EIGRP Stub Routing

Problem

You want to stabilize your network by sending smaller routing tables out to stub branches and reducing the scope of EIGRP queries.

Solution

To enable stub routing, use the eigrp stub configuration command:

Router1#configure terminal 
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#router eigrp 55
Router1(config-router)#eigrp stub 
Router1(config-router)#exit
Router1(config)#end
Router1#

Discussion

This feature became available starting in IOS 12.0(15)S. It is most commonly used in hub-and-spoke network designs where a remote router connects to the rest of the network through only one or two central routers, and where the remote router is the only connection for a small number of LAN segments. In general, you would configure the central routers in this case to send only a default route, as discussed in Recipe 7.9.

In situations when a route suddenly goes away, by default, every router will ask all of its neighbors if they have a path to that remote network. However, it is never going to be fruitful to ask these remote branch routers if they can reach the missing network. If these stub routers are reachable at all, they will already be exchanging information about the few networks that they can access. No trick of topology will allow them to find the missing route if the central routers don't have it. So the EIGRP stub feature disables these queries. This should help to improve overall network stability. And, in particular, this feature could significantly improve the stability of hub-and-spoke WANs:

Router2#show ip eigrp neighbors detail
IP-EIGRP neighbors for process 55
H Address Interface Hold Uptime SRTT RTO Q Seq Type
 (sec) (ms) Cnt Num
1 172.25.2.2 Se0/0.2 6 00:15:57 787 4722 0 148 
 Version 12.2/1.2, Retrans: 0, Retries: 0
 Stub Peer Advertising ( CONNECTED SUMMARY ) Routes
2 172.25.1.7 Fa0/0.1 70 1w0d 12 200 0 405 
 Version 12.0/1.0, Retrans: 1, Retries: 0
0 172.22.1.4 Fa0/1 12 1w0d 1 200 0 258 
 Version 12.2/1.2, Retrans: 2, Retries: 0
Router2#

The eigrp stub command can take six different keywords:

Receive-only

The router becomes a receive-only neighbor. This router will not share its routing information with its neighbors.

Connected

This router will only advertise connected networks. Note that you must configure the appropriate network statements for these connected networks, or alternatively use the redistribute connected command.

Static

The router will advertise static routes. Note that with this option, you must also configure the redistribute static command

Summary

The router will advertise summary routes. This function is enabled by default. Please refer to Recipe 7.9 for details on route summarization.

Leak-map

The router will advertise summary routes, as well as certain specific route prefixes. Please refer to Recipe 7.9 for details on leak maps.

Redistributed

The router will advertise external routes that have been redistributed from other protocols. By default, the stub feature will not advertise external routes. Please refer to Recipe 7.3 for details on route redistribution.