SingleArea-OSPF-Review.ppt

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Single Area OSPF - Review

CIS 185 Advanced Routing

Rick GrazianiCabrillo College

[email protected]

Last Updated: Fall 2009



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Note

My web site is www.cabrillo.edu/~rgraziani.

For access to these PowerPoint presentations and othermaterials, please email me at [email protected].



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For further information

This presentation is an

overview of what iscovered in thecurriculum/book.

For further explanationand details, please readthe chapter/curriculum.

Book:

Routing Protocolsand Concepts

By Rick Graziani and Allan Johnson

ISBN: 1-58713-206-0

ISBN-13: 978-58713-206-3



Introduction to OSPF



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Introduction to OSPF

OSPF is:

Classless

Link-state routing protocol

Uses the concept of areas for scalability

RFC 2328 defines the OSPF metric as an arbitrary value called cost.

Cisco IOS software uses bandwidth to calculate the OSPF cost metric.



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The network Command

The area area-id refers to the OSPF area .

A group of OSPF routers that share link-state information.

All OSPF routers in the same area must have the same link-state

information in their link-state databases.

This is accomplished by routers flooding their individual link

states to all other routers in the area.

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



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1 – Flooding of link-state

information

2 – Building a

Topological

Database

3 – SPF Algorithm

4 – SPF Tree

5 – Routing Table

Link State Concepts



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Before two routers can form an OSPF neighbor adjacency, they

must agree on three values:

Hello interval Dead interval

Network type

Both the interfaces must be part of the same network, including

having the same subnet mask.

IP MTU must match

Neighbors and

Adjacencies



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Hello Intervals

By default, OSPF Hello packets are sent:

10 seconds on multiaccess and point-to-point segments

30 seconds on nonbroadcast multiaccess (NBMA) segments (FrameRelay, X.25, ATM).

In most cases, use multicast address ALLSPFRouters at 224.0.0.5.



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Dead Intervals

Cisco uses a default of four times the Hello interval.

40 seconds - Multiaccess and point-to-point segments.

120 seconds - NBMA networks.

Dead interval expires

OSPF removes that neighbor from its link-state database.

Floods the link-state information about the “down” neighbor out

all OSPF-enabled interfaces.



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Modifying OSPF Intervals

Dead time is counting down from 40 seconds.

Refreshed every 10 seconds when R1 receives a Hello from the neighbor.

R1# show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

10.3.3.3 0 FULL/ - 00:00:35 192.168.10.6 Serial0/0/1

10.2.2.2 0 FULL/ - 00:00:36 192.168.10.2 Serial0/0/0



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Modifying OSPF Intervals

Router(config-if)# ip ospf hello-interval seconds

Router(config-if)# ip ospf dead-interval seconds



Basic OSPF Configuration

Lab Topology

The router ospf command

The network command

OSPF Router ID

Verifying OSPF

Examining the Routing Table



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OSPF Router ID is an IP address used to uniquely identify an OSPF router.

Also used in the DR and BDR process.

1. Use the IP address configured with the OSPF router-id command.

2. Highest IP address of any of its loopback interfaces.

3. Highest act ive IP address of any of its physical interfaces.

OSPF Router ID

Router ID?

Router ID?

Router ID?



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Verifying New Router IDs (Loopbacks)R1# show ip protocols

Routing Protocol is “ospf 1”

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Router ID 10.1.1.1

<output omitted>

R2# show ip protocolsRouting Protocol is “ospf 1”



Router ID 10.2.2.2

<output omitted>

R3# show ip protocols

Routing Protocol is “ospf 1”



Router ID 10.3.3.3<output omitted>



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Verifying OSPF

Neighbor ID: The router ID of the neighboring router.

Pri: The OSPF priority of the interface.

State: The OSPF state of the interface.

Dead Time:

Address: The IP address of the neighbor’s interface

Interface: Local interface

R1# show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

10.3.3.3 1 FULL/ - 00:00:30 192.168.10.6 Serial0/0/1

10.2.2.2 1 FULL/ - 00:00:33 192.168.10.2 Serial0/0/0



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R1# show ip ospf interface serial 0/0/0

Serial0/0/0 is up, line protocol is upInternet Address 192.168.10.1/30, Area 0

Process ID 1, Router ID 10.1.1.1, Network Type POINT_TO_POINT, Cost: 64

Transmit Delay is 1 sec, State POINT_TO_POINT,

Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

<output omitted>

Verifying OSPF



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Verifying OSPF

R1# show ip protocols

Routing Protocol is “ospf 1” Outgoing update filter list for all interfaces is not set


Router ID 10.1.1.1

Number of areas in this router is 1. 1 normal 0 stub 0 nssa

Maximum path: 4Routing for Networks:

172.16.1.16 0.0.0.15 area 0

192.168.10.0 0.0.0.3 area 0

192.168.10.4 0.0.0.3 area 0

Reference bandwidth unit is 100 mbps

Routing Information Sources:

Gateway Distance Last Update

10.2.2.2 110 11:29:29

10.3.3.3 110 11:29:29

Distance: (default is 110)

OSPF Process ID

OSPF Router ID

Networks OSPF is

advertising that are

originating from this router

OSPF Neighbors

Administrative Distance



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Verifying OSPFR1# show ip ospf

<some output omitted>

Routing Process “ospf 1” with ID 10.1.1.1

Start time: 00:00:19.540, Time elapsed: 11:31:15.776

Supports only single TOS(TOS0) routes

Supports opaque LSA

Supports Link-local Signaling (LLS)

Supports area transit capabilityRouter is not originating router-LSAs with maximum metric

Initial SPF schedule delay 5000 msecs

Minimum hold time between two consecutive SPFs 10000 msecs

Maximum wait time between two consecutive SPFs 10000 msecs

Incremental-SPF disabled

Minimum LSA interval 5 secs

Minimum LSA arrival 1000 msecs

Area BACKBONE(0)

Number of interfaces in this area is 3

Area has no authentication

SPF algorithm last executed 11:30:31.628 agoSPF algorithm executed 5 times



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Verifying OSPF

Any time a router receives new information about the topology (addition,deletion, or modification of a link), the router must:

Rerun the SPF algorithm

Create a new SPF tree

Update the routing table

The SPF algorithm is CPU intensive, and the time it takes for calculationdepends on the size of the area.

R1# show ip ospf

<some output omitted>Initial SPF schedule delay 5000 msecs

Minimum hold time between two consecutive SPFs 10000 msecs

Maximum wait time between two consecutive SPFs 10000 msecs





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Verifying OSPF


Serial0/0/0 is up, line protocol is up

Internet Address 192.168.10.1/30, Area 0




<output omitted>



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Examining the Routing Table

Unlike RIPv2 and EIGRP, OSPF does not automatically summarize at major

network boundaries.

R1# show ip route

Codes: <some code output omitted>

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

192.168.10.0/30 is subnetted, 3 subnets

C 192.168.10.0 is directly connected, Serial0/0/0


O 192.168.10.8 [110/128] via 192.168.10.2, 14:27:57, Serial0/0/0172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

O 172.16.1.32/29 [110/65] via 192.168.10.6, 14:27:57, Serial0/0/1

C 172.16.1.16/28 is directly connected, FastEthernet0/0

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

O 10.10.10.0/24 [110/65] via 192.168.10.2, 14:27:57, Serial0/0/0

C 10.1.1.1/32 is directly connected, Loopback0



The OSPF Metric

OSPF Metric

Modifying the Cost of the Link



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OSPF Metric

The OSPF metric is called cost. The following passage is from RFC 2328:

A cost is associated with the output side of each router interface. This

cost is configurable by the system administrator. The lower the cost, the

more likely the interface is to be used to forward data traffic.

RFC 2328 does not specify which values should be used to determine the

cost.



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OSPF Metric

Cisco IOS software uses the cumulative bandwidths of the outgoing

interfaces from the router to the destination network as the cost value.

108 is known as the reference bandwidth

Cisco IOS Cost for OSPF = 108 /bandwidth in bps



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Reference Bandwidth

When this command is necessary, it is recommended that it is used on all

routers so the OSPF routing metric remains consistent .

R1(config-router)# auto-cost reference-bandwidth ?

1-4294967 The reference bandwidth in terms of Mbits per second.

R1(config-router)# auto-cost reference-bandwidth 10000

To increase it to 10GigE (10 Gbps Ethernet) speeds, you need to change the reference

bandwidth to 10,000.



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T1 cost 64 + Fast Ethernet cost 1 = 65

The “Cost = 64” refers to the default cost of the serial interface,

108/1,544,000 bps = 64, and not to the actual 64-Kbps “speed” of the link.

R1# show ip route

O 10.10.10.0/24 [110/65] via 192.168.10.2, 14:27:57, Serial0/0/0

OSPF

Accumulates Cost

Serial interfaces bandwidth value

defaults to T1 or 1544 Kbps.



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Default Bandwidth on Serial Interfaces

On Cisco routers, the bandwidth value on many serial interfaces

defaults to T1 (1.544 Mbps).

R1# show interface serial 0/0/0

Serial0/0/0 is up, line protocol is upHardware is GT96K Serial

Description: Link to R2

Internet address is 192.168.10.1/30

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255



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Modifying the Cost of the Link

The bandwidth command is used to modify the bandwidth value

used by the Cisco IOS software in calculating the OSPF cost metric.

Same as with EIGRP

Router(config-if)# bandwidth bandwidth-kbps

R1(config)# inter serial 0/0/0

R1(config-if)# bandwidth 64

R1(config-if)# inter serial 0/0/1


R1(config-if)# endR1# show ip ospf interface serial 0/0/0

Serial0/0 is up, line protocol is up




<output omitted>

100,000,000/64,000 = 1562



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The ip ospf cost Command

An alternative method to using the bandwidth command is to use

the ip ospf cost command, which allows you to directly specify

the cost of an interface.

This will not change the output of the show ip ospf interface

command,

R1(config)# interface serial 0/0/0

R1(config-if)# ip ospf cost 1562

R1(config)# inter serial 0/0/0


R1(config-if)# end


Serial0/0 is up, line protocol is up



<output omitted>

100,000,000/64,000 = 1562



OSPF and Multiaccess

Networks Challenges in Multiaccess Networks

DR/BDR Election Process

OSPF Interface Priority



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Solution: Designated Router

OSPF elects a Designated Router (DR) to be the collection and distribution

point for LSAs sent and received.

A Backup Designated Router (BDR) is also elected in case the DR fails.

All other routers become DROthers.



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DROthers only form full adjacencies with the DR and BDR in the network.

send their LSAs to the DR and BDR

using the multicast address 224.0.0.6 ( ALLDRouters, all DR routers).

R1 sends LSAs to the DR. The BDR listens, too.

The DR is responsible for forwarding the LSAs from R1 to all other routers.

DR uses the multicast address 224.0.0.5 ( AllSPFRouters, all OSPF routers).

Only one router doing all the flooding.

DROther

DROther DROther DROther

DROther

DROther

224.0.0.6

224.0.0.5



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DR/BDR Election

The following criteria are applied:

1. DR: Router with the highest OSPF interface priority.

2. BDR: Router with the second highest OSPF interface priority.

3. If OSPF interface priorities are equal, the highest router ID is used to

break the tie.

Default OSPF interface priority is 1.

Current configuration, the OSPF router ID is used to elect the DR and BDR.

DR

BDR

DROther

Verif ing Ro ter States



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RouterA# show ip ospf interface fastethernet 0/0

FastEthernet0/0 is up, line protocol is up


Process ID 1, Router ID 192.168.31.11, Network Type BROADCAST, Cost: 1Transmit Delay is 1 sec, State DROTHER , Priority 1

Designated Router (ID) 192.168.31.33, Interface address 192.168.1.3

Backup Designated router (ID) 192.168.31.22, Interface address192.168.1.2


<output omitted>

Verifying Router States



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Timing of DR/BDR Election

If I booted first and startedthe election before the

others were ready, I would

be the DR!



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When the DR is elected, it remains the DR until one of the following

conditions occurs:

The DR fails. The OSPF process on the DR fails.

The multiaccess interface on the DR fails.

If the DR fails, the BDR assumes the role of DR, and an election is held to

choose a new BDR.

DR failed! I am now the

DR! Elections will now

happened for BDR

I am now

the BDR!

DR

BDR



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If a new router enters the network after the DR and BDR have been

elected, it will not become the DR or the BDR even if it has a higher

OSPF interface priority or router ID than the current DR or BDR.

DR

BDR

Timing of

DR/BDR

Election

DROther

I am a new router with the highest

Router ID. I cannot force a new

DR or BDR election, so I am a

DROther .

I’m back but I don’t



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A previous DR does not regain DR status if it returns to the network.

DR

BDR

Timing of

DR/BDR

Election

DROther

get to become DR

again. I am now just a

DROther .

DROther



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If the BDR fails, an election is held among the DROthers to see which router

will be the new BDR.

DR

BDR

Timing of

DR/BDR

Election

BDR

Amongst the

DROthers I have the

highest Router ID, so

I am the new BDR!

DROther

DR



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RouterB fails.

Because RouterD is the current BDR, it is promoted to DR.

RouterC becomes the BDR.

DR

BDR

Timing of

DR/BDR

Election

BDR

I am now the new DR!

DROther

I am now the new

BDR!



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We can change the OSPF interface priority to better control our DR/BDRelections.

How can we make sure

RouterB is the DR andRouterA is the BDR,

regarless of RouterID

values?

Want to be DR

Want to be

BDR

Highest Router ID

To simplify our discussion, we

removed RouterD from the topology.



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Control the election of these routers with the ip ospf priority interface

command.

Priority (Highest priority wins):

0 = Cannot become DR or BDR 1 = Default

Therefore, the router ID determines the DR and BDR.

Priorities are an interface-specific value, they provide better control of the

OSPF multiaccess networks.

They also allow a router to be the DR in one network and a DROther inanother.

Router(config-if)# ip ospf priority {0 - 255}



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The OSPF interface priority can be viewed using the show ip ospf

interface command.

RouterA# show ip ospf interface fastethernet 0/0

FastEthernet0/0 is up, line protocol is upInternet Address 192.168.1.1/24, Area 0

Process ID 1, Router ID 192.168.31.11, Network Type BROADCAST, Cost: 1

Transmit Delay is 1 sec, State DROTHER, 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


<output omitted>



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After doing a shutdown and a no shutdown on the Fast Ethernet

0/0 interfaces of all three routers, we see the result of the change of

OSPF interface priorities.

RouterA(config)# interface fastethernet 0/0

RouterA(config-if)# ip ospf priority 200

RouterB(config)# interface fastethernet 0/0

RouterB(config-if)# ip ospf priority 100

Pri = 200

Pri = 100

Highest priority wins



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Clarifications regarding DR/BDR

Hello packets are still exchanged between all routers on a multi-access segment (DR, BDR, DROthers,….) to maintain neighbor

adjacencies.

OSPF LSA packets (coming) are packets which are sent from the

BDR/DROthers to the DR, and then from the DR to the

BDR/DROthers. (The reason for a DR/BDR.) Normal routing of IP packets still takes the lowest cost route, which

might be between two DROthers.



More OSPF Configuration

Redistributing an OSPF Default Route

Fine-tuning OSPF



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Redistributing

an OSPF

Default Route

If the default-information originate command is not used, the

default “quad zero” route will not be propagated to other routers in the

OSPF area.

R1(config)# interface loopback 1

R1(config-if)# ip add 172.30.1.1 255.255.255.252

R1(config-if)# exit

R1(config)# ip route 0.0.0.0 0.0.0.0 loopback 1

R1(config)# router ospf 1

R1(config-router)# default-information originate

The static default route is using the

loopback as an exit interface

because the ISP router in this

topology does not physically exist.



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R3’s Routing Table

R3# show ip route

Gateway of last resort is 192.168.10.5 to network 0.0.0.0

192.168.10.0/30 is subnetted, 3 subnets

O 192.168.10.0 [110/1952] via 192.168.10.5, 00:00:38, S0/0/0




C 172.16.1.32/29 is directly connected, FastEthernet0/0

O 172.16.1.16/28 [110/391] via 192.168.10.5, 00:00:38, S0/0/0


C 10.3.3.3/32 is directly connected, Loopback0O 10.10.10.0/24 [110/782] via 192.168.10.9, 00:00:38, S0/0/1

O*E2 0.0.0.0/0 [110/1] via 192.168.10.5, 00:00:27, Serial0/0/0



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External Type 2 Route

E2 denotes that this route is an OSPF External Type 2 route.

OSPF external routes fall in one of two categories:

External Type 1 (E1) External Type 2 (E2)

OSPF accumulates cost for an E1 route as the route is being propagated

throughout the OSPF area.

This process is identical to cost calculations for normal OSPF internal routes.

E2 route is always the external cost, irrespective of the interior cost to reach thatroute.

In this topology, because the default route has an external cost of 1 on the

R1 router, R2 and R3 also show a cost of 1 for the default E2 route.

E2 routes at a cost of 1 are the default OSPF configuration.

More later

R3# show ip route

O*E2 0.0.0.0/0 [110/1] via 192.168.10.5, 00:00:27, Serial0/0/0



Steps to OSPF Operation with States

1. Establishing router adjacencies (Routers are adjacent)

Down State – No Hello receivedInit State – Hello received, but not with this router’s Router ID

“Hi, my name is Carlos.” “Hi, my name is Maria.”

Two-way State – Hello received, and with this router’s Router ID

“Hi, Maria, my name is Carlos.” “Hi, Carlos, my name is Maria.”

2. Electing DR and BDR – Multi-access (broadcast) segments onlyExStart State with DR and BDR

Two-way State with all other routers

3. Discovering Routes

ExStart State

Exchange State

Loading State

Full State (Routers are “fully adjacent”)

4. Calculating the Routing Table

5. Maintaining the LSDB and Routing Table



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Hello 10.6.0.1

Hello 10.5.0.1

Hello 10.6.0.1 10.5.0.1

Hello 10.5.0.1 10.6.0.1

DownInit DownInit2-way 2-way

Down State - Init State – Two Way State

Down State - OSPF routers send Hello packets at regular intervals (10 sec.) to establish

neighbors.

When a router (sends or) receives its first Hello packet, it enters the init state. When the router sends a Hello packet to the neighbor with its RouterID and the neighbor

sends a Hello packet packet back with that Router ID, the router’s interface will transition

to the two-way state.

Now, the router is ready to take the relationship to the next level.

1. Establishing Adjacencies



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Steps to OSPF Operation with States (cont)

Explanations in Notes Section



Couple of notes on link state flooding…

OSPF is a link state routing protocol and does not send periodic updates like RIP.

OSPF only floods link state state advertisements when there is a changein topology (this includes when a routers are first booted).

OSPF uses hop-by-hop flooding of LSAs; an LSA received on oneinterface are flooded out other OSPF enabled interfaces.

If a link state entry in the LSDB (Link State DataBase) reaches an age of 60

minutes (MaxAge) without being updated, it is removed and SPF isrecalculated.

Every 30 minutes (LSRefreshTime), OSPF routers flood only their linkstates to all other routers (in the area).

This is known as a “ paranoid update”

These do not trigger SPF recalculations.

Special note: When a link goes down and a router wants to send a LSA totell other routers to remove this link state, it sends this link state with avalue of 60 minutes (MAXAGE).



Single Area OSPF - Review

CIS 185 Advanced Routing

Rick Graziani

Cabrillo College

[email protected]

Last Updated: Fall 2009

Documents

SingleArea-OSPF-Review.ppt