the OSPF process. Router(config-router)#area <area-id> authentication The example below can be used for our understanding

The basic interface configurations and OSPF configurations are done which was shown in the running configuration of the two routers as seen below:

Now, we can do the configuration of simple password authentication: CONFIGURATION: Router1(config)#interface serial 0 Router1(config-if)#ip ospf authentication Router1(config-if)#ip ospf authentication-key cisco


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Router1(config-if)#exit Router1(config)#router ospf 1 Router1(config-router)#area 0 authentication Router3(config)#interface serial 0 Router3(config-if)#ip ospf authentication Router3(config-if)#ip ospf authentication-key cisco Router3(config-if)#exit Router3(config)#router ospf 1 Router3(config-router)#area 0 authentication Neighbor relationship between the two routers is formed only when authentication is successful, not otherwise ! Router1# 00:31:29: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on Serial0 from LOADING to FULL, Loading Done Router3# 00:47:17: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on Serial0 from LOADING to FULL, Loading Done The below output displays the type of authentication enabled on a particular interface Router1#sh ip ospf interface Loopback0 is up, line protocol is up Internet Address 1.1.1.1/8, Area 0 Process ID 1, Router ID 1.1.1.1, Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host Serial0 is up, line protocol is up Internet Address 25.0.0.1/8, Area 0 Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:08 Index 1/1, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 1, maximum is 1 Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 3.3.3.3 Suppress hello for 0 neighbor(s) Simple password authentication enabled Router3#show ip ospf interface Serial0 is up, line protocol is up Internet Address 25.0.0.2/8, Area 0 Process ID 1, Router ID 3.3.3.3, Network Type POINT_TO_POINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT,


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Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:02 Index 2/2, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 1, maximum is 1 Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 1.1.1.1 Suppress hello for 0 neighbor(s) Simple password authentication enabled Loopback0 is up, line protocol is up Internet Address 3.3.3.3/8, Area 0 Process ID 1, Router ID 3.3.3.3, Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host NOTE: The Password for the authentication on the both routers should be the same. If a different password is configured, neighbor relationship is not formed. The debug ip ospf adj output, shows "Mismatch Authentication Key - Clear Text" as displayed below: Router1#debug ip ospf adj OSPF adjacency events debugging is on 00:58:25: OSPF: Rcv pkt from 25.0.0.2, Serial0 : Mismatch Authentication Key - Clear Text 00:58:35: OSPF: 3.3.3.3 address 25.0.0.2 on Serial0 is dead 00:58:35: OSPF: 3.3.3.3 address 25.0.0.2 on Serial0 is dead, state DOWN 00:58:35: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on Serial0 from FULL to DOWN, Neighbor Down: Deadtimer expired Router3#debug ip ospf adj OSPF adjacency events debugging is on 01:14:14: OSPF: Rcv pkt from 25.0.0.1, Serial0 : Mismatch Authentication Key - Clear Text 01:14:24: OSPF: 1.1.1.1 address 25.0.0.1 on Serial0 is dead 01:14:24: OSPF: 1.1.1.1 address 25.0.0.1 on Serial0 is dead, state DOWN 01:14:24: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on Serial0 from FULL to DOWN, Neighbor Down: Deadtimer expired Configuration of MD5 AUTHENTICATION: The configuration of MD5 authentication is given in steps as follows: Step1: Specify the MD5 authentication type for an interface Router(config-if)#ip ospf authentication [message-digest] Step2: Assign a key ID and key (password) to be used with neighboring routers. Router(config-if)#ip ospf message-digest-key <key-id> md5 <key>


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Where, key-id is an identifier in the range from 1 to 255 and key is an alphanumeric password Note : Enable the md5 authentication for an area in the OSPF process using Router(config-router)#area <area-id> authentication [message-digest] MD5 configuration example:

The basic interface configurations and OSPF configurations are done as shown in the running configuration of the two routers :

Now, we can do the configuration of md5 authentication: CONFIGURATION: Router1(config)#interface serial 0 Router1(config-if)#ip ospf authentication message-digest Router1(config-if)#ip ospf message-digest-key 1 md5 cisco Router1(config-if)#exit Router1(config)#router ospf 1 Router1(config-router)#area 0 authentication message-digest Router3(config)#interface serial 0 Router3(config-if)#ip ospf authentication message-digest Router3(config-if)#ip ospf message-digest-key 1 md5 cisco Router3(config-if)#exit Router3(config)#router ospf 1 Router3(config-router)#area 0 authentication message-digest


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When the authentication is successful, we receive the message - Router1# 00:31:29: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on Serial0 from LOADING to FULL, Loading Done Router1#sh ip ospf interface Loopback0 is up, line protocol is up Internet Address 1.1.1.1/8, Area 0 Process ID 1, Router ID 1.1.1.1, Network Type LOOPBACK, Cost: 1 Loopback interface is treated as a stub Host Serial0 is up, line protocol is up Internet Address 25.0.0.1/8, Area 0 Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 64 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:09 Index 1/1, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 1, maximum is 1 Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 3.3.3.3 Suppress hello for 0 neighbor(s) Message digest authentication enabled Youngest key id is 1 The lowest key no. will be used if multiple keys ids are used – as revealed above NOTE: The Key-id and key(Password) for the authentication on the both Routers should be the same. If they are different, neighbor is not formed and OSPF process fails. Let’s observe debug ip ospf adj output when we configure wrong key id / key : 1) If Key-id is different, then the Debug shows: Router1#debug ip ospf adj OSPF adjacency events debugging is on 01:49:45: OSPF: Send with youngest Key 1 01:49:27: OSPF: Rcv pkt from 25.0.0.2, Serial0 : Mismatch Authentication Key - No message digest key 2 on interface Router3#debug ip ospf adj OSPF adjacency events debugging is on 02:05:06: OSPF: Send with youngest Key 2 02:05:04: OSPF: Rcv pkt from 25.0.0.1, Serial0 : Mismatch Authentication Key - No message digest key 1 on interface 2) If the Key is different, then the Debug shows:


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Router1#debug ip ospf adj OSPF adjacency events debugging is on 01:57:15: OSPF: Send with youngest Key 1 01:57:18: OSPF: Rcv pkt from 25.0.0.2, Serial0 : Mismatch Authentication Key - Message Digest Key 1 Router3#debug ip ospf adj OSPF adjacency events debugging is on 02:13:46: OSPF: Send with youngest Key 1 02:13:54: OSPF: Rcv pkt from 25.0.0.1, Serial0 : Mismatch Authentication Key - Message Digest Key 1 TROUBLESHOOTING IN AUTHENTICATION: First, we have to know the authentication type number: TYPE0 - Null Authentication (i.e., no authentication configured) TYPE1 - Simple Password Authentication TYPE2 - MD5 Authentication 1) For the above Scenario, if Router1 is configured with simple password authentication and Router3 has no authentication, then the debug output shows: Router1#debug ip ospf adj OSPF adjacency events debugging is on 02:18:09: OSPF: Rcv pkt from 25.0.0.2, Serial0 : Mismatch Authentication type. Input packet specified type 0, we use type 1 Router3#debug ip ospf adj OSPF adjacency events debugging is on 02:33:54: OSPF: Rcv pkt from 25.0.0.1, Serial0 : Mismatch Authentication type. Input packet specified type 1, we use type 0 2) For the above Scenario, if Router1 is configured with MD5 authentication and Router3 has no authentication, then the debug output shows: Router1#debug ip ospf adj OSPF adjacency events debugging is on 02:18:06: OSPF: Send with youngest Key 1 02:18:09: OSPF: Rcv pkt from 25.0.0.2, Serial0 : Mismatch Authentication type. Input packet specified type 0, we use type 2 Router3#debug ip ospf adj OSPF adjacency events debugging is on 02:33:54: OSPF: Rcv pkt from 25.0.0.1, Serial0 : Mismatch Authentication type. Input packet specified type 2, we use type 0

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