EIGRP automatically summarizes routes at the classful boundary. This is the boundary where the network address ends, as defined by class-based addressing. This means that even though RTC is connected only to the subnet 2.1.1.0, it will advertise that it is connected to the entire Class A network, 2.0.0.0. In most cases auto summarization is beneficial because it keeps routing tables as compact as possible.

However, automatic summarization may not be the preferred option in certain instances. For example, if there are discontiguous subnetworks auto-summarization must be disabled for routing to work properly. To turn off auto-summarization, use the following command:

router(config-router)#no auto-summary

With EIGRP, a summary address can be manually configured by configuring a prefix network. Manual summary routes are configured on a per-interface basis, so the interface that will propagate the route summary must be selected first. Then the summary address can be defined with the ip summary-address eigrp command:

router(config-if)#ip summary-address eigrp autonomous-system-number ip-address mask administrative-distance

EIGRP summary routes have an administrative distance of 5 by default. Optionally, they can be configured for a value between 1 and 255.

In Figure , RTC can be configured using the commands shown:

RTC(config)#router eigrp 2446
RTC(config-router)#no auto-summary
RTC(config-router)#exit
RTC(config)#interface serial 0/0
RTC(config-if)#ip summary-address eigrp 2446 2.1.0.0 255.255.0.0

Therefore, RTC will add a route to its table as follows:

D 2.1.0.0/16 is a summary, 00:00:22, Null0

Notice that the summary route is sourced from Null0 and not from an actual interface. This is because this route is used for advertisement purposes and does not represent a path that RTC can take to reach that network. On RTC, this route has an administrative distance of 5.

RTD is not aware of the summarization but accepts the route. The route is assigned the administrative distance of a normal EIGRP route, which is 90 by default.

In the configuration for RTC, auto-summarization is turned off with the no auto-summary command. If auto-summarization was not turned off, RTD would receive two routes, the manual summary address, which is 2.1.0.0 /16, and the automatic, classful summary address, which is 2.0.0.0 /8.

In most cases when manually summarizing, the no auto-summary command should be issued.