Route summarization is a method of representing multiple networks with a single summary address. It is often use in large networks with many subnets because it reduces the number of routes that a router must maintain and minimizes the traffic used for routing updates. Two methods for summarizing routes exist: automatic summarization and manual summarization.
EIGRP automatic summarization
By default, EIGRP has the 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, because of the auto-summary feature, 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:
EIGRP manual summarization
One of the advantages of EIGRP over some other routing protocols (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 the concept of manual summarization:
Router R1 and R2 are running EIGRP. Router R1 (on the left) 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:
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: