EIGRP routes, external and internal, can be summarized manually or automatically. (This is called autosummarization.) Manual summarization can be configured at any bit boundary using an interface level command such as the following:
ip summary-address eigrp autonomous system summary address mask
With this configured, EIGRP will do the following:
Build a routing table entry for this summarized network to the interface null0.
Advertise the summary out of the interface it is configured on.
Advertise the components of this summary as unreachable out of the interface it is configured on.
The route will be marked as a summary in both the routing table and the topology table on the router where the summarization takes place (the router ...