Examine the routing table to verify that the summarized route was received.

Figure 1 shows R3’s routing table prior to automatic summarization, and then with automatic summarization enabled using the auto-summary command. Notice that with automatic summarization enabled, R3’s routing table now only contains the single class B network address The successor or next-hop router is R1 via

Note: Automatic summarization is only an option with EIGRP for IPv4. Classful addressing does not exist in IPv6; therefore, there is no need for automatic summarization with EIGRP for IPv6.

When enabling automatic summarization , it is also necessary to understand the Null interface. Figure 2 shows the routing table for R1. Notice the two entries highlighted are using an exit interface of Null0. EIGRP has automatically included a summary route to Null0 for two classful networks and

The Null0 interface is a virtual IOS interface that is a route to nowhere, commonly known as "the bit bucket." Packets that match a route with a Null0 exit interface are discarded.

EIGRP for IPv4 automatically includes a Null0 summary route whenever the following conditions exist:

The purpose of the Null0 summary route is to prevent routing loops for destinations that are included in the summary, but do not actually exist in the routing table.