EIGRP auto-summary

In EIGRP, the auto-summary feature is enabled by default. This means that EIGRP will automatically summarize networks at their classful boundaries. The auto-summary feature can be useful, but it can also cause problems with discontiguous networks (which are two or more subnetworks of a classful network connected together by different classful networks).

Consider the following example:

eigrp auto summary example

In the example above we have a network of two routers and one host. Both routers are running EIGRP and have established the neighbor relationship. The router R1 has a directly connected subnet 10.5.10.0/24 that it advertises to R2. Because the auto-summary feature is turned on, R1 will summarize the subnet 10.5.10.0/24 and send the classful route 10.0.0.0/8 to R2.

Here is the routing table on R2:

eigrp auto summary

Note that R2 has a route to the classful network 10.0.0.0/8. R2 will therefore send all packets destined for any IP address inside the 10.0.0.0 – 10.255.255.255 range to R1. This can cause problems with discontiguous networks. That is why the auto-summary feature is usually turned off with the no auto-summary command:

no auto summary command

Let’s take a look at the routing table on R2:

show ip route no auto summary

You can see in the picture above that R2 now has the more specific route. Note that the EIGRP neighbor relationship has to be re-established after the no auto-summary command was entered.

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