EIGRP for IPv4 autosummarization is disabled by default beginning with Cisco IOS Release 15.0(1)M and 12.2(33). Prior to this, autosummarization was enabled by default. This meant that EIGRP performed auto-summarization each time the EIGRP topology crossed a border between two different major class networks.
In Figure 1, the output from the show ip protocols command on R1 indicates that EIGRP autosummarization is disabled. This router is running IOS 15.2; therefore, EIGRP autosummarization is disabled by default. Figure 2 shows the current routing table for R3. Notice that the IPv4 routing table for R3 contains all of the networks and subnets within the EIGRP routing domain.
To enable autosummarization for EIGRP, use the auto-summary command in router configuration mode, as shown in Figure 3:
R1(config)# router eigrp as-number
R1(config-router)# auto-summary
The no form of this command is used to disable autosummarization .
Use the Syntax Checker in Figure 4 to enable autosummarization for R3.