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 172.16.0.0/16. The successor or next-hop router is R1 via 192.168.10.5.
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 192.168.10.0/24 and 172.16.0.0/16.
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:
- There is at least one subnet that was learned via EIGRP.
- There are two or more network EIGRP router configuration mode commands.
- Automatic summarization is enabled.
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.