In Figure 1, notice the EIGRP routing domain has three classful networks:
- 172.16.0.0/16 class B network consisting of 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/30 subnets
- 192.168.10.0/24 class C network consisting of the 192.168.10.4/30 and 192.168.10.8/30 subnets
- 192.168.1.0/24 class C network, which is not subnetted
The output from R1’s show ip protocols command in Figure 2 shows that autosummarization is now enabled. The output also indicates the networks that are summarized and on which interfaces. Notice that R1 summarizes two networks in its EIGRP routing updates:
- 192.168.10.0/24 sent out the GigabitEthernet 0/0 and Serial 0/0/0 interfaces
- 172.16.0.0/16 sent out the Serial 0/0/1 interface
R1 has the subnets 192.168.10.4/30 and 192.168.10.8/30 in its IPv4 routing table.
As indicated in Figure 3, R1 summarizes the 192.168.10.4/30 and 192.168.10.8/30 subnet. It forwards the summarized address of 192.168.10.0/24 to its neighbors on its Serial 0/0/0 and GigabitEthernet 0/0 interfaces. Because R1 does not have any EIGRP neighbors on its GigabitEthernet 0/0 interface, the summarized routing update is only received by R2.
As indicated in Figure 4, R1 also has the 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/30 subnets in its IPv4 routing table. R3 selects R1 as the successor to 172.16.0.0/16 because it has a lower feasible distance. The R3 S0/0/0 interface connecting to R1 uses a default bandwidth of 1,544 kb/s. The R3 link to R2 has a higher feasible distance because the R3 S0/0/1 interface has been configured with a lower bandwidth of 1,024 kb/s.
Notice that the 172.16.0.0/16 summarized update is not sent out R1’s GigabitEthernet 0/0 and Serial 0/0/0 interfaces. This is because these two interfaces are members of the same 172.16.0.0/16 class B network. The 172.16.1.0/24 non-summarized routing update is sent by R1 to R2. Summarized updates are only sent out interfaces on different major classful networks.