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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.
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