Figure
shows an example
of a network that could produce a large routing table. In this example, there
are only three routes. In a real network, however, the number of networks could
be much higher and the problem of a large routing table, much worse.
R2
is announcing several subnets of 131.108.0.0 network. Notice that the link
between R1 and R2 is also part of the 131.108.0.0 network, so
auto-summarization cannot play any role to solve the problem of receiving a
subnet route that could be summarized. The auto-summarization feature can only
work if the link between R1 and R2 is on a different major network.
Debugs and Verification
Figure
shows
that in the configuration of R2, the ip summary-address
command is not used under the Serial 1 interface to summarize the routes.
Figure
shows the
routing table of R1.
Solution
In this situation, autosummary
is on but is not helpful because the whole network in within one major network.
Autosummary cannot play any role here because no major network boundary is
crossed. A new feature of summarization was introduced in RIP starting with
Cisco IOS Software Release 12.0.7T. This feature is similar to EIGRP manual
summarization.
Figure
shows the new
configuration that solves this problem using the ip summary-address
rip command. This configuration reduces the size of the routing
table. This command can be used with different masks, so if a network has
contiguous blocks of a subnet, the router could be configured to summarize
subnets into smaller blocks. This would reduce the routes advertised to the RIP
network.
Based on the preceding configuration, router R2 will summarize
the RIP routes on the Serial 1 interface. Any network subnet that falls in the
131.108.0.0 to 131.108.3.0 range of networks will be summarized as a single
131.108.0.0/22 major network. This means that R2 will announce only a single
summarized route as 131.108.0.0/22 and suppress the 131.108.1.0/24,
131.108.2.0/24, and 131.108.3.0/24 subnets.
Figure
shows the
routing table of router R1 with a reduced number of entries as a result of the
summarization.