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.