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By default, EIGRP has auto summary feature enabled. Because of this, routes are summarized to classful address at network boundaries in the routing updates.
To better understand the concept of auto-summarization, consider the following example.
Router R1 and R2 are running EIGRP. Router R1 has the locally connected subnet 10.0.1.0/24 that is advertised to the router R2. Because of the auto summary feature, the router R1 summarizes the network 10.0.1.0/24 before sending the route to R2. With the auto summary feature turned on, R1 sends the classful route 10.0.0.0/8 to R2 instead of the more specific 10.0.1.0/24 route.
On R1, we have configured the following network statement:
But R2 receives the route to the classful network 10.0.0.0/8:
The auto summary feature can cause problems with discontiguous networks. This is why this feature is usually turned off. This is done by using the no auto-summary command:
Now R2 has the classless route to reach the subnet 10.0.1.0/24:
NOTE – after typing the no auto-summary command, a neighbor relationship has to be re-established.
One of the advantages of EIGRP over other routing protocol (like OSPF) is that manual summarization can be done on any router within a network. A single route can be used to represent multiple routes, which reduces the size of routing tables in a network.
Manual summarization is configured on a per-interface basis. The syntax of the command is:
(config-if) ip summary-address eigrp ASN SUMMARY_ADDRESS SUBNET_MASK
An example will help you to understand this.
Router R1 and R2 are running EIGRP. Router R1 has two directly connected subnets: 10.0.0.0/24 and 10.0.1.0/24. EIGRP advertises these subnets as two separate routes. R2 now has two routes for two subnets, which can be confirmed by using the show ip route command on R2:
We could configure R1’s to advertise only one summary route for both subnets, which helps reduce R2’s routing table. To do this, the following command could be used:
Now, R1 is sending only one route to reach both subnets to R2. We can verify that by using the show ip route command on R2:
Now R2 has only one route to reach both subnets on R1.
NOTE – in the example above, the ip summary command included two subnets on R1, but also some other addresses that are not in these subnets. The range of the summarized addresses is 10.0.0.0 – 10.0.255.255, so R2 thinks that R1 has the routes for all addresses inside that range. That could cause some problems if these addresses exist somewhere else in the network.