EIGRP automatic & manual summarization

EIGRP automatic & manual summarization

Route Summary is a method of displaying multiple networks with a single summary URL. It is often used in large networks with many subnetworks, as it reduces the number of routes a router must maintain and minimizes the traffic used to route updates. There are two methods for route summary: automatic summary and manual summary.

EIGRP auto summary

In EIGRP, the automatic summary function is enabled by default. For this reason, routes are grouped into routing updates in class-related addresses at network boundaries.

Consider the following example to better understand the concept of auto summary.

Routers R1 and R2 perform EIGRP. The router R1 has the locally connected subnet 10.0.1.0/24, which is communicated with the router R2. Due to the automatic resume function, the router R1 completes the network 10.0.1.0/24 before the box is sent to R2. If the autoscroll function is enabled, R1 sends the classified route 10.0.0.0/8 to R2 instead of the more specific 10.0.1.0/24 route.

At R1, we have configured the following network declaration:

Due to the auto summary function, R2 gets the route to the 10.0.0.0 / 8 network:

The auto-summary feature can cause problems with non-neighboring networks. Because of this, this feature is usually disabled. This is done with the command no automatic summary:

Now, R2 has the classless way to reach subnet 10.0.1.0/24:

EIGRP manual summary

One of the advantages of EIGRP over some other routing protocols (like OSPF) is that manual resync can be done on any router within a network. A single route can be used to represent multiple routes, reducing the size of routing tables in a network.

The manual summary is configured for each interface. The syntax command is:

(config-if) IP Summary Address Eigrp ASN SUMMARY_ADDRESS SUBNET_MASK

An example helps you understand the concept of manual summary:

Routers R1 and R2 perform EIGRP. The router R1 (left) has two online subnet networks: 10.0.0.0/24 and 10.0.1.0/24. EIGRP announces these subnetworks as two separate routes. R2 now has two routes for two subnetworks that can be confirmed by the show ip-rut command of R2:

We could configure R1 to only announce a summary route for both subnetworks, reducing R2's routing table. The following command can be used for this:

EIGRP Manual Summary

Now R1 only sends one route to reach both subnet to R2. We can verify this with the show ip routing command on R2:

Displays the summary of the IP box manually

Now R2 only has a route to reach both subnet of R1.