EIGRP manual summarization

The manual summarization is a process of creating a summary route that will be used to represent multiple routes and can be used to reduce the sizes of routing tables in a network. EIGRP, unlike some other routing protocols such as OSPF, supports the manual summarization on any router within a network.
The manual summarization in EIGRP is configured on the per-interface basis. The syntax of the command is:
(config-if)ip summary-address eigrp ASN SUMMARY_ADDRESS SUBNET_MASK
Consider the following example:

In the example above we have a network of three routers running EIGRP. R1 has two directly connected subnets: 10.5.10.0/24 and 10.20.15.0/24. Assuming that no automatic summarization is enabled, R1 will advertise these two networks to R2. Here is the routing table on R2:


Note that R2 has two routes to reach the 10.5.10.0/24 and 10.20.15.0/24 networks. To reduce the size of the R2’s routing table, we can advertise a summary route that will include both subnets. We will use the following command on the Fa0/1 interface on R1:

R1 should now send only a single route (10.0.0.0) for both subnets. We can verify that by running the show ip route command on R2:

Notice how R2 now has only a single route to reach both subnets.

In the example above, the ip summary command included two subnets directly connected to R1, but also plenty of other addresses that are not in these subnets. The range of the summarized addresses is 10.0.0.0 – 10.255.255.255, so R2 will send all packets destined for these addresses to R1. This could cause some problems if these addresses exist somewhere else in the network.