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?

access-list 1 deny 192.168.0.0 0.0.0.255
access-list 1 permit any
router eigrp 1
 distribute-list 1 out

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

ip prefix-list PL1 deny 192.168.0.0/24
ip preffix-list PL1 permit 0.0.0.0/0 le 32
router eigrp 1
 distribute-list prefix PL1 out

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

access-list 1 permit 192.168.0.0 0.0.0.255
route-map RM1 deny 10
 match ip address 1
route-map RM1 permit 999
router eigrp 1
 distribute-list route-map RM1 out

-OR-

ip prefix-list PL1 permit 192.168.0.0/24
route-map RM1 deny 10
 match ip address prefix-list PL1
route-map RM1 permit 999
router eigrp 1
 distribute-list route-map RM1 out

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