EIGRP for IPv4 automatic summarization is disabled by default beginning with Cisco IOS Release 15.0(1)M and 12.2(33). Prior to this, automatic summarization was enabled by default. This meant that EIGRP performed automatic 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 automatic summarization is disabled. This router is running IOS 15.2; therefore, EIGRP automatic summarization 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 automatic summarization for EIGRP, use the auto-summary command in router configuration mode, as shown in Figure 3:
R1(config)# router eigrp as-number
The no form of this command is used to disable automatic summarization .
Use the Syntax Checker in Figure 4 to enable automatic summarization for R3.