Configuring Eigrp Summarization

EIGRP allows you to manually summarize multiple contiguous routes into a more simplified and generalized route. Summarization is key for network scalability, routing efficiency, and router resource conservation. Consider the application for route summarization shown in Figure 3-14.

Figure3-14 Applying EK¡Rl} Route Summarization

Big Corporate Network 172.20.0.0/16

Figure3-14 Applying EK¡Rl} Route Summarization

Big Corporate Network 172.20.0.0/16

172.20.16.0/20 Summary route

Region Z 172.20.16.0/24 172.20.17.0/24 172.20.18.0/24

Region Z 172.20.16.0/24 172.20.17.0/24 172.20.18.0/24

172.20.16.0/20 Summary route

172.20.31.0/24

16 routes

In Figure 3-14, Router F. is responsible for advertising Region Z's routes to the big corporate network. Normally, Router L advertises each /24 route separately and injects 16 routes into the big corporate network—this is the default behavior.

Due to some good address planning, the routes in Region Z are contiguous. Subsequently, Router E can summarize and reduce the number of routes it advertises—that is, it can advertise a single route that covers all of Region Z's address space. The rest of the organization, such as Regions X and Y, can still reach everything in Region Z. and every router benefits from sending and maintaining fewer routes.

The following is Router E's configuration with summarization applied:

inter-face Seriall ip address 172.20.77.2 255.255.255.0 ip summary-address eigrp 100 172.20.16.0 255.255.240.0

j router eigrp 100 network 172.20.0.0

no auto summary where ip summary-address eigrp 100 172.20.16.0 255.255.240.0 is the key command and summarizes all 16 routes inside Region Z with 172.20.16.0/20 (mask 255.255.240.0 equals /20). Interface Serial I gets this command because it is the interface pointing to the big corporate network the summary route exits out this interface. Keywords eigrp 100 define the associated EIGRP autonomous system (necessary because a router might route for more than one autonomous system).

The command no auto-summary might not be necessary. If major net 172.20.0.0 has discontiguous subnets, the command is needed. See "Understanding EIGRP Auto-Summarization," earlier in this chapter.

NOTE EIGRP route summarization is performed at the interface level. Although OSPF has separate summarization commands for internal (area-to-area) and external (redistributed) routes, EIGR P summarizes both internal and external routes with ip summary-address eigrp at the interface.

To understand why 172.20.16.0/20 adequately describes all 16 routes in Region Z, examine the octet where the mask falls (the third octet). Expand the third octet (decimal 16) into binary and apply the /20 mask, as shown in Example 3-1.

Example 3-1 Analysis of Summarization with J 72.20.16.0/20

Third octet = Decimal 16 = 0001-0000 Third octet of /20 mask - 1111 -0000

Route summarizes: 0001 -xxxx (where x is 0 or 1)

As shown in Example 3-1, the /20 mask sets the third octet to 0001-xxxx, where .v can be a zero or a one. This means all possible combinations for the third octet are 0001-0000 through 0001-1111 (decimal 16 through 31). Putting the range of the third octet into the rest of the address yields an address range of 172.20.16.0 through 172.20.31.255 (the last octet can be anything from 0 to 255). This equals the address space of the subnets in Region Z; therefore, 172.20.16/ 20 is the correct summary route for Region Z.

A look at Router E's routing table shows that the summary route is installed:

RTE#sh ip ro

Codes: C • connected, S - static, I - IGRP, R - RIP, M - mobile, B BGP D - EIGRP, EX - EIGRP external, 0 OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 ■ OSPF NSSA external type 2 E1 - OSPF external type 1, E2 OSPF external type 2, E EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * • candidate default U - per-user static route, o ■ 0DR T - traffic engineered route

Gateway of last resort is not set

C 172.20.16.0/24 is directly connected, Serial©.1

C 172.20.17.0/24 is directly connected, Serial0.2

C 172.20.18.0/24 is directly connected, Serial0.3

C 172.20.19.0/24 is directly connected, Serial0.4

<l±nes deleted for brevity>

The entry shown in boldface in the preceding output is the EIGRP summary route for

Region Z. Because this router is the originator, the summary route points to NullO.

Likewise, Router F receives and installs the summary route: RTF#sh ip ro

Codes: C - connected, S - static, I - IGRP, R ■ RIP, M - mobile, B • BGP D • EIGRP, EX • EIGRP external, 0 - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E • EGP i • IS-IS, Li - IS-IS level-1, L2 - IS-IS level-2, * - candidate default U - per-user static route, o • ODR T - traffic engineered route

Gateway of last resort is not set

172.20.0.0/16 is variably subnetted, 2 subnets, 2 masks D 172.20.16.9 20 [99/1920000] via 172.29.77.2, 00:16:48, Seriall

C 172.20.77.0/30 is directly connected, Seriall

<lines deleted for brevity>

The boldface entry in the preceding output is the summary route sent to this router (Router F) by Router E.

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