When this feature is enabled, EIGRP will check if you have any subnets that fall within the range of your network command(s) and if so, it will advertise the classful A, B or C network to its neighbors. It will only summarize routes from subnets on its own interfaces, not the ones you learn from other routers.
Auto summarization is configured like so:
R1(config)#router eigrp 1 R1(config-router)#auto-summary
and is disabled like so:
R1(config)#router eigrp 1 R1(config-router)#no auto-summary
Depending on your IOS version, auto-summary might be enabled or disabled by default:
- The behavior of this command is enabled by default on older IOS versions.
- From Cisco IOS Release 15.0(1)M, 12.2(33)SRE, 12.2(33)XNE, Cisco IOS XE Release 2.5, Cisco IOS Release 12.2(33)SXI4 and later releases, the behavior of this command is disabled by default.