Understanding Eigrp Auto Summarization

By default, EIGRP automatically summarizes across major net boundaries. Often, this is nothing to worry about; however, when you deploy discontiguous subnets, as EIGRP supports, you might have to disable EIGRP auto-summarization. Consider the following scenario with auto-summarization enabled (see Figure 3-13). Assume EIGRP is the only routing protocol.

Fig u re 3-13 EIGRP Auto-Summarization

In the network depicted in Figure 3-13, major net is broken up and separated by major net in other words, the subnets of are discontiguous. Router C thinks it's sitting on a major net boundary because it has direct connections to both and

With FIGRP auto-summarization enabled. Router C sends a summary route for all of into because it is a major net boundary. It has no need to tell the routers in about the internal subnets of—or so it thinks.

NOTE Classful protocols RIP and IGRP also automatically summarize across major net boundaries.

They are classful, so they naturally assume all subnets of a major net are contiguous. You cannot disable auto-summarization in RIP or IGRP.

After Router C sends upstream, the details of the subnets beneath Router C are now lost. The summary route generalizes the four subnets with a single route.

Router D is also an FIGRP router and automatically summarizes its subnets into Router D sends into the network. This is the same route advertised by Router C.

In this scenario, you can identify the following problems caused by EIGRP auto-summarization:

• Router D never learns the details of the subnets beneath Router C. Router C's summary route throws away the specific routes to,,, and

• In the same way. Router C never learns the details of the subnets beneath Router D.

• Routers in the cloud receive a summary route for but from different directions. When they choose the route with the lowest metric, that route reaches the destinations beneath either Router C or Router D, but not both.

To solve the problems with auto-summarization and discontiguous subnets, disable auto-summarization with the EIGRP no auto-summary command. The following is Router C's configuration:

router eigrp 100 network network no auto-summary where the command no auto-summary disables EIGRP auto-summarization and forces the router to advertise specific routes.

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