EIGRP Auto-Summary

Lessons Learned:

EIGRP-Auto Summary

-Like RIPv2, EIGRP is classless but does automatic classful summarization by default.

- Disabeld with # no auto-summary command under the eigrp process.

VLSM

-VLSM is supported within the same major network.

Advertisements between major network boundaries are summarized to the classful boundary

-will not result in traffic black hole due to discard route.
EIGRP uses a Null route or discard route to help to prevent traffic black holes

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With the Null routes – EIGRP will summarize the classful boundary locally then generate the route out to the neighbors. The local router generating the route will have the classful boundary as a summary address going to the Null0 interface.

EIGRP knows if it is running in a classful manor, it does not make sense for it to install same summary that it is generating as it is learned from other neighbors.

Note: auto-summary feature indeed works when a router advertises prefixes between major networks, but ONLY directly connected ones. It does not affect learned networks.

When a router has multiple working interfaces, and those interfaces use IP addresses in different classful networks, the router advertises a summary route for each classful network on interfaces attached to a different classful network.