OSPF- Multi area

Ch.6 – OSPFPart 2 of 2: Multi-Area OSPF

CCNP version 3.0

Rick Graziani

Cabrillo College

Rick Graziani [email protected] 2

Note

• Optional: This lab coincides with my Multi-area OSPF Lab handout, Rick's OSPF Scenarios (Word doc)

• Most of the slides in this presentation do not come directly from CCNP 1 version 3.0, OSPF, but slides which contain a little more detail and explanation of OSPF.


Optional: Rick’s OSPF Scenarios

Optional: We will be using the following handout for this presentation:

Rick’s OSPF Handout:

1. OSPF Multi-Area - All Normal Areas

2. OSPF Multi-Area - Stub Area

3. OSPF Multi-Area - Totally Stubby Area

This handouts can be downloaded from (Word doc):

• http://www.cabrillo.cc.ca.us/ciscoacad/curriculum/presentations/semester5/OSPF_Scenario_Handout.doc


Part I - LSAs using all normal areas

Multi Area OSPF

Normal Areas

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Suggestion to Instructors: Draw this network on the white-board as it will be used for discussion throughout these slides.


Topics

• Areas• LSAs• show ip ospf database (summary of link state database)• show ip route• Stub Areas• Totally Stubby Areas • E1 and E2 routes• Default Routes • Route Summarization• NSSA (Not So Stubby Areas)• Multiple ABR Scenario• Multiple ASBR Scenario• Virtual Links• Load Balancing• show commands


Issues with large OSPF nets

• Large link-state table

– Each router maintains a LSDB for all links in the area

– The LSDB requires the use of memory

• Frequent SPF calculations

– A topology change in an area causes each router to re-run SPF to rebuild the SPF tree and the routing table.

– A flapping link will affect an entire area.

– SPF re-calculations are done only for changes within that area.

• Large routing table

– Typically, the larger the area the larger the routing table.

– A larger routing table requires more memory and takes more time to perform the route look-ups.

Solution: Divide the network into multiple areas


OSPF uses “Areas”

• Hierarchical routing enables you to separate large internetworks (autonomous systems) into smaller internetworks that are called areas.

• With this technique, routing still occurs between the areas (called inter-area routing), but many of the smaller internal routing operations, such as recalculating the database – re-running the SPF algorithm, are restricted within an area.


• Question: I understand the routing table is recalculated every time the router receives an new version of an LSA. Does OSPF recalculate its routing table when their is a topology change in another area? show ip ospf displays no change in SPF execution, but show ip ospf database shows a change in the topology?

• Answer: Good question! OSPF areas are designed to keep issues like flapping links within an area. SPF is not recalculated if the topology change is in another area. The interesting thing is that OSPF distributes inter-area (between areas) topology information using a distance-vector method. OSPF uses link-state principles only within an area. ABRs do not announce topological information between areas, instead, only routing information is injected into other areas. ABRs relay routing information between areas via distance vector technique similar to RIP or IGRP. This is why show ip ospf does not show a change in the number of times SPF has been executed when the topology change is in another area.

• Note: It is still a good idea to perform route summarization between areas, announcing multiple routes as a single inter-area route. This will hide any changes in one area from affecting routing tables in other areas.

An advantage of Multiple Areas


OSPF uses “Areas”


OSPF Router Types


OSPF Router Types

InternalInternal: Routers with all their interfaces within the same area

BackboneBackbone: Routers with at least one interface connected to area 0

ASBRASBR: (Autonomous System Boundary Router): Routers that have at least one interface connected to an external internetwork (another autonomous system)

ABRABR: (Area Border Router): Routers with interfaces attached to multiple areas.


OSPF Packet Types

• Last week we discussed various OSPF packets, used for:– Means for dynamic neighbor discovery– Detect unreachable neighbors within a finite period of time– Ensure two-way communications between neighbors– Ensure correctness of basic interace parameters between neighbors– Provide necessary information for the election of the Designated and

Backup Designated routers on a LAN segement– Request link state information from another router– Sharing data base summary and detailed information– Acknowledge the receipt of an OSPF packet

OSPF packet types


OSPF Type 4 - Link State Advertisements

• This week we will look at OSPF Type 4 packets more closely

OSPF packet types


OSPF packet types

OSPF Type-4 packets have 7 LSA packets (later)


LSAs used for discovering routes and reaching Full State, along with Maintain Routes

LSA Types


LSA Types

LSA Types 1 through 5• We will look at these in detail as we discuss areas in this chapter.

LSA Type 6 MOSPF (Multicast OSPF) • Not supported by Cisco. • MOSPF enhances OSPF by letting routers use their link-state

databases to build multicast distribution trees for the forwarding of multicast traffic.

LSA Type 7 NSSA External Link Entry• Originated by an ASBR connected to an NSSA. • Type 7 messages can be flooded throughout NSSAs and translated

into LSA Type 5 messages by ABRs. • Routes learned via Type-7 LSAs are denoted by either a “N1” or and

“N2” in the routing table. (Compare to E1 and E2).• We will discuss this more later when we look at NSSA areas.


Area Types

• Standard or Normal Areas– Backbone– Non-Backbone

• Stub– Stub Area– Totally Stubby Area (TSA)– Not-so-stubby-area (NSSA)


Area Types



Multi Area OSPF

Normal Areas

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32


Routes Received on all OSPF Routers

Overview of Normal Areas – This will all be explained!

Receives all routes from within A.S.:• Within the local area – LSA 1 and LSA 2• From other areas (Inter-Area) – LSA 3, LSA 4, LSA 5

Receives all routes from External A.S.’s (External AS means routes not from this OSPF routing domain):

• From external AS’s – LSA 5• As long as routes are being redistributed by the ASBR (more later)

Default Routes• Received only if default-information-originate command was used (later)• If default-information-originate command is not used, then the default

route is not received



• Your Turn - In groups, examine running-configs

• Look at the running-configs for “1. OSPF Multi-Areas - All Normal Areas”

• Look at the OSPF network statements!



1. OSPF Multi-Areas - All Normal Areas

ASBRrouter ospf 1

redistribute static

network 172.16.1.0 0.0.0.255 area 0

!

ip classless

ip route 11.0.0.0 255.0.0.0 Null0

ip route 12.0.0.0 255.0.0.0 Null0

ip route 13.0.0.0 255.0.0.0 Null0

ABR-1interface FastEthernet0/0

ip address 172.16.1.2 255.255.255.0

ip ospf priority 200

router ospf 1

network 172.16.1.0 0.0.0.255 area 0

network 172.16.51.0 0.0.0.255 area 51

ABR-2interface FastEthernet0

ip address 172.16.1.3 255.255.255.0

ip ospf priority 100

!

router ospf 1

network 172.16.1.0 0.0.0.255 area 0

network 172.16.10.4 0.0.0.3 area 1

Internalrouter ospf 1

network 172.16.0.0 0.0.255.255 area 1

ABR contains network statements for each area it belongs to, using the proper area value.


Understanding LSAs

• show ip ospf database– This is not the link state database, only a summary.– It is a tool to help determine what routes are included in the routing table.– We will look at this output to learn the tool as well as become familiar with

the different types of LSAs.– To view the link state database use: show ip ospf database [router|

network|…]

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| LS age | Options | LS type |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link State ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Advertising Router |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| LS sequence number |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| LS checksum | length |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LSA Header


• LSA 1 – Router LSA

• Generated by each router for each area it belongs to.

• Describes the states of the links in the area to which this router belongs.

• Tells the other routers in the area about itself and its links to adjacent OSPF routers, and “leaf networks.”

• Flooded only within the area. On multi-access networks, sent to the DR.

• Denoted by just an “O” in the routing table or “C” if the network is directly connected.

• ABR will include a set of LSA 1’s for each area it belongs to.

A C

D

2

5

B

15 Router A’s LSA 1s which are flooded to all other routers in this area.

“Leaf” network

LSA 1 - Router Link States



• When a new LSA 1 is received and installed in the LSDB, the router forwards that LSA, using hop-by-hop or asynchronous flooding.

• The LSA is sent out all OSPF interfaces that are in the Exchange State or a higher state.

• For interfaces in Exstart or lesser state, the router will wait until it is out of Exstart.


0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| LS age | Options | 1 |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link State ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| 0 |V|E|B| 0 | # links |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link Data |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Type | # TOS | metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| ... |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| TOS | 0 | TOS metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link Data |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| ... |



ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1’s being sent within Area 0

LSA 1LSA 1LSA 1



ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1’s being sent within other areas

LSA 1

LSA 1

LSA 1

LSA 1



ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1 Originated

LSA 1’s are flooded out other interfaces within the same area.

LSA 1 flooded




For Router Links:

• The Link State ID is always the same as the Advertising Router

• Advertising Router is the Router ID of the router that created this LSA 1

Internal#show ip ospf data

OSPF Router with ID (192.168.4.1) (Process ID 1)

Router Link States (Area 1) <- Note the Area!

(LSA 1 - Links in the area to which this router belongs.)

Link ID ADV Router Age Seq# Checksum Link count

192.168.3.1 192.168.3.1 898 0x80000003 0xCE56 2

192.168.4.1 192.168.4.1 937 0x80000003 0xFD44 3

• Bottom line: Router Link States (LSA1’s) should display all the RouterIDs of routers in that area, including its own.

• Rick’s reminder: LSA 1 -> “my one area”


ABR-2#show ip ospf data


Router Link States (Area 1) <- Note the Area!

(LSA 1 - Links in the area to which this router belongs.)


192.168.3.1 192.168.3.1 786 0x80000003 0xCE56 2

192.168.4.1 192.168.4.1 828 0x80000003 0xFD44 3

• Bottom line: Router Link States (LSA1’s) should display all the RouterIDs of routers in that area, including its own.

• Rick’s reminder: LSA 1 -> “my one area”



ABR-2 - show ip route

172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks

O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:11:44, FastEthernet0

O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:12:29, Serial0

C 172.16.10.4/30 is directly connected, Serial0

C 172.16.1.0/24 is directly connected, FastEthernet0

O E2 11.0.0.0/8 [110/20] via 172.16.1.1, 00:11:44, FastEthernet0



192.168.3.0/32 is subnetted, 1 subnets

C 192.168.3.1 is directly connected, Loopback1


• Denoted by just an “O” in the routing table, or a “C”• Why is there only just an”O” for this network and not the other

networks?– Directly connected or via another area.


Your Turn -Discuss in groups (LSA 1s)

• Using the Multi-area OSPF Lab Handout: “Lab 1. – OSPF Normal Areas” verify these results.

• Look at the link state database summary (show ip ospf database) commands and the Router Links States (LSA1s) for each router.

• Look at the routing tables (show ip route) and notice the routes within that router’s area.

• Why do some routers have more than one set of Router Links States?

• Where does “show ip ospf database” tell you the RouterID.

• Where does “show ip ospf database” tell you the Area.


• LSA 2 – Network LSA

• Generated by the DR on every multi-access network

• Denoted by just an “O” in the routing table or “C” if the network is directly connected.

• Flooded only within the originating area.

• LSA 2’s are in link state database for all routers within area, even those routers on not on multi-access networks or DRs on other multi-access networks in the same area.

• ABR may include a set of LSA 2s for each area it belongs to.

LSA 2 - Network Link States


0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link State ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Network Mask |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Attached Router |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| ... |



ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 2

No LSA 2’s for ABR-1 in Area 51, or for Internal because no other routers on multi-access segment.

flooded

DR

DR

LSA 2

LSA 2flooded

LSA 2

LSA 2



ASBR#show ip ospf data


Net Link States (Area 0)

(LSA 2 - Generated by the DR)

Link ID ADV Router Age Seq# Checksum

172.16.1.2 192.168.2.1 201 0x8000000D 0xCFE8


• Link ID 172.16.1.2 = IP address of DR on MultiAccess Network• ADV Router 192.168.2.1 = Router ID of DR• Bottom line: Net Link States (LSA2’s) should display the RouterIDs of

the DRs on all multi-access networks in the area and their IP addresses.

• Rick’s reminder: LSA 2 -> “Ethernet = Layer 2 or D R” 1 2



• Look at the link state database summary (show ip ospf database) commands and the Net Links States (LSA2s) for each router.

• Look at the routing tables (show ip route) and notice the multi-access routes within that router’s area.

• Could a router have more than one entry in its listing of Net Links States?

• Could an area with a broadcast segment, still have no LSA 2’s?



• LSA 3 – Summary LSA

• Originated by the ABR.

• Describes links between ABR and Internal Routers of the Local Area

• ABR will include a set of LSA 3’s for each area it belongs to.

• LSA 3s are flooded throughout the backbone (Area 0) and to other ABRs.

• Routes learned via LSA type 3s are denoted by an “IA” (Inter-area) in the routing table.

LSA 3 – Summary Net Link States


0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| LS age | Options | 3 or 4 |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link State ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Network Mask |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| 0 | metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| TOS | TOS metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| ... |





ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1’s are sent as LSA 3’s into other areas by the ABRs.

LSA 1LSA 1

LSA 3LSA 3

LSA 1



ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas


LSA 1

LSA 1

LSA 1

LSA 3

LSA 3

LSA 3

LSA 3

LSA 1

LSA 3

LSA 3



ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1

LSA 1

LSA 1

LSA 3

LSA 3

LSA 3

LSA 3

LSA 1

LSA 3

LSA 3

Area 1


• Routers only see the topology of the area they belong to.• When a link in one area changes, the adjacent routers originate in LSA 1’s and

flood them within the area, causing intra-area (internal) routers to re-run the SPF and recalculating the routing table.

• ABRs do not announce topological information between areas.• ABRs only inject routing information into other areas, which is basically a

distance-vector technique.


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas


LSA 1

LSA 1

LSA 1

LSA 3

LSA 3

LSA 3

LSA 3

LSA 1

LSA 3

LSA 3

Area 1

• ABRs calculate intra-area routes for directly attached areas and announce them to all other areas as inter-area routes, using LSA 3’s.

• OSPF ABRs will only announce inter-area routes that were learned from the backbone area, area 0.

• The backbone area serves as a repository for inter-area routes.• This keeps OSPF safe from routing loops.


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Don’t forget about the LSA 1’s from Area 0.

Multi Area OSPF

Normal Areas

LSA 1LSA 1LSA 3

LSA 3

LSA 1



Area 0Backbone Area

Area 51Area 1

RTA RTB

RTC

LSA 3 LSA 3

Normal AreasLSA 1’s

Not ABR

• ABRs calculate intra-area routes for directly attached areas and announce them to all other areas as inter-area routes, using LSA 3’s.

• In normal operation, OSPF ABRs will only announce inter-area routes that were learned from the backbone area, area 0.

• RTC does not forward LSA 3’s from Area 1 to Area 51, and does not forward LSA 3’s from Area 51 to Area 1.

• The backbone area serves as a repository for inter-area routes.• This keeps OSPF safe from routing loops.


Area 0Backbone Area

Area 51Area 1

RTA RTB

RTC

LSA 3

LSA 3

Normal Areas

LSA 1’s

Not ABR

• Example of an LSA 1 originated in Area 1, sent to Area 0 as an LSA 3, and the sent to Area 51 as an LSA 3.

• RTC does not forward the LSA 3’s back into Area 1, or routing loops may develop.

• Again, in normal operations, OSPF ABRs will only announce inter-area routes that were learned from the backbone area, area 0.

• Note: RTC will create LSA 1’s and flood them within the appropriate area.


Area 0Backbone Area

Area 51Area 1

RTA RTB

RTC

Normal Areas

LSA 1’s LSA 1’s

LSA 3 LSA 3

LSA 3LSA 3

Not ABR

• RTC does not forward LSA 1’s from Area 1 as LSA 3’s into Area 51.

• RTC does not forward LSA 1’s from Area 51 as LSA 3’s into Area 1.

• Any LSA 3’s from RTC are not forwarded into Area 0 by RTA or RTB.

• OSPF specification states that ABRs are restricted to considering LSA 3’s only from the backbone area to avoid routing information loops.


Area 0Backbone Area

Area 51Area 1

RTA RTB

RTC

Normal Areas

LSA 1’s

LSA 3

LSA 3

X

Area 1 routers re-run SPF, creates new SPF tree and updates routing table.

Update is sent to Area 0 and Area 51 routers using a “distance vector update technique.” SPF not re-run, but routers update routing table.

Topology Change: Down Link• When a router detects a topology change it immediately sends out LSA

1’s (Router LSAs) with the change.• In the case of a down link, the age of the LSA is set to MaxAge (3,600

seconds) – Routers that receive LSAs with the age equal to MaxAge remove this entry from their LSDB (Link State Data Base).

• Routers that receive the LSA 1’s, within the area of the change, re-run their SPF algorithm, to build a new SPF tree and then make the changes to their IP routing tables. (Continued next slide)


Area 0Backbone Area

Area 51Area 1

RTA RTB

RTC

Normal Areas

LSA 1’s

LSA 3

LSA 3

X

Area 1 routers re-run SPF, creates new SPF tree and updates routing table.

Update is sent to Area 0 and Area 51 routers using a “distance vector update technique.” SPF not re-run, but routers update routing table.

Topology Change: Down Link• ABR RTA receives the LSA 1 and recalculate their SPF for that area,

Area 1.• RTA floods the change as a LSA 3 within its other area, Area 0.• RTB receives the LSA 3 and floods it within Area 51.• Area 0 and Area 51 routers do not recalculate their SPFs, but inject the

change into their routing tables.Note: LSA 3’s (and other Inter-Area routes) are viewed as “leaf nodes” in

the SPF tree.


Multi Area OSPF

Normal Areas

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32



ASBR ASBR# show ip ospf database

Summary Net Link States (Area 0)


(Area 1 networks - Advertising Router ABR-2)

172.16.10.4 192.168.3.1 278 0x80000001 0xD126

172.16.20.0 192.168.3.1 278 0x80000001 0xA746


172.16.51.1 192.168.2.1 206 0x80000005 0xA832


• Link ID = IP network addresses of networks in other areas• ADV Router = ABR Router ID sending the LSA-3• Divided by ABR• Bottom line: Should see networks in other areas and the ABR advertising that

route. • Rick’s reminder: LSA 3 -> “networks sent by the A B R” 1 2 3


ASBR ASBR# show ip route


O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:02:54, FastEthernet0/0



C 172.16.1.0/24 is directly connected, FastEthernet0/0


C 10.1.0.0 is directly connected, FastEthernet0/1

S 11.0.0.0/8 is directly connected, Null0






• Routes learned via LSA type 3s are denoted by an “IA” (Inter-Area Routes) in the routing table.


Another example: non-area 0 router, InternalInternal# show ip ospf database LSA 3 - Generated by the ABR. Describes links between ABR and

Internal Routers of the Local Area




172.16.1.0 192.168.3.1 848 0x80000005 0xD339

172.16.51.1 192.168.3.1 843 0x80000001 0xB329

Internal# show ip route 172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks

O IA 172.16.51.1/32 [110/783] via 172.16.10.5, 00:13:48, Serial0



O IA 172.16.1.0/24 [110/782] via 172.16.10.5, 00:13:53, Serial0



O E2 11.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0

O E2 12.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0

O E2 13.0.0.0/8 [110/20] via 172.16.10.5, 00:14:42, Serial0



• Look at the link state database summary (show ip ospf database) commands and the Summary Net Links States (LSA3s) for each router.

• Look at the routing tables (show ip route) and notice the Interarea (IA) routes.

• Why do some routers have more than one set of Summary Net Links States?



• LSA 4 – ASBR Summary LSA• Originated by the ABR. • Flooded throughout the backbone area to the other

ABRs.• Describes the reachability to the ASBRs• Advertises an ASBR (Router ID) not a network• Included in routing table as an “IA” route.• Same format as a LSA 3 - Summary LSA, except LSA 4

ASBR Summary LSA the Network Mask field is always 0

Exceptions• Not flooded to Stub and Totally Stubby networks.• More on this later

LSA 4 – ASBR Summary Link States


0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| LS age | Options | 3 or 4 |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link State ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Network Mask |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| 0 | metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| TOS | TOS metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| ... |



ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Normal Areas

LSA 4

LSA 4

LSA 4

LSA 5’s flooded

LSA 4

Area 1

• Flooded throughout the backbone area to the other ABRs.

• Describes the reachability to the ASBRs

• How do the ABRs know about the ASBR? I am still researching this, but I believe when routers receive an LSA 5 (AS External LSA) with external route information, the routers denote the Router ID being the ASBR.


ABR-2 ABR-2# show ip ospf database

Summary ASB Link States (Area 1)

LSA 4 - Reachability to ASBR.

Not flooded to Stub and Totally Stubby networks.


192.168.1.1 192.168.3.1 801 0x80000003 0x93CC


• Link ID 192.168.1.1 = Router ID of ASBR• ADV Router 192.168.3.1 = Router ID ABR advertising route

• Bottom line: Routers in non-area 0, should see Router ID of ASBR and its ABR to get there .

• Rick’s reminder: LSA 4 -> “Reachability to the A S B R” 1 2 3 4


Internal

Internal# show ip ospf database Summary ASB Link States (Area 1) LSA 4 - Reachability to ASBR. Not flooded to Stub and Totally Stubby networks.

Link ID ADV Router Age Seq# Checksum192.168.1.1 192.168.3.1 912 0x80000003 0x93CC


• Link ID 192.168.1.1 = Router ID of ASBR

• ADV Router 192.168.3.1 = Router ID ABR advertising route

• Note: No LSA 4s for Area 0 on Router B



• Look at the link state database summary (show ip ospf database) commands and the Summary Net Links States (LSA4s) for each router.

• Why do some routers have more than one set of Summary ASB Links States and others may not (like RouterA and ASBR)?

• Which Area 0 routers have LSA 4’s in their LSDB?

• Why don’t some Area 0 routers have LSA 4’s in their LSDB?



• LSA 5 – AS External LSA

• Originated by the ASBR.

• Describes destination networks external to the Autonomous System (This OSPF Routing Domain)

• Flooded throughout the OSPF AS except to stub and totally stubby areas

• Denoted in routing table as E1 or E2 (default) route (soon)

• We will discuss default routes later.

• ASBR – Router which “redistributes” routes into the OSPF domain.

Exceptions

• Not flooded to Stub and Totally Stubby networks.

• More on this later

LSA 5 - AS External Link States


0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Link State ID |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Network Mask |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

|E| 0 | metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Forwarding address |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| External Route Tag |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

|E| TOS | TOS metric |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| Forwarding address |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| External Route Tag |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| ... |



ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

ASBR

router ospf 1

redistribute static

network 172.16.1.0 0.0.0.255 area 0

ip route 11.0.0.0 255.0.0.0 Null0

ip route 12.0.0.0 255.0.0.0 Null0

ip route 13.0.0.0 255.0.0.0 Null0

Normal Areas


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Normal Areas

LSA 5

LSA 5

LSA 5LSA 5

LSA 5’s floodedLSA 5

ASBR

router ospf 1

redistribute static

network 172.16.1.0 0.0.0.255 area 0

ip route 11.0.0.0 255.0.0.0 Null0

ip route 12.0.0.0 255.0.0.0 Null0

ip route 13.0.0.0 255.0.0.0 Null0

• “Redistribute” command creates an ASBR router.• Originated by the ASBR. • Describes destination networks external to the OSPF Routing Domain• Flooded throughout the OSPF AS except to stub and totally stubby

areas



ABR-2 ABR-2# show ip ospf database

AS External Link States <- Note, NO Area!

LSA 5 - External Networks originated by the ASBR, Flooded throughout A.S. except to Stub and Totally Stubby

Link ID ADV Router Age Seq# Checksum Tag11.0.0.0 192.168.1.1 1191 0x80000001 0x3FEA 012.0.0.0 192.168.1.1 1191 0x80000001 0x32F6 013.0.0.0 192.168.1.1 1191 0x80000001 0x2503 0

• Link ID = External Networks• ADV Router = Router ID of ASBR• Note: For ABRs: There is only one set of “AS External Link States” in

database summary. In other words, an ABR router will only show one set of “AS External Link States,” not one per area.

• Bottom line: All Routers should see External networks and the Router ID of ASBR to get there .

• Rick’s reminder: LSA 5 -> O T H E R networks 1 2 3 4 5


ABR-2 ABR-2# show ip route 172.16.0.0/16 is variably subnetted, 4 subnets, 3 masksO IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:11:44, FastEthernet0O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:12:29, Serial0C 172.16.10.4/30 is directly connected, Serial0C 172.16.1.0/24 is directly connected, FastEthernet0O E2 11.0.0.0/8 [110/20] via 172.16.1.1, 00:11:44, FastEthernet0O E2 12.0.0.0/8 [110/20] via 172.16.1.1, 00:11:44, FastEthernet0O E2 13.0.0.0/8 [110/20] via 172.16.1.1, 00:11:44, FastEthernet0 192.168.3.0/32 is subnetted, 1 subnetsC 192.168.3.1 is directly connected, Loopback1

• Designated by “E2”• Notice that the cost is 20 for all three routes, we will see why later. • It has to do with E2 routes and where the default cost is 20.

– Redistribute command (Route Optimization chapter): If a value is not specified for the metric option, and no value is specified using the default-metric command, the default metric value is 0, except for OSPF where the default cost is 20.



Internal# show ip ospf database

Type-5 AS External Link States

LSA 5 - External Networks originated by the ASBR,

Flooded throughout A.S. except to Stub and Totally StubbyLink ID ADV Router Age Seq# Checksum Tag

11.0.0.0 192.168.1.1 1191 0x80000001 0x3FEA 0

12.0.0.0 192.168.1.1 1191 0x80000001 0x32F6 0

13.0.0.0 192.168.1.1 1191 0x80000001 0x2503 0


Internal# show ip route 172.16.0.0/16 is variably subnetted, 4 subnets, 3 masks

O IA 172.16.51.1/32 [110/783] via 172.16.10.5, 00:13:48, Serial0



O IA 172.16.1.0/24 [110/782] via 172.16.10.5, 00:13:53, Serial0



O E2 11.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0

O E2 12.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0

O E2 13.0.0.0/8 [110/20] via 172.16.10.5, 00:14:42, Serial0


E1 vs. E2 External Routes

• External routes fall under two categories:

– external type 1

– external type 2 (default)

• The difference between the two is in the way the cost (metric) of the route is being calculated.

• The cost of a type 2 route is always the external cost, irrespective of the interior cost to reach that route.

• A type 1 cost is the addition of the external cost and the internal cost used to reach that route.

• A type 1 route is always preferred over a type 2 route for the same destination.

• More later…




• Look at the link state database summary (show ip ospf database) commands and the AS External Links States (LSA5s) for each router.

• Also, look at the routing tables for each router.

• How many sets of LSA 5s does the ABRs have in their link state summary database? Notice the ASBRs entries.



Stub Areas

Considerations for both Stub and Totally Stubby Areas

• An area could be qualified a stub when:

– There is a single exit point (a single ABR) from that area. More than one ABR can be used, but be ready to “accept non-optimal routing paths.”

– If routing to outside of the area does not have to take an optimal path.

• The area is not needed as a transit area for virtual links (later).

• The ASBR is not within the stub area

• The area is not the backbone area (area 0)

• Stub areas will result in memory and processing savings depending upon the size of the network.


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Stub Area


Receives all routes from within A.S.:• Within the local area - LSA 1s and LSA 2s (if appropriate)• From other areas (Inter-Area) - LSA 3s

Does not receive routes from External A.S. (External Routes).

ABR:• ABR blocks all LSA 4s and LSA 5s. • ‘If LSA 5s are not known inside an area, LSA 4s are not necessary.’• LSA 3s are propagated by the ABR.

Note: Default route is automatically injected into stub area by ABR– External Routes: Once the ABR gets a packet headed to a default route, it

must have a default route, either static or propagated by the ASBR via default information originate (coming!)

Configuration:• All routers in the area must be configured as “stub”

Stub Areas


ABR-2 router ospf 1 network 172.16.1.0 0.0.0.255 area 0 network 172.16.10.4 0.0.0.3 area 1 area 1 stub << Command: area area stub

Internal router ospf 1 network 172.16.0.0 0.0.255.255 area 1 area 1 stub << Command: area area stub

Stub Areas

• All routers in the area must be configured as “stub” including the ABR


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Stub Area

LSA 4

LSA 4

X Blocked

LSA 5

Blocked XLSA 5

LSA 3

LSA 3

Default route to ABR injected Area 1

• LSA 3s (Inter-Area routes) are propagated by the ABR.• ABR blocks all LSA 4s (reachabilitly to ASBR) and LSA 5s (External routes) • The ABR injects a default route into the stub area, pointing to the ABR. (This

does not mean the ABR has a default route of its own.)• Essentially, internal routers in a Stub Area only see Inter-Area OSPF routes

and the default route to the ABR – No External routes.


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Stub Area

LSA 4

LSA 4

X Blocked

LSA 5

Blocked XLSA 5

LSA 3

LSA 3

Default route to ABR injected

• Changes in External routes no longer affect Stub Area routing tables.


Internal Internal# show ip ospf database


LSA 3 - Generated by the ABR.

Describes links between ABR and Internal Routers of the Local Area


Default Route – Advertised by ABR-1

0.0.0.0 192.168.3.1 243 0x80000001 0x8A46

Area 0 networks - Advertised by ABR-1

172.16.1.0 192.168.3.1 243 0x80000006 0xEF1E

Area 51 networks - Advertised by ABR-1

172.16.51.1 192.168.3.1 243 0x80000002 0xCF0E

• Notice that there are no LSA 4s or LSA 5s for stub area routers.

• Default Route injected by ABR (LSA 3)

Stub Areas


Internal Internal# show ip route

Gateway of last resort is 172.16.10.5 to network 0.0.0.0


O IA 172.16.51.1/32 [110/783] via 172.16.10.5, 00:03:08, Serial0



O IA 172.16.1.0/24 [110/782] via 172.16.10.5, 00:03:08, Serial0



O*IA 0.0.0.0/0 [110/782] via 172.16.10.5, 00:03:08, Serial0

Stub Areas

NOTE on default route:

• ABR will advertise a default route with a cost of 1

• cost of 65 = 1 + 64 (serial link)

• Using bandwidth of 128K, not 64K: 782 = (100,000,000/128,000) + 1


ABR-2 ABR-2# show ip route

Gateway of last resort is not set


O IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:01:59,FastEthernet0

O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:01:59, Serial0








Stub Areas

• Notice, there is no automatic default route on ABR, as there are with the internal stub routers.


• Using the Multi-area OSPF Lab Handout: “Lab 2. – OSPF Stub Area.

• Look at the link state database summary (show ip ospf database) commands and the Summary Net Links States (LSA 3s).

Note: A Stub area may have more than one ABR, but because of the default route, the internal routers will not be able to determine which router is the optimal gateway outside the AS and end up load balancing between the multiple ABRs.

Your Turn -Discuss in groups (Stub)


Totally Stubby Areas

“Cisco proprietary”, however the RFC does make some provisions for this as an optional feature..

Same considerations as with Stub areas:

• An area could be qualified a stub when there is a single exit point (a single ABR) from that area or if routing to outside of the area does not have to take an optimal path.




• Stub areas will result in memory and processing savings depending

upon the size of the network. - This is even more true with Totally Stubby areas


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Totally Stubby Area


Receives routes from within A.S.:

• Only from within the local area - LSA 1s and LSA 2s (if appropriate)

• Does not receive routes from other areas (Inter-Area) - LSA 3s

Does not receive routes from External A.S. (External Routes)

ABR:

• ABR blocks all LSA 4s and LSA 5s.

• ABR blocks all LSA 3s, except propagating a default route.

• Default route is injected into totally stubby area by ABR.

Configuring:

• All routers must be configured as “stub”

• ABR must be configured as “stub no-summary”



ABR-2 router ospf 1 network 172.16.1.0 0.0.0.255 area 0 network 172.16.10.4 0.0.0.3 area 1 area 1 stub no-summary ^^ Command: area area stub no-summary

Internal router ospf 1 network 172.16.0.0 0.0.255.255 area 1 area 1 stub

^^ Command: area area stub



ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Totally Stubby Area

LSA 4

LSA 4

X Blocked

LSA 5

Blocked XLSA 5

LSA 3

LSA 3


X

• LSA 3s (Inter-Area routes) are blocked by the ABR.• ABR blocks all LSA 4s (reachability to ASBR) and LSA 5s (External routes) • The ABR injects a default route (LSA 3) into the stub area, pointing to the ABR.

(This does not mean the ABR has a default route of its own.)• Essentially, internal routers in a Totally Stubby Area only see the default route

to the ABR.


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Totally Stubby Area

LSA 4

LSA 4

X Blocked

LSA 5

Blocked XLSA 5

LSA 3

LSA 3


X

• Changes in any networks outside the Totally Stubby Area no longer affects the Totally Stubby routing tables.


Internal Internal# show ip ospf database

Summary Net Link States (Area 1) LSA 3 - Generated by the ABR.

Describes links between ABR and Internal Routers of the Local Area


Default Route – Advertised by ABR-2

0.0.0.0 192.168.3.1 205 0x80000003 0x8648

Default Route injected by ABR (LSA 3)


• Default route is injected into totally stubby area by ABR for all other networks (inter-area and external routes)

• Does not receive routes from other areas (Inter-Area)

• Does not receive routes from External A.S. (External Routes)


Internal Internal# show ip route

Gateway of last resort is 172.16.10.5 to network 0.0.0.0






O*IA 0.0.0.0/0 [110/782] via 172.16.10.5, 00:03:09, Serial0

• Default route is injected into totally stubby area by ABR for all other networks (inter-area and external routes)

• Does not receive routes from other areas (Inter-Area)

• Does not receive routes from External A.S. (External Routes)



ABR-2 ABR-2# show ip routeGateway of last resort is not set

172.16.0.0/16 is variably subnetted, 4 subnets, 3 masksO IA 172.16.51.1/32 [110/2] via 172.16.1.2, 00:02:35,FastEthernet0O 172.16.20.0/24 [110/782] via 172.16.10.6, 00:02:35, Serial0C 172.16.10.4/30 is directly connected, Serial0C 172.16.1.0/24 is directly connected, FastEthernet0O E2 11.0.0.0/8 [110/20] via 172.16.1.1, 00:02:35, FastEthernet0O E2 12.0.0.0/8 [110/20] via 172.16.1.1, 00:02:35, FastEthernet0O E2 13.0.0.0/8 [110/20] via 172.16.1.1, 00:02:35, FastEthernet0 192.168.3.0/32 is subnetted, 1 subnetsC 192.168.3.1 is directly connected, Loopback1


• ABR will forward Intra-Area routes (to other areas within AS)

• Notice, there is not an automatic default route in the ABR’s routing table like there is with the internal Totally Stubby routers.


OSPF Design Tips

• Different people have different approaches to designing OSPF networks.

• The important thing to remember is that any protocol can fail under pressure.

• “The idea is not to challenge the protocol but rather to work with it in order to get the best behavior.” CCO

OSPF design considerations


Number of Routers per Area

The maximum number of routers per area depends on several factors, including the following:

What kind of area do you have?

What kind of CPU power do you have in that area?

What kind of media?

Will you be running OSPF in NBMA mode?

Is your NBMA network meshed?

Do you have a lot of external LSAs in the network?

Are other areas well summarized?

For this reason, it's difficult to specify a maximum number of routers per area.



Stub and Totally Stubby Areas:

• An area could be qualified a stub when there is a single exit point (a single ABR) from that area or if routing to outside of the area does not have to take an optimal path.




• Stub areas will result in memory and processing savings depending

upon the size of the network. - This is even more true with Totally Stubby areas

• Totally Stubby areas is a Cisco enhancement.



Quick Review

• Areas

• LSAs

• Stub Area

• Totally Stubby Area


LSA-1 - Router LSA


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1’s being sent within Area 0

LSA 1LSA 1LSA 1



ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1’s being sent within other areas

LSA 1

LSA 1

LSA 1

LSA 1



ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 1 Originated

LSA 1’s are flooded out other interfaces within the same area.

LSA 1 flooded



LSA-2 - Network LSA


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas

LSA 2

No LSA 2’s for ABR-1 in Area 51, or for Internal because no other routers on multi-access segment.

flooded

DR

DR

LSA 2

LSA 2flooded

LSA 2

LSA 2



LSA-3 - Summary LSA


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas


LSA 1LSA 1

LSA 3LSA 3

LSA 1



ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Multi Area OSPF

Normal Areas


LSA 1

LSA 1

LSA 1

LSA 3

LSA 3

LSA 3

LSA 3

LSA 1

LSA 3

LSA 3



ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Don’t forget about the LSA 1’s from Area 0.

Multi Area OSPF

Normal Areas

LSA 1LSA 1LSA 3

LSA 3

LSA 1



LSA-4 – ASBR Summary LSA


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Normal Areas

LSA 4

LSA 4

LSA 4

LSA 5’s flooded

LSA 4

Area 1

• Flooded throughout the backbone area to the other ABRs.

• Describes the reachability to the ASBRs


LSA-5 - External LSA


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

ASBR

router ospf 1

redistribute static

network 172.16.1.0 0.0.0.255 area 0

ip route 11.0.0.0 255.0.0.0 Null0

ip route 12.0.0.0 255.0.0.0 Null0

ip route 13.0.0.0 255.0.0.0 Null0

Normal Areas


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Normal Areas

LSA 5

LSA 5

LSA 5LSA 5

LSA 5’s floodedLSA 5

ASBR

router ospf 1

redistribute static

network 172.16.1.0 0.0.0.255 area 0

ip route 11.0.0.0 255.0.0.0 Null0

ip route 12.0.0.0 255.0.0.0 Null0

ip route 13.0.0.0 255.0.0.0 Null0

• “Redistribute” command creates an ASBR router.• Originated by the ASBR. • Describes destination networks external to the OSPF Routing Domain• Flooded throughout the OSPF AS except to stub and totally stubby

areas


Stub Area


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Stub Area

LSA 4

LSA 4

X Blocked

LSA 5

Blocked XLSA 5

LSA 3

LSA 3

Default route to ABR injected


Totally Stubby Area


ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Totally Stubby Area

LSA 4

LSA 4

X Blocked

LSA 5

Blocked XLSA 5

LSA 3

LSA 3


X


Next

• E1 and E2 routes

• Default Routes

• Route Summarization

• NSSA (Not So Stubby Areas)

• Multiple ABR Scenario

• Multiple ASBR Scenario

• Virtual Links

• Load Balancing

• show commands


NSSA (Not So Stubby Area)

NSSAArea 2

Backbone AreaArea 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH


• Relatively new, standards based OSPF enhancement, RFC 1587.

• NSSA allows an area to remain a stub area, but carry external routing information (Type 7 LSAs) from its stubby end back towards the OSPF backbone.

• ASBR in NSSA injects external routing information into the backbone and the NSSA area, but rejects external routing information coming from the ABR.

• The ABR does not inject a default route into the NSSA. – This is true for a NSSA Stub, but a default route is injected for a

NSSA Totally Stubby area.

• Note: RFC 1587, “A default route must not be injected into the NSSA as a summary (type-3) LSA as in the stub area case.”

• What???

• Following scenario is only example of how NSSA works. For the purposes of learning about NSSAs, don’t get hung up on the why’s and what if’s.



NSSAArea 2

Backbone AreaArea 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH

Default route via RTG

NSSA Stub Area• Area 2 would like to be a stub network.

• RTH only supports RIP, so RTG will run RIP and redistribute those routes in OSPF.

• Unfortunately, this makes the area 2 router, RTG, an ASBR and therefore area 2 can no longer be a stub area.

• RTH does not need to learn routes from OSPF, a default route to RTG is all it needs.

• But all OSPF routers must know about the networks attached to the RIP router, RTH, to route packets to them.


NSSA Stub Area (cont.)• NSSA allow external routes to be advertised into the OSPF AS while retaining the

characteristics of a stub area to the rest of the OSPF AS.

• ASBR RTG will originate Type-7 LSAs to advertise the external destinations.

• These LSA 7s are flooded through the NSSA but are blocked by the NSSA ABR.

• The NSSA ABR translates LSA 7s into LSA 5s and flood other areas.

NSSAArea 2

Backbone AreaArea 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH


LSA 7LSA 7

LSA 7

LSA 7

LSA 7

LSA 7LSA 5

LSA 7s Blocked


Type 7 LSA NSSA External Link Entry

• Originated by an ASBR connected to an NSSA.

• Type 7 messages can be flooded throughout NSSAs and translated into LSA Type 5 messages by ABRs.

• Routes learned via Type-7 LSAs are denoted by either a default “N1” or an “N2” in the routing table. (Relative to E1 and E2).

NSSAArea 2

Backbone AreaArea 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH


LSA 7LSA 7

LSA 7

LSA 7

LSA 7

LSA 7LSA 5

LSA 7s Blocked


Configuring NSSA Stub AreaConfigured for all routers in Area 2:

router ospf 1

network 172.16.2.0 0.0.0.255 area 2

area 2 nssa

NSSAArea 2

Backbone AreaArea 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH


LSA 7LSA 7

LSA 7

LSA 7

LSA 7

LSA 7LSA 5

LSA 7s Blocked

NSSA Generic


NSSA Stub and NSSA Totally Stubby

• There are two flavors in NSSA:

– Stub

– Totally Stubby

• Area 2 routers may or may not receive Inter-area routes from RTA, depending upon NSSA configuration

• NSSA areas have take on the same characteristics as stub and totally stubby areas, along with the characteristics of NSSA areas.



NSSA –Stub

NSSA stub areas:

• NSSAs that block type 4 and 5, but allow type 3.

• To make a stub area into an NSSA, use the following command under the OSPF configuration.

• This command must be configured on all routers in area 2.

router ospf 1 area 2 nssa


NSSAArea 2

Backbone AreaArea 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH

LSA 7LSA 7

LSA 7

LSA 7

LSA 7

LSA 7LSA 5

LSA 7s Blocked

NSSA Stub Areas

0.0.0.0/0

LSA 3s

XRTH routes:E1/E2

RTH routes:N1/N2LSA 4s & LSA 5s

X


NSSA Stub Area Routing Tables: • RTG: Area 2 routes, Area 0 routes (IA), RTH RIP routes

– No 0.0.0.0/0 (IA) route from RTB (ABR), despite documentation• Area 2 Internal Routers: Area 2 routes, RTH routes (N1/N2), Area 0 routes (IA)

– No 0.0.0.0/0 (IA) route from RTB (ABR), despite documentation• RTB: Area 2 routes, Area 0 routes, RTH routes (N1/N2), External routes if redistributed from

RTA ASBR (E1/E2)• RTA: Area 0 routes, Area 2 routes, RTH routes (E1/E2), External routes if redistributed from

RTA (E1/E2)• Note: Area 2 routers may or may not receive E1/E2 routes from RTA, depending upon NSSA

configuration (next).


Area 2 routers:

router ospf 1

network 172.16.2.0 0.0.0.255 area 2

area 2 nssa

NSSAArea 2

Backbone AreaArea 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH


LSA 7LSA 7

LSA 7

LSA 7

LSA 7

LSA 7LSA 5

LSA 7s Blocked

NSSA Stub Areas

0.0.0.0/0

LSA 3s

XRTH routes:E1/E2

RTH routes:N1/N2LSA 4s & LSA 5s

X


NSSA Totally Stubby Area

• NSSA totally stub areas: Allow only summary default routes and filters everything else.

• To configure an NSSA totally stub area, use the following command under the OSPF configuration on the NSSA ABR:

router ospf 1 area 2 nssa no-summary

• Configure this command on NSSA ABRs only.

• All other routers in area 2 (internal area 2 routers):

router ospf 1 area 2 nssa

• After defining the NSSA totally stub area, area 2 has the following characteristics (in addition to the above NSSA characteristics):

No type 3 or 4 summary LSAs are allowed in area 2. This means no inter-area routes are allowed in area 2.

A default route is injected into the NSSA totally stub area as a type 3 summary LSA by the ABR.

NSSA – Totally Stubby


NSSAArea 2

Backbone AreaArea 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH


LSA 7LSA 7

LSA 7

LSA 7

LSA 7

LSA 7LSA 5

LSA 7s Blocked

NSSA Totally Stubby Areas

0.0.0.0/0

LSA 3sX

RTH routes:E1/E2

RTH routes: N1/N2LSA 4s & LSA 5s

X

RTB (ABR):

router ospf 1

network 172.16.1.0 0.0.0.255 area 0

network 172.16.2.0 0.0.0.255 area 2 ...

area 2 nssa no-summary

Area 2 routers:

router ospf 1

network 172.16.2.0 0.0.0.255 area 2

area 2 nssa


NSSAArea 2

Backbone AreaArea 0

ASBR

ABR(Possible

ASBR)

RIP

RTARTB

RTC

RTD

RTE

RTF

RTG

RTH


LSA 7LSA 7

LSA 7

LSA 7

LSA 7

LSA 7LSA 5

LSA 7s Blocked

NSSA Totally Stubby Areas

0.0.0.0/0

LSA 3sX

RTH routes:E1/E2

RTH routes: N1/N2LSA 4s & LSA 5s

X

NSSA Totally Stubby Area Routing Tables: • RTG: Area 2 routes, RTH RIP routes, 0.0.0.0/0 (IA) route from RTB (ABR)

– Totally Stubby: No Area 0 routes or external routes from RTA• Area 2 Internal Routers: Area 2 routes, RTH routes (N1/N2), 0.0.0.0/0 (IA) route from RTB

(ABR)

– Totally Stubby: No Area 0 routes or external routes from RTA• RTB: Area 2 routes, Area 0 routes, RTH routes (N1/N2), External routes from RTA ASBR

(E1/E2) if redistributed by ASBR• RTA: Area 0 routes, Area 2 routes, RTH routes (E1/E2), External routes (E1/E2)


Area 2NSSA Area 0

RTARTBRTCRTD

RTE

RIPDefault Route

172.16.3.0/24 172.16.2.0/24 172.16.1.0/24

10.0.0.0/8

200.200.200.0/24

222.222.222.0/24

For More on NSSA

For more examples

• See NSSA document on my web site for more info.


Virtual Links


• All areas in an OSPF autonomous system must be physically connected to the backbone area (area 0).

• In some cases where this is not possible, you can use a virtual link to connect to the backbone through a non-backbone area.

• As mentioned above, you can also use virtual links to connect two parts of a partitioned backbone through a non-backbone area.

• The area through which you configure the virtual link, known as a transit area, must have full routing information.

• Must be configured between two ABRs.

• The transit area cannot be a stub area.

Virtual Links


• A virtual link has the following two requirements:– It must be established between two routers that share a common

area and are both ABRs.– One of these two routers must be connected to the backbone.

• Doyle, “should be used only as a temporary fix to an unavoidable topology problem.”

Virtual Links


The command to configure a virtual link is as follows:

area <area-id> virtual-link <remote-router-id>

RTA(config)#router ospf 1RTA(config-router)#network 192.168.0.0 0.0.0.255 area 51RTA(config-router)#network 192.168.1.0 0.0.0.255 area 3RTA(config-router)#area 3 virtual-link 10.0.0.1...

RTB(config)#router ospf 1RTB(config-router)#network 192.168.1.0 0.0.0.255 area 3RTB(config-router)#network 192.168.2.0 0.0.0.255 area 0RTB(config-router)#area 3 virtual-link 10.0.0.2

Virtual Links


• OSPF allows for linking discontinuous parts of the backbone using a virtual link.

• In some cases, different area 0s need to be linked together. This can occur if, for example, a company is trying to merge two separate OSPF networks into one network with a common area 0.

• In other instances, virtual-links are added for redundancy in case some router failure causes the backbone to be split into two. (CCO)

• Whatever the reason may be, a virtual link can be configured between separate ABRs that touch area 0 from each side and having a common area.

Virtual Links


Virtual Links – Another Example


Route Summarization

Inter-Area Route Summarization - Area Range• By default ABRs do not summarize routes between areas.

• Route summarization is the consolidation of advertised addresses.

• This feature causes a single summary route to be advertised to other areas by an ABR.

• In OSPF, an ABR will advertise networks in one area into another area.

• If the network numbers in an area are assigned in a way such that they are contiguous, you can configure the ABR to advertise a summary route that covers all the individual networks within the area that fall into the specified range.

On the ABR (Summarizes routes before injecting them into different area)

Router(config-router)# area area-id range network-address subnet-mask

• area-id - Identifier of the area about which routes are to be summarized. (From area)


• RTB is summarizing the range of subnets from 128.213.64.0 to 128.213.95.0 into one range: 128.213.64.0 255.255.224.0.

• This is achieved by masking the first three left most bits of 64 using a mask of 255.255.224.0.

128.213.64.0/24 - 010 00000

128.213.95.0/24 – 010 11111

-----------------------------------------

128.213.64.0/19 - 01000000

Route Summarization


Route Summarization

• In the same way, RTC is generating the summary address 128.213.96.0 255.255.224.0 into the backbone.

• Note that this summarization was successful because we have two distinct ranges of subnets, 64-95 and 96-127.

128.213.96.0/24 - 011 00000

128.213.127.0/24 – 011 11111

-----------------------------------------

128.213.96.0/19 - 01100000


128.213.64.0/24 - 010 00000

128.213.95.0/24 – 010 11111

-----------------------------------------

128.213.64.0/19 - 01000000

RTB

router ospf 100

area 1 range 128.213.64.0 255.255.224.0

Route Summarization


128.213.96.0/24 - 011 00000

128.213.127.0/24 – 011 11111

-----------------------------------------

128.213.96.0/19 - 01100000

RTC

router ospf 100

area 2 range 128.213.96.0 255.255.224.0

Route Summarization


External Route Summarization - summary-address

• When redistributing routes from other protocols into OSPF (later), each route is advertised individually in an external link state advertisement (LSA).

• However, you can configure the Cisco IOS software to advertise a single route for all the redistributed routes that are covered by a specified network address and mask.

• Doing so helps decrease the size of the OSPF link state database.

On the ASBR only (Summarizes external routes before injecting them into the OSPF domain.)

Router(config-router)# summary-address network-address subnet-mask

Route Summarization


RTA router ospf 100 summary-address 128.213.64.0 255.255.224.0 redistribute bgp 50 metric 1000 subnets (later)

RTD router ospf 100 summary-address 128.213.96.0 255.255.224.0 redistribute bgp 20 metric 1000 subnets (later)

Route Summarization


Injecting Default Routes into OSPF

• By default, 0.0.0.0/0 route is not propagated from the ASBR to other routers.

• An autonomous system boundary router (ASBR) can be forced to generate a default route into the OSPF domain.

• As discussed earlier, a router becomes an ASBR whenever routes are redistributed into an OSPF domain.

• However, an ASBR does not, by default, generate a default route into the OSPF routing domain.


The way that OSPF generates default routes (0.0.0.0) varies depending on the type of area the default route is being injected into.

Stub and Totally Stubby Areas

• For stub and totally stubby areas, the area border router (ABR) to the stub area generates a summary link-state advertisement (LSA) with the link-state ID 0.0.0.0.

• This is true even if the ABR doesn't have a default route.

• In this scenario, you don't need to use the default-information

originate command.



All routers in the area must be configured as “stub” including the ABR: router ospf 1

area 1 stub

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Stub Area

LSA 4

LSA 4

X Blocked

LSA 5

Blocked XLSA 5

LSA 3

LSA 3


Stub Area


All routers in the area must be configured as “stub” except the ABR “stub no summary”:

ABR: router ospf 1 Other: router ospf 1 area 1 stub no-summary area 1 stub

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

Totally Stubby Area

LSA 4

LSA 4

X Blocked

LSA 5

Blocked XLSA 5

LSA 3

LSA 3


X

Totally Stubby Area


Normal Areas

• By default, in normal areas routers don't generate default routes.

• To have an OSPF router generate a default route, use the default-information originate command.

• This generates an external type-2 link with link-state ID 0.0.0.0 and network mask 0.0.0.0.

• This command should only be used on the ASBR.

– Some documentation states this command works only on an ASBR while other documentation states this command turns a router into an ASBR.



To have OSPF generate a default route use the following:

router ospf 10default-information originate [always] [metric metric-

value] [metric-type type-value] [route-map map-name]



There are two ways to generate a default.

1) default-information originate

• If the ASBR already has the default route (ip route 0.0.0.0 0.0.0.0), you can advertise 0.0.0.0 into the area.

2) default-information originate always

• If the ASBR doesn't have the route (ip route 0.0.0.0 0.0.0.0), you can add the keyword always to the default-information originate

command, and then advertise 0.0.0.0. • You should be careful when using the always keyword. If your router

advertises a default (0.0.0.0) inside the domain and does not have a default itself or a path to reach the destinations, routing will be broken.



ASBRrouter ospf 1

redistribute static

network 172.16.1.0 0.0.0.255 area 0

default-information originate

ip route 0.0.0.0 0.0.0.0 10.0.0.2

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

0.0.0.0/00.0.0.0/00.0.0.0/0

0.0.0.0/0 0.0.0.0/0

0.0.0.0/0

0.0.0.0/0



ASBRrouter ospf 1

redistribute static

network 172.16.1.0 0.0.0.255 area 0

default-information originate always

ip route 0.0.0.0 0.0.0.0 10.0.0.2

ASBR

ABR-1 ABR-2

InternalArea 51

Area 1

Area 0

172.16.0.0/16

172.16.1.0/24

172.16.51.0/24172.16.10.4/30

172.16.20.0/24

10.1.0.0/24

11.0.0.0/812.0.0.0/813.0.0.0/8

.1

.1

.2 .3

.5

.6

.1

Lo - RouterID192.168.2.1/32

Lo - RouterID192.168.1.1/32

Lo - RouterID192.168.3.1/32

Pri 100Pri 200

Lo - RouterID192.168.4.1/32

0.0.0.0/00.0.0.0/00.0.0.0/0

0.0.0.0/0 0.0.0.0/0

0.0.0.0/0

No 0.0.0.0/0 route, but propagated anyway or “always”



E1 vs. E2 External Routes

• External routes fall under two categories, external type 1 and external type 2.

• The difference between the two is in the way the cost (metric) of the route is being calculated.

• A type 1 (E1) cost is the addition of the external cost and the internal cost used to reach that route.

• The cost of a type 2 (E2) route is always the external cost, irrespective of the interior cost to reach that route.

• Type 2 (E2) is the default!

Redistributing External Routes


router ospf 1

redistribute routing-protocol metric-type [1|2]

• metric-type 1 - A type 1 cost is the addition of the external cost and the internal cost used to reach that route.

redistribute rip [metric value] metric-type 1

• metric-type 2 - The cost of a type 2 route is always the external cost, irrespective of the interior cost to reach that route.


• We will look at this command, along with internal/external costs, later in the chapter discussion route redistribution.




metric-type 2 - The cost of a type 2 route is always the external cost, irrespective of the interior cost to reach that route.


More later, but here is a taste of the metric value option …

• If a value is not specified for the metric value option, and no value is specified using the default-metric command, the default metric value is 0, except for OSPF where the default cost is 20.

• 0 is only understood by IS-IS and not by RIP, IGRP and EIGRP.

• RIP, IGRP and EIGRP must have the appropriate metrics assigned to any redistributed routes, or redistribution will not work.

• Use a value consistent with the destination protocol.


RouterELoop 1.10.202.206/24

RouterALoop 1.0.202.206/24

RouterBLoop 2.0.202.206/24

RouterCLoop 1.2.202.206/24

RouterDLoop 2.2.202.206

Switch

Switch

RouterFLoop 2.10.202.206/24

ASBRLoop 1.5.202.206/24

AS-Rem ote

10.0.0.0/8

192.10.10.0

/24206.202.0.0/24

192.10.5.0/24

206.202.1.0/24206.202.2.0/24

OSPFArea 1

OSPFArea 51

OSPFArea 0

.2

.1

.2

.1

.2.1

.3

.4.1

.2

.1

Loop 162.10.5.1/16

RIPRIP

.1

.2

574 510

510 574

510

510574

574 584584

ASBRrouter ospf 1

redistribute rip metric 500 metric-type 1

network 206.202.0.0 0.0.0.255 area 0

RIP routes redistributed with a metric (cost) of 500 plus the outgoing cost of the interface and a metric-type 1

metric-type 1Redistributing External Routes


RouterELoop 1.10.202.206/24

RouterALoop 1.0.202.206/24

RouterBLoop 2.0.202.206/24

RouterCLoop 1.2.202.206/24

RouterDLoop 2.2.202.206

Switch

Switch

RouterFLoop 2.10.202.206/24

ASBRLoop 1.5.202.206/24

AS-Rem ote

10.0.0.0/8

192.10.10.0

/24206.202.0.0/24

192.10.5.0/24

206.202.1.0/24206.202.2.0/24

OSPFArea 1

OSPFArea 51

OSPFArea 0

.2

.1

.2

.1

.2.1

.3

.4.1

.2

.1

Loop 162.10.5.1/16

RIPRIP

.1

.2

500

500

500

500500

500 500

ASBRrouter ospf 1

redistribute rip metric 500 metric-type 2

network 206.202.0.0 0.0.0.255 area 0

RIP routes redistributed with a metric (cost) of 500 and a metric-type 2 (default)

metric-type 2



So when should you redistribute a Type-1 (E1) External route?

• If there is more than one ABR for the area and the area is not a stub or totally stubby area.

– In this case one of the ABRs may provide a shorter path for certain non-area 0 internal routers, than other ABRs.

– E1 routes will include all internal costs from the internal router to the ABR and to the ASBR, allowing each router to choose which ABR provides the shorter path.

• Multiple ASBRs redistributing the same networks.

– In this case the routers’ cost to each ASBR can be used to choose the shortest path to the destination.



Know your outputs

• show ip route

• show ip ospf

• show ip ospf neighbor

• show ip ospf border-router

• show ip database

• show ip interface


Internal#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

<text omitted>

Gateway of last resort is not set


O IA 172.16.51.1/32 [110/783] via 172.16.10.5, 00:13:48, Serial0



O IA 172.16.1.0/24 [110/782] via 172.16.10.5, 00:13:53, Serial0



O E2 11.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0

O E2 12.0.0.0/8 [110/20] via 172.16.10.5, 00:14:41, Serial0

O E2 13.0.0.0/8 [110/20] via 172.16.10.5, 00:14:42, Serial0

Internal#

• LSA 1 and LSA 2: Denoted by “O” or “C”

• LSA 3: Denoted by “IA”

• LSA 5: Denoted by “E1” or “E2” (default)

show ip route


ABR-2#show ip ospf Routing Process "ospf 1" with ID 192.168.3.1 Supports only single TOS(TOS0) routes 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 3. Checksum Sum 0x97E3 Number of DCbitless external LSA 0 Number of DoNotAge external 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 1 Area has no authentication SPF algorithm executed 8 times <text omitted>

Area 1 Number of interfaces in this area is 1 Area has no authentication

SPF algorithm executed 5 times <text omitted>

show ip ospf


ASBR#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

192.168.3.1 100 FULL/BDR 00:00:37 172.16.1.3 FastEthernet0/0

192.168.2.1 200 FULL/DR 00:00:33 172.16.1.2 FastEthernet0/0

• Displays a list of neighbors and their link state status

show ip ospf neighbor


show ip ospf border-router• To display the internal OSPF routing table entries to an Area Border Router (ABR) and

Autonomous System Boundary Router (ASBR), use the show ip ospf border-routers privileged EXEC command.

• LSA 4’s (routes to ASBRs) are not installed in the main IP routing table but in the special internal OSPF routing table.

This command will displays any ABRs in the area or any ASBRs in the OSPF routing domain.

• Destination - Router ID of the destination.

• Next Hop - Next hop toward the destination.

• Cost - Cost of using this route.

• Type - The router type of the destination; it is either an ABR or ASBR or both.

• Rte Type - The type of this route; it is either an intra-area or interarea route.

• Area - The area ID of the area from which this route is learned.

• SPF No - The internal number of the shortest path first (SPF) calculation that installs this route.

ABR-1#show ip ospf border

OSPF Process 1 internal Routing Table

Codes: i - Intra-area route, I - Inter-area route

i 192.168.1.1 [1] via 172.16.1.1, FastEthernet0/0, ASBR, Area 0, SPF 38

i 192.168.3.1 [1] via 172.16.1.3, FastEthernet0/0, ABR, Area 0, SPF 38

ABR-1#


Internal#show ip ospf data OSPF Router with ID (192.168.4.1) (Process ID 1)

Router Link States (Area 1)Link ID ADV Router Age Seq# Checksum Link count192.168.3.1 192.168.3.1 898 0x80000003 0xCE56 2192.168.4.1 192.168.4.1 937 0x80000003 0xFD44 3

Summary Net Link States (Area 1) Link ID ADV Router Age Seq# Checksum172.16.1.0 192.168.3.1 848 0x80000005 0xD339 172.16.51.1 192.168.3.1 843 0x80000001 0xB329

Summary ASB Link States (Area 1)Link ID ADV Router Age Seq# Checksum192.168.1.1 192.168.3.1 912 0x80000003 0x93CC

Type-5 AS External Link StatesLink ID ADV Router Age Seq# Checksum Tag11.0.0.0 192.168.1.1 1302 0x80000001 0x3FEA 012.0.0.0 192.168.1.1 1303 0x80000001 0x32F6 013.0.0.0 192.168.1.1 1303 0x80000001 0x2503 0

show ip ospf database


Router Link States (LSA 1)

• Router Link States (LSA1’s) should display all the RouterIDs of routers in that area, including its own.

• Link State ID is always the same as the Advertising Router.

• ADV Router is the Router ID of the router that created this LSA 1.

Net Link States (LSA 2)

• Net Link States (LSA2’s) should display the RouterIDs of the DRs on all multi-access networks in the area and their IP addresses.

• Link ID is the IP address of DR on MultiAccess Network.

• ADV Router is the Router ID of the DR.

Summary Link States (LSA 3)

• Should see networks in other areas and the ABR advertising that route.

• Link ID is the IP network addresses of networks in other areas.

• ADV Router is the ABR Router ID sending the LSA-3.

Summary ASB Link States (LSA 4)

• Routers in non-area 0, should see Router ID of ASBR and its ABR to get there.

• Link ID is the Router ID of ASBR

• ADV Router is the Router ID of the ABR advertising route

Type-5 AS External Link States (LSA 5)

• All Routers should see External networks and the Router ID of ASBR to get there

• Link ID is the External Network

• ADV Router is the Router ID of ASBR advertising the LSA 5.


SanJose3#show ip ospf interface fa 0

FastEthernet0 is up, line protocol is up

Internet Address 192.168.1.3/24, Area 0

Process ID 1, Router ID 192.168.31.33, Network Type BROADCAST, Cost: 1

Transmit Delay is 1 sec, State DR, Priority 1

Designated Router (ID) 192.168.31.33, Interface address 192.168.1.3

Backup Designated router (ID) 192.168.31.22, Interface address 192.168.1.2

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 2

Last flood scan time is 0 msec, maximum is 0 msec

Neighbor Count is 2, Adjacent neighbor count is 2

Adjacent with neighbor 192.168.31.11

Adjacent with neighbor 192.168.31.22 (Backup Designated Router)

Suppress hello for 0 neighbor(s)

SanJose3#

Do you know these?

show ip ospf interface


OSPF Extra’s, FAQs, and FYIs

• The following sections contain information to help you understand OSPF.

• This information is not necessarily on the CCNP Advanced Routing Exam.


• OSPF Hello traffic can keep an ISDN link up indefinitely.• By entering the command “ip ospf demand-circuit” on one side of a BRI,

adjacencies will be formed and:– Ongoing OSPF Hellos will be suppressed– The DNA (Do-Not-Age) bit is set in the LSA so that this entry is not aged in

the router’s LSDB.• LSA is not flooded when reaching LSRefresh• LSA is not flooded if there is a new version but the contents are the

same

show ip ospf interface bri 0• “Run as demand circuit”• “(Hello Suppressed)”

show ip ospf neighbor• Dead Time: “-”

Extra: OSPF over ISDN


Note: You need to configure the demand circuit at one end of the link only. However, if you configure this command on both ends it does not cause any harm.

Suggestion: To reduce the affect of link flaps on the demand circuit, configure the area that contains the demand circuit as totally stub.

• In this case configure Area 1 to be a totally stubby area.

• Summarizing routes on Router 1 can also help if the flapping link is within the summarized range.

Router1

interface BRI1/1

ip address 192.158.254.13 {/30}

ip ospf demand-circuit

router ospf 20

network 192.158.254.0 0.0.0.255 area 0

Router2

interface BRI1/0

ip address 192.158.254.14 {/30}

router ospf 20

network 192.158.254.0 0.0.0.255 area 0

Extra: OSPF over ISDN


Extra: OSPF and Load Balancing

• OSPF only supports equal-cost load balancing.

• By default, four equally good routes to the same destination are kept in the routing table for load balancing.

• This can be increased up to six with the maximum-paths command.

• The bandwidth and/or ip ospf cost (or in the case of serial links [1.544 Mbps] the lack of) commands can be used to make unequal-cost links look like equal-cost links to OSPF for load balancing.– This should be done with caution, as it may burden slower

links and/or not make efficient use of faster links.


Extra: OSPF and DNS Lookups

• Loopback interfaces simplify the management and troubleshooting of OSPF routing domains by providing predictable Router Ids.

• This can be taken one step further by recording the Router Ids in a Domain Name Service (DNS) database.

• The router can then be configured to consult the server address-to-name mappings, or Reverse DNS lookups, and then display the routers by name instead of by Router ID.


ASBR was configured to perform DNS lookups as follows: ip name-server 172.16.1.100 ip ospf name-lookup

• The first command specifies the DNS server.• The second command enables the OSPF process to perform DNS lookups.• This can also be used for identifying router interfaces such as ABR-1 and ABR-2

Extra: OSPF and DNS Lookups

ASBR#show ip ospf data


Router Link States (Area 0)


172.16.10.5 ABR-1 412 0x8000000F 0x6F9C 1

192.168.1.1 ABR-2 201 0x80000012 0x8D3D 1

192.168.2.1 ABR-2 205 0x80000016 0x7E46 1

192.168.3.1 ABR-2 205 0x80000005 0x9C36 1


Extra: IOS 12.01(T) – router-id

router-id

• To use a fixed router ID, use the router-id router configuration command.

• To force OSPF to use the previous OSPF router ID behavior, use the no form of this command.

• Takes precedence over Loopback address

router ospf 1

router-id ip-address


OSPF and Redistribution (later)

• “Before Cisco IOS Software Release 12.1.3, when redistributing connected routes into OSPF, connected networks included in the network statements under router OSPF advertised in Type-1, Type-2, or Type-3 link-state advertisements (LSAs) were also announced in Type-5 LSAs.”

• In other words, if you are using the redistributed connected command, any connected networks included using the OSPF network command, were not only advertised as normal using LSA Type 1, 2, or 3, but also as an external LSA Type-5.

• “Memory is required to store those Type-5 LSAs. The storage also requires a CPU to process the LSAs during full or partial Shortest Path First (SPF) runs and to flood them when some instability occurs.”

• “In Cisco IOS Software Release 12.1(3) and later, the Type-5 LSAs are no longer created for connected networks included in the network statements under router OSPF.”

Redistributing Connected Networks into OSPF• http://www.cisco.com/warp/public/104/redist-conn.html


• I was teaching redistribution of RIP into OSPF and couldn't explain the Baypoint output on p. 132 of the lab manual. Since both 172.16.8.0 and 172.16.7.0 are being redistributed into OSPF with a default metric of 10, why does the .7.0 network have a metric of 20?

• The reason for the difference is that when using the redistribute connected command, those connected networks are not affected by the default-metric command. So once the ospf, 'redistribute connected subnets' command is entered on SJ1, those directly connected networks, being redistributed into rip, are no longer affected by the default-metric value.

For more info see: http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fiprrp_r/ind_r/1rfindp2.htm

"You cannot use the 'default-metric' command to affect the metric used to advertise 'connected' routes."

Usually, it is not advisable to redistribute connected networks into area 0, but thats another story.

• In other words, if you are using the redistributed connected command, any connected networks included using the OSPF network command, were not only advertised as normal using LSA Type 1, 2, or 3, but also as an external LSA Type-5.

We saw this in our multi-area OSPF labs when some students were using 12.0.5 and others were using 12.2. With 12.0.5 an internal OSPF route, LSA Type-1, was also being advertised as a LSA Type-5, when they used the "redistribute connected" command.

Question from another instructor (later)…

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fiprrp_r/ind_r/1rfindp2.htm

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fiprrp_r/ind_r/1rfindp2.htm


OSPF FAQs and FYIs

Q: Why are loopbacks advertised as /32 host routes in OSPF?

A: Loopbacks are considered host routes in OSPF, and they're advertised as /32. For more information, see section 9.1 of RFC 2328. In Cisco IOS ® version 11.3T and 12.0, if the ip ospf network point-to-point command is configured under loopbacks, then OSPF advertises the loopback subnet as the actual subnet configured on loopbacks.

http://www.cisco.com/warp/public/104/9.html

Q: Can a virtual link cross more than one area.

A: No.

http://www.cisco.com/warp/public/104/9.html


Q: What happens within OSPF if there is more than one route to a destination? What is the preference of OSPF in choosing a best route?

A: Here is the OSPF preference rules:• Intra-area routes area always most preferred.• Inter-area routes are preferred over AS or NSSA external routes.• AS-external routes and NSSA-external routes are of equal preference.

Within these routes, preferences are as follows:– External Type-1 routes are always preferred

• If equal, route-metric (cost) is the tie-breaker– External Type-2 routes

• If equal, route metric and distance to the originating router are used as tie-breakers.

– If still a tie (Type-1 or Type-2), AS-external (LSA 5) routes are preferred over NSSA external (LSA 7) routes.

• If these rules do not solve the tie, routes are installed as parallel routes.

OSPF FAQs and FYIs


OSPF Packet Pacing

• Introduced in Cisco IOS 11.3

• Helps avoid packet drops at the receiving side, caused by uncontrolled bursts of link-state updates.

• The receiving router may not be able to queue and process all of the packets so some packets are dropped.

• To make matters worse, when the sending router does not receive LSAcks for all of the LSAs sent, so retransmits along with other LSAs needed to be sent.

• Currently Cisco IOS Packet Pacing, every 33 milliseconds (non-configurable) the router builds a link-state update and sends it to its neighbors.

• The next group of LSAs is transmitted after another 33 milliseconds.

• This speeds up convergence and decreases the length of the transition period.

OSPF FAQs and FYIs


OSPF Group Pacing

• Introduced in Cisco IOS 11.3

• Every LSA is aged whiled stored in the LSDB.

• ALL LSAs are aged independently of one another.

• When an LSA reaches LSRefreshTime (30 minutes) the router that originated the it floods the LSA.

• When an LSA reaches MaxAge (60 minutes) the router floods the LSA, even if it did not originate the LSA.

• If a router has a lot of LSAs, maintaining a separate timer can be expensive.

• With Cisco OSPF Group Pacing, LSAs are collected into groups by their ages, with ages within 4 minutes by default (can be configured).

• The router maintains timers for LSA groups instead of individual LSAs.

• This is used for all LSA operations including LSA aging and LSA refreshing.

OSPF FAQs and FYIs


Cisco SPF Scheduling (Review)• SPF algorithm is CPU intensive and takes some time depending upon

the size of the area (coming next week), the number of routers, the size of the link state database.

• A flapping link can cause an OSPF router to keep on recomputing a new routing table, and never converge.

• To minimize this problem:– SPF calculations are delayed by 5 seconds after receiving an

LSU (Link State Update)– Delay between consecutive SPF calculations is 10 seconds

• You can configure the delay time between when OSPF receives a topology change and when it starts a shortest path first (SPF) calculation (spf-delay).

• You can also configure the hold time between two consecutive SPF calculations (spf-holdtime).

Router(config-router)#timers spf spf-delay spf-holdtime

OSPF FAQs and FYIs (Know this!)


OSPF Design Issues – Some extra info!

• Number of Routers per Area

• Number of Neighbors

• Number of Areas per ABR

• Full Mesh vs. Partial Mesh

• Memory Issues


• The following information is taken from Cisco CCO.

• http://www.cisco.com/warp/public/104/3.html

• The OSPF RFC (1583) did not specify any guidelines for the number of routers in an area or number the of neighbors per segment or what is the best way to architect a network.

• Different people have different approaches to designing OSPF networks.

• The important thing to remember is that any protocol can fail under pressure.

• The idea is not to challenge the protocol but rather to work with it in order to get the best behavior.

• The following are a list of things to consider.

– Number of Routers per Area

– Number of Neighbors

– Number of Areas per ABR

– Full Mesh vs. Partial Mesh

– Memory Issues

OSPF Design Issues


Number of Routers per Area

The maximum number of routers per area depends on several factors, including the following:

• What kind of area do you have?

• What kind of CPU power do you have in that area?

• What kind of media?

• Will you be running OSPF in NBMA mode?

• Is your NBMA network meshed?

• Do you have a lot of external LSAs in the network?

• Are other areas well summarized?

• For this reason, it's difficult to specify a maximum number of routers per area.

OSPF Design Issues


Number of Neighbors

• The number of routers connected to the same LAN is also important.

• Each LAN has a DR and BDR that build adjacencies with all other routers.

• The fewer neighbors that exist on the LAN, the smaller the number of adjacencies a DR or BDR have to build.

• That depends on how much power your router has. You could always change the OSPF priority to select your DR.

• Also if possible, try to avoid having the same router be the DR on more than one segment.

• If DR selection is based on the highest RID, then one router could accidentally become a DR over all segments it is connected to.

• This router would be doing extra effort while other routers are idle.

OSPF Design Issues


Number of Areas per ABR

• ABRs will keep a copy of the database for all areas they service.

• If a router is connected to five areas for example, it will have to keep a list of five different databases.

• The number of areas per ABR is a number that is dependent on many factors, including type of area (normal, stub, NSSA), ABR CPU power, number of routes per area, and number of external routes per area.

• For this reason, a specific number of areas per ABR cannot be recommended.

• Of course, it's better not to overload an ABR when you can always spread the areas over other routers.

• The following diagram shows the difference between one ABR holding five different databases (including area 0) and two ABRs holding three databases each.

• Again, these are just guidelines, the more areas you configure per ABR the lower performance you get. In some cases, the lower performance can be tolerated.

OSPF Design Issues


Full Mesh vs. Partial Mesh

• Non Broadcast Multi-Access (NBMA) clouds such as Frame Relay or X.25, are always a challenge.

• The combination of low bandwidth and too many link-states is a recipe for problems.

• A partial mesh topology has proven to behave much better than a full mesh.

• A carefully laid out point-to-point or point-to-multipoint network works much better than multipoint networks that have to deal with DR issues.

OSPF Design Issues


Memory Issues• It is not easy to figure out the memory needed for a particular OSPF configuration. Memory

issues usually come up when too many external routes are injected in the OSPF domain. A backbone area with 40 routers and a default route to the outside world would have less memory issues compared with a backbone area with 4 routers and 33,000 external routes injected into OSPF.

• Memory could also be conserved by using a good OSPF design. Summarization at the area border routers and use of stub areas could further minimize the number of routes exchanged.

• The total memory used by OSPF is the sum of the memory used in the routing table (show ip route summary) and the memory used in the link-state database. The following numbers are a rule of thumb estimate. Each entry in the routing table will consume between approximately 200 and 280 bytes plus 44 bytes per extra path. Each LSA will consume a 100 byte overhead plus the size of the actual link state advertisement, possibly another 60 to 100 bytes (for router links, this depends on the number of interfaces on the router). This should be added to memory used by other processes and by the IOS itself. If you really want to know the exact number, you can do a show memory with and without OSPF being turned on. The difference in the processor memory used would be the answer (keep a backup copy of the configs).

• Normally, a routing table with less than 500K bytes could be accommodated with 2 to 4 MB RAM; Large networks with greater than 500K may need 8 to 16 MB, or 32 to 64 MB if full routes are injected from the Internet.

OSPF Design Issues


Whew!

Ch.6 – OSPFPart 2 of 2: Multi-Area OSPF

CCNP version 3.0

Rick Graziani

Cabrillo College

Education

OSPF- Multi area