ROUTE Notes – Controlling Routes in EIGRP

Corrections welcome.

Study Questions

  • Why would you ever want to summarize routes?

Summarizing routes minimizes the routes advertised to the network.  For example, instead of advertising 192.168.0.0/24, 192.168.1.0/24…192.168.n.0/24, a router can advertise a single route to 192.168.0.0/16.  Keeping routing tables small saves hardware resources, minimizes convergence times, helps avoid route flapping, and makes the routing table easier to read for humans.

  • When will an EIGRP router auto-summarize a route?

If a router has interfaces that that are in different classes of network (Class A, B, C), then that router will auto-summarize those routes up to the classful boundary.  For example, if you have a 10.0.0.1/24 and a 192.168.100.1/30, the router will advertise 10.0.0.0/8 and 192.168.100.0/24.

  • You have two routers advertising the same summarized route.  How do you make one preferred over the other?

Adjust the delay or bandwidth so one is favored.

  • What is suboptimal forwarding in regards to summarization?

A summary route could be advertised from a router that’s not in the optimal path to the destination.  If that route is chosen by a downstream router, traffic is passed to that router instead of a more optimal path through another router.

  • How do you avoid suboptimal forwarding in regards to summarization?

Disable summarization.  Advertising real networks will result in the optimal path being calculated.

  • How do you manually summarize the route 192.168.100.0/22?

R1(config-if)#ip summary-address eigrp 1 192.168.100.0 255.255.252.0

  • When will a summarized route stop being advertised by a router?

When the router no longer has any routes that fall inside the summary route, the summary is removed.  That is, if a router is advertising 192.168.100.0/22, the route will be removed if the router no longer has ANY routes that are in the 192.168.10[0123].0 networks.

  • What’s the biggest route a router can summarize?

The default route.

  • What are two ways to advertise a default route in EIGRP?

Advertise a static default route through redistribute static
Summarize 0/0 out of an interface

  • How do you keep one part of the network from having a route to another part of the network?

This is done with route filtering.

  • When configuring route filtering in EIGRP, what’s the big keyword?

distribute-list

  • What are the three techniques for filtering routes?

ACLs
Prefix lists
Route-maps

  • Do you mean to tell me that route filtering also uses ACLs and route-maps?

Yes.  Welcome to Cisco Systems.

  • How do you use an ACL to filter out the route 192.168.0.0/24?

  • How do you use a prefix list to filter out the route 192.168.0.0/24?

  • How do you use a route-map to filter out the route 192.168.0.0/24?

-OR-

What Command Was That?

What command…

  • …shows you the prefix lists configured?

show ip prefix-list

  • …shows the summary route being advertised for a particular network?

show ip route 192.168.0.0 255.255.0.0 longer-prefixes

  • …shows what networks are being summarizes on a router?

show ip protocols

  • …shows what route a router considers to be a default candidate?

show ip route (look for the *)

  • …shows the default network?

show ip route

Aaron Conaway

I shake my head around sometimes and see what falls out. That's what lands on these pages.

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