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2004 Cisco Systems, Inc. All rights reserved. ICND v2.23-1

Determining IP Routes

Enabling OSPF

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Outline

Overview

OSPF Features

OSPF and Distance Vector Routing ProtocolComparison

Shortest Path Algorithm

Single Area OSPF Configuration

Loopback Interfaces

OSPF Configuration Verification OSPF Configuration Troubleshooting

Summary

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Introducing OSPF

Open standard

Shortest path first (SPF) algorithm

Link-state routing protocol (vs. distance vector)

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OSPF as a Link-State Protocol

OSPF propagates link-state advertisements ratherthan routing table updates.

Link = router interface

State = description of an interface and itsrelationship to neighboring routers

LSAs are flooded to all OSPF routers in the area.

The OSPF link-state database is pieced together

from the LSAs generated by the OSPF routers. OSPF uses the SPF algorithm to calculate the

shortest path to a destination.

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OSPF Hierarchical Routing

Consists of areas and autonomous systems

Minimizes routing update traffic

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Shortest Path First Algorithm

Places each router at the root of a tree and calculates theshortest path to each destination based on the cumulative cost

Cost = 108/bandwidth (bps)

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7/14 2004 Cisco Systems, Inc. All rights reserved. ICND v2.23-7

Configuring Single-Area OSPF

Router(config-router)#network wildcard-maskarea area-id

Assigns networks to a specific OSPF area

Router(config)#router ospfprocess-id Defines OSPF as the IP routing protocol

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OSPF Configuration Example

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Configuring Loopback Interfaces

Router ID

Number by which the router is known to OSPF Default: The highest IP address on an active interface at the moment

of OSPF process startup

Can be overridden by a loopback interface: Highest IP address of anyactive loopback interface

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Router#show ip ospf interface

Verifying the OSPF Configuration

Displays area ID and adjacency information

Router#show ip protocols

Verifies that OSPF is configured

Router#show ip route

Displays all the routes learned by the router

Router#show ip ospf neighbor

Displays OSPF neighbor information on a per-interface basis

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Router#debug ip ospf events

OSPF:hello with invalid timers on interface Ethernet0

hello interval received 10 configured 10

net mask received 255.255.255.0 configured 255.255.255.0dead interval received 40 configured 30

Router#debug ip ospf packet

OSPF: rcv. v:2 t:1 l:48 rid:200.0.0.117

aid:0.0.0.0 chk:6AB2 aut:0 auk:

Router#debug ip ospf packet

OSPF: rcv. v:2 t:1 l:48 rid:200.0.0.116

aid:0.0.0.0 chk:0 aut:2 keyid:1 seq:0x0

OSPF debug Commands

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Summary

OSPF is an interior gateway protocol similar toIGRP, but based on link states rather than distancevectors.

OSPF advertises information about each of its links

rather than sending routing table updates like adistance vector protocol does.

Hierarchical routing enables separation of a largeinternetwork into smaller internetworks, calledareas.

The SPF algorithm places each router at the root ofa tree and calculates the shortest path to eachdestination based on the cumulative cost requiredto reach that destination.

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Summary (Cont.)

The router ospfcommand starts an OSPF routingprocess. The network command is used to associateaddresses to an OSPF area.

The interface loopback command is used to modify

the OSPF router ID to a loopback address. Any one of a number ofshow commands can be

used to display information about an OSPFconfiguration.

The debug ip ospf events privileged EXEC commandcan be used to display information on OSPF-relatedevents, such as adjacencies, flooding information,designated router selection, and SPF calculation.

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