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

    Determining IP Routes

    Enabling OSPF

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    2004 Cisco Systems, Inc. All rights reserved. ICND v2.23-2

    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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    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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