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. If you have any questions, the best way to contact me is through Twitter at @aconaway.

More Posts

Follow Me:
Twitter

Leave a Reply

Your email address will not be published. Required fields are marked *