When a RIP summary address is configured on an interface with the ip summary-address rip network mask configuration command, a corresponding summary route pointing to Null 0 is not installed in the IP routing table, resulting in routing loops in networks where the summarization is improperly configured.
The summary prefix 10.0.1.0/24 advertised from A1 to R1 is propagated to R2 and A2, which accepts it, as there is no corresponding summary address configured on the A2-R2 link (see the inbound processing rules section of the RIP route database document) and propagates it back to A1.
Since the summary prefix is received through the Ethernet interface, A1 accepts it and the routing loop is complete.
If you perform a trace from any of the routers to an unreachable destination in the summary address range, the packet will loop around the network continuously.
To fix the routing loop, you have to configure a summary address on the A2-R2 link. The summarization prefix has to be identical to the one configured on the A1-R1 link.
After the ip summary-address rip 10.0.1.0 255.255.255.0 has been configured, A2 ignores the 10.0.1.0/24 prefix received from R2, breaking the routing loop.
This page was last modified on 16 October 2008, at 18:47.