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SSA CATC

The Routing Table: ACloser Look

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Agenda

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Common OSPFProblems

Routing TableStructure

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Starting Configuration

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SSA CATC

Sample Routing Table

R2

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SSA CATC

debug ip routing

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SSA CATC

Hierarchical Routing

Routing table is hierarchical structure used to speed up thelookup process

Level 1 and Level 2

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Level 1 Routes

Route with a subnet mask equal to or less than the classful mask of thenetwork address

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Ultimate Routes

route thatincludes either a next-hop IP address and/or an exitinterface

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Parent and Child Routes

no next-hop (L1 Parent)

L2 Child

Parent route is created whenever a route with a mask greaterthan the classfulmask is entered into the routing table

L1 parent route exists only when there is at least one L2 child route!

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Parent and Child Relationships

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Parent and Child Relationships

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Parent and Child: Classless Routing

VLSM

Ultimate Routes

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Common OSPFProblemsRouting Table LookupProcess

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Stepsin Route Lookup Process

Step 1: Router examines level 1 routesfor the best matchwith the destination address

Step 1a: If the best match is a level 1 ultimate route - this route isused to forward the packet

Step 1b: If the best match is a level 1 parent route, proceed to Step 2

Step 2: Router examines child routes (the subnet routes) ofthe parent route for a best match

Step 2a: If there is a match with a level 2 child route, that subnet willbe used to forward the packet

Step 2b: If there is not a match with any of the level 2 child routes,proceed to Step 3

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Stepsin Route Lookup Process

Step 3: Is the router implementing classful or classless routingbehavior?

Step 3a: Classful routing behavior: terminate the lookup process and dropthe packet

Step 3b: Classless routing behavior: continue searching level 1supernet routes in the routing table for a match, including thedefault route

Step 4: Match with a level 1 supernet or default routes, therouter uses that route to forward the packet

Step 5: If there is not a match with any route in the routing table, therouterdrops the packet

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LongestMatch Preferred

Mostspecific

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LongestMatch: Level 1 NetworkRoutes

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SSA CATC

LongestMatch: Level 1 NetworkRoutes

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LongestMatch: Parent and Child

22 bit match

23 bit match

24 bit match

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Common OSPFProblemsRouting Behaviour

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Classful and Classless RoutingBehaviour

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Classful and Classless RoutingBehaviour

Discontiguous Networks:

R2(config-router)# default-information originate

R3(config)# ip route 172.16.0.0 255.255.0.0 s0/0/1

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R2 Routing Table

Destination of IP Packet: 172.16.4.10

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Classful Search Process

Because router R2 is using classful routing behavior (no ip classless)the routerwill notsearch beyond the child routes

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ClasslessSearch Process

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Questions?

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Labs

8.4.1 Investigating the Routing Table LookupProcess

8.4.2 show ip route Challenge Lab