When a major network is separated by another major network, this is called a
discontiguous network. Figure
shows an example
of a discontiguous network.
Debugs and Verification
Figure
shows
the configuration for routers R1 and R2. RIP, with the default of Version 1, is
enabled on the Ethernet interfaces of R1 and R2 with the correct
network statements.
Figure
shows the
debug ip rip command output for routers R1 and R2. The
debugs show that both routers are sending to each other the summarized major
network address for 137.99.0.0, instead of their specific network addresses. As
a result, both routers will ignore the less specific 137.99.0.0 update because
they are already connected to this major network.
Solution
RIP is not installing the route 137.99.0.0 in the
routing table because RIP v1 does not support discontiguous subnets. Several
solutions to this problem exist. Figure
shows that a
quick solution is to configure each router with a static route to the specific
137.99.0.0 subnet on the other router. Since RIP v1 does not include the subnet
information in its routing updates, configuring the static routes is a
"patch" to fix this problem. This may be necessary if the routers in
question can only run RIP v1.
Another solution is to change the address
on the link between routers R1 and R2 to be part of the 137.99.0.0 network. In
other words, assign another subnet on this link, which is part of 137.99.0.0.
Figure
shows a better
solution, to enable the classless routing version of RIP v2 with no
auto-summary configured on both routers. The no
auto-summary command will disable auto-summarization of RIP v2
routes when crossing a major network boundary. It is important to disable the
auto-summarization or the same unreachability problem will continue to exist.
With no auto-summary, the specific subnet information is
also included in these updates. Figure
shows the
routing table of R2 after using this solution.