7.3 Static Routes and Gateways of Last Resort
7.3.4 Fixing a default gateway loop
In the scenario shown in the main figure, a situation has been created in which two routers have installed a gateway of last resort that points to the other router.

The network administrator placed the static route for the gateway of last resort in router A instead of router B, where it belongs.

Router A has the redistribute static command configured, as well as a default metric. This causes router A to advertise the route for 10.0.0.0 to router B. This is not a violation of split horizon because router A did not actually learn about 10.0.0.0 from router B. Having a static route pointing to another router and having a route learned from another router are not the same thing.

In other words, if router B had dynamically advertised a route for 10.0.0.0 to routers A and router A had advertised it back to router B, that would be a violation of split horizon. In this case, router A is telling router B about a route that router A actually uses router B to reach.

Because router B doesn't have the static route to 10.0.0.0 statically configured as it should have had, it installs the dynamic route learned from router A with the next hop of 168.71.6.1 and tags it as the gateway of last resort.

Packets to unknown destinations that should go to the rest of the world now bounce between the two routers. The routing table from router A shows that its gateway of last resort is pointing to router B. (see RouterA#show ip route command output).

The routing table from router B shows that its gateway of last resort is pointing to router A. (see RouterB#show ip route command output).

In the output of the debug ip packet command from router B, you can see the packets bouncing between the two routers, a scenario that continues until the TTL expires. (see RouterB#debug ip packet command output).

Clearly, the use of gateways of last resort must be carefully planned. Misuse can lead to loss of connectivity and routing loops. If a sufficient number of packets start looping the routers, the links can be overwhelmed and a routing loop storm can be created. A quick fix is a temporary static route pointing to null0 in one of the routers, vacuuming up all the looping packets. The problem can then be fixed by placing the default routes where they belong.