EIGRP Support of VLSM and Route Summarization

EIGRP automatically summarizes routes at classful major network boundaries on attached interfaces. This is one of the attributes of its distance-vector behavior. Route summarization reduces the number of routing table entries that are advertised to neighbor routers, which results in fewer routing table lookups and enhances overall network performance. For example, routes for 10 contiguous Class C networks, such as 192.168.1.0/24 through 192.168.10.0/24, would be advertised by EIGRP with just a single summary route: 192.168.1.0/16.

EIGRP supports both automatic route summarization and manual summarization. EIGRP, like its predecessor IGRP, auto-summarizes at major network boundaries by default, although this behavior can be disabled. Manual summarization enables you to implement VLSMs and force a summary route from an interface at any bit boundary. This is a major advantage of EIGRP over older distance-vector routing protocols. The router configuration command auto-summary is used to enable automatic summarization of IP subnets at major network boundaries. Again, this is the default setting.

One negative issue that can arise with automatic classful route summarization is the potential for routing discrepancies when discontiguous network numbers are implemented on multiple routers. For example, EIGRP would automatically advertise the subnetted Class B network 172.16.5.0/24 as 172.16.0.0. If another subnet with the address 172.16.10.0 were to be deployed on a subnet connected to another router, EIGRP would also advertise the classful summary route 172.16.0.0. This could result in real problems when the router attempts to access some networks. In certain circumstances, you may want to disable the default EIGRP route summarization at classful boundaries. To enable variable-length subnet masking and route summarization at arbitrary network boundaries, you need to first disable auto-summary by issuing the no auto-summary command from the router configuration prompt:

 RouterA(config-router)#  no auto-summary  

To configure manual route summarization, use the following command:

 RouterA(config-router)#  summary-address eigrp   autonomous-system-number address mask  

The autonomous-system-number parameter is obviously the EIGRP AS number. This number is the global designator for all the routers within the EIGRP internetwork and must be identical on all the routers configured within the internetwork. The address keyword is the IP address that is being advertised as a summary (at any bit boundary). The mask keyword is the IP mask that is creating the summary address. To turn off the auto-summarization of EIGRP, execute the no auto-summary command. This router configuration command is used with EIGRP to support variable-length subnet masks and route summarization at classless network boundaries. Using no auto-summary disables automatic route summarization altogether for the EIGRP process. Any summarized routes, whether classful or classless, thereafter must be manually configured with the ip summary-address eigrp interface configuration command. In Listing 7.4, RouterA advertises a single summary route entry for all networks that match prefix 10.10.0.0/16. First, you select the interface that you want to propagate the route summary, in this case interface e0/0. Next, you use the proper command to designate the EIGRP process number and route summarization format.

Listing 7.4 A Sample Manual EIGRP Summarization

 RouterA#config t Enter configuration commands, one per line. End with CNTL/Z RouterA(config)# router eigrp 10 RouterA(config-router)# no auto-summary RouterA(config-router)# network 10.10.0.0 RouterA(config-router)# exit RouterA(config)# interface e0/0 RouterA(config-if)# ip summary address eigrp 10 10.10.0.0 255.255.0.0 RouterA(config-if)# exit RouterA(config)# 

Remember that normal EIGRP routes have an administrative distance (AD) value of 90 and that external EIGRP routes have a much higher AD of 170. In addition, IP EIGRP summary routes have a default AD value of 5.