EIGRP Summarization

Two types of summarization take place in EIGRP—autosummarization and manual summarization. Autosummarization is the default behavior for EIGRP, just as it is for RIP and IGRP. Basically, when the router sends out a routing update, it automatically summarizes the route to its natural major network when the route is advertised across a major network boundary. Figure 6-9 shows an example of autosummarization. In Figure 6-9, Router R1 needs to send an update about the network 132.168.1.0 to R2 across a major network of 192.168.2.0. R1 then autosummarizes the update to its classful network of 132.168.0.0 and sends it to R2. The problem of autosummarization is that the design of the network cannot be discontiguous.

Figure 6-9. Example of Autosummarization

Figure 6-9. Example of Autosummarization

Manual summarization in EIGRP is configurable on a per-interface basis in any router within the network. The command for EIGRP manual summarization is ip summary-address eigrp autonomous-system-number address mask. With EIGRP, summarization can be done on any interface and any router in the network, compared to OSPF, which can summarize only on an area border router (ABR) and an autonomous system border router (ASBR). When manual summarization is configured on the interface, the router will immediately create a route to null 0 with an administrative distance of 5. This is to prevent routing loops of summary address. Finally, when the last specific route of the summary goes away, the summary route is deleted. Example 6-2 shows the configuration for EIGRP manual summarization for the network in Figure 6-10.

Example 6-2 Configuring EIGRP Manual Summarization interface s0

ip address 192.168.11.1 255.255.255.252 ip summary-address eigrp 1 192.168.8.0 255.255.252.0

Example 6-2 demonstrates how R1 in Figure 6-10 is summarizing addresses of 192.168.8.0/24, 192.168.9.0/24, and 192.168.10.0/24 into one update of 192.168.8.0/22. Summarization in EIGRP reduces the size of the routing table and the number of updates. It also limits the query range, which is crucial in terms of making a large EIGRP network more stable and more scalable.

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