Core Issue
Route summarization filters more specific routes and advertises a less specific summary route to neighboring routers, covering the range of the specific networks. Enhanced Interior Gateway Routing Protocol (EIGRP) can summarize both internal routes and external routes. Internal routes are those networks natively included in the EIGRP process by issuing the network ip-address [subnet-mask] command. External routes are networks redistributed into the EIGRP process by issuing the redistribute protocol command. Depending on the type of route summarized, the EIGRP process on a router can be configured for automatic or manual summarization.
Resolution
EIGRP automatically summarizes networks at major network boundaries when the auto-summary command is issued in router configuration mode. For example, if a router has two interfaces belonging to 10.1.1.0/24 and 172.16.1.0/24 under EIGRP, these networks are summarized to their major network boundaries, 10.0.0.0/8 and 172.16.0.0/16 respectively, when the updates are advertised by EIGRP. Automatic summarization summarizes internal routes, but it does not summarize external routes.
Manual summarization can be configured in EIGRP instead of automatic summarization by configuring a summary aggregate address for a specified interface. This is useful if the summary route is sent only out of certain interfaces, but more specific routes are advertised over other interfaces. Manual summarization allows you to summarize routes at any bit boundary, but automatic summarization always summarizes at the major network boundary. In the previous example, manual summarization can be configured to advertise summary route 10.1.0.0/16 instead of 10.0.0.0/8.
Manual summarization allows you to summarize both internal and external EIGRP routes. When summarizing external routes in EIGRP using this method, the summary route becomes an internal route. Also, manual summarization can be configured on a specific interface belonging to the same major network as the specific routes being summarized, which is not possible using automatic summarization. For example, an interface belonging to 10.1.1.0/24 can be configured to summarize specific networks belonging to the major network 10.0.0.0/8.
To configure a summary aggregate address for a specified interface using manual summarization, issue the ip summary-address eigrp as-number network-address subnet-mask [admin-distance] command in interface configuration mode. When a router running EIGRP summarizes routes either due to manual or automatic summarization, a summary route pointing to null 0 interface is automatically created in the routing table of the router. This is done to avoid routing loops or sending packets to a black hole when a default route is also available on the router performing the summarization.Configuring route summarization is also to prevent the EIGRP process from triggering each time an interface changes state or a specific route is lost, which causes high CPU utilization.
For more information on configuring automatic and manual summarization, refer to these documents:
