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 172.20.0.0 is broken up and separated by major net 10.0.0.0 in other words, the subnets of 172.20.0.0 are discontiguous. Router C thinks it's sitting on a major net boundary because it has direct connections to both 10.0.0.0 and 172.20.0.0.
With FIGRP auto-summarization enabled. Router C sends a summary route for all of 172.20.0.0 into 10.0.0.0 because it is a major net boundary. It has no need to tell the routers in 10.0.0.0 about the internal subnets of 172.20.0.0—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 172.20.0.0/16 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 10.0.0.0. Router D sends 172.20.0.0/16 into the 10.0.0.0 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 172.20.16.0/24, 172.20.17.0/24, 172.20.18.0/24, and 172.20.19.0/24.
• In the same way. Router C never learns the details of the subnets beneath Router D.
• Routers in the 10.0.0.0 cloud receive a summary route for 172.20.0.0 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 172.20.0.0 network 10.0.0.0 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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