Early EIGRP implementation treated changes in EIGRP summary address configuration (configured with the ip summary-address eigrp interface configuration command) very disruptively: all EIGRP sessions across the affected interface were cleared, sometimes resulting in a large number of routes entering active state, potentially leading to a stuck-in-active condition.

Newer Cisco IOS releases are more lenient: router with a change in summary address requests a resync (logged as graceful-restart on adjacent routers). A lot of updates and queries are still sent, but the adjacencies themselves are preserved:

• When configuring a summary route, all more specific prefixes on downstream routers enter active state.
• When a summary is removed, only the summary prefix itself enters active state and the affected router sends queries to all its neighbors, while the more specific prefixes are sent as regular EIGRP updates to the neighbors across the affected interface.

A change in EIGRP summary generates the following output on the router under configuration:

a1(config-subif)#ip summary-address eigrp 1 0.0.0.0 0.0.0.0
%DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 172.16.1.2 (Serial0/0/0.100) is resync: summary configured
a1(config-subif)#no ip summary-address eigrp 1 0.0.0.0 0.0.0.0
%DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 172.16.1.2 (Serial0/0/0.100) is resync: summary configured

The downstream router generates similar log messages:

%DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 172.16.1.1 (Serial0/0/0.100) is resync: peer graceful-restart