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Route summarization is the most common tuning method of EIGRP. It allows a router to group multiple networks and advertises them as one large group using a single, summarized route. The capability to summarize routes is very important due to the rapid growth of networks.

Route summarization reduces the number of routes in a larger network with many subnets and the traffic used for routing updates. We can summarize the routes by automatic summarization or manual summarization. The auto summarization feature is enabled by default on EIGRP. Using default auto summarization, routes are summarized to classful addresses at network boundaries in the routing updates.

EIGRP will check for any subnets that fall within the range of your network command, and if found, it will advertise the classful A, B, or C network to its neighbors. The default auto-summary only summarizes routes from subnets on its interfaces, not the routes learned from other routers.

Figure 1 illustrates the process of route summarization. Routers R1, R2, and R3 are configured using EIGRP for IPv4 with automatic summarization. R1 has four subnets in its routing table: 10.10.10.0/30, 10.10.11.0/24, 10.10.12.0/24 and 10.10.13.0/24. These subnets are all considered part of a larger class A network, 10.0.0.0/8. Because automatic summarization is enabled on router R1, it summarizes all subnets as a single network of 10.0.0.0/8. Summarization reduces the number of routing updates sent and the number of entries in R2’s IPv4 routing table. Traffic destined for the three subnets 10.10.11.0/24, 10.10.12.0/24, and 10.10.12.0/24 travels across one common path. R2 never maintains routes to individual subnets.

Configuring EIGRP Automatic Summarization

Before Cisco IOS Release 15.0(1)M and 12.2(33), automatic summarization was enabled by default. However, after the release of these IOS versions, automatic summarization is disabled by default. Therefore, we should configure automatic summarization in the latest IOS release.

Figure 2 illustrates the output of the “show ip protocols” command on R1. The figure illustrates that the automatic summarization is disabled on R1. This is because the router running IOS Version 15.1(4)M4 therefore, automatic summarization is disabled

Figure 3 shows the current routing table for R2, the edge router for R1 and R2. Notice that the EIGRP IPv4 routing table for R2 contains all of the networks and subnets within the EIGRP routing domain. We can enable automatic summarization for EIGRP using the “auto-summary” command in router configuration mode.

R1(config)# router eigrp 100

R1(config-router)# auto-summary

Use the same commands for automatic summarization on all routers. “100” is the autonomous system number of the EIGRP domain. We can negate automatic summarization using the router configuration mode’s “no auto-summary” command.

Verifying Auto-Summary

We can verify the auto-summary configuration by using the command “show ip protocols.” Figure 4 illustrates the output of this command, which enables the auto-summary on R1.

We can examine and verify the summarization from R2’s routing table. Using the “show ip route eigrp” command, we can display R2’s routing table for the EIGRP domain. Figure 5 illustrates R2’s routing table after auto-summary configuration.

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.

Verifying Auto-Summary from Topology Table

In Figure 6, routers R1 will send R2 a summarized EIGRP routing update of 172.16.0.0/16. Routing tables for R1, R2, and R3 contain the 172.16.0.0/16 network subnets, so all routers send the summary advertisement across a different major network.

We can verify the auto-summary configuration on the R2 and R3 topology table. This will verify that R2 and R3 have received the 172.16.0.0/16 summary route. We can use the all-links parameter with the “show ip eigrp topology” to show all received updates.

Null Interface

Figure 5 shows the routing table for R2. The highlighted entries use an exit interface of Null0. EIGRP automatically adds a summary route to Null0 for two classful networks: 10.0.0.0/8 and 172.16.0.0/16. It is an IOS virtual interface route to nowhere, also known as “the bit bucket.” Packets that match a route with a Null0 exit interface are discarded.

The Null0 summary route prevents routing loops for destinations integrated into the summary but not in the routing table. EIGRP for IPv4 automatically includes a Null0 summary route in the following conditions:

• In the case of one subnet that was learned via EIGRP.
• In the case of two or more network EIGRP router configuration mode commands.
• Automatic summarization is enabled.

Notice – Automatic summarization is unavailable for IPv6 because classful addressing does not exist in IPv6.