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Controlling Redistribution
In some cases, it is not always desirable to redistribute all the routing
announcements from one protocol into another. To restrict the flow of dynamic
route announcements between routers belonging to the same routing policy or
between protocol redistributions, IOS provides route filtering. Route filtering is
particularly advantageous for administrators who need to keep strict control over
the route announcements exchanged between routers.
Filtering with Distribution Lists
The route filter's responsibility is to suppress unwanted routing information from
being redistributed, entered in or advertised out of the routing table. Although the
result is the same as redistribution, this process behaves differently if a link state or
distance-vector protocol is being used. When you use a route filter with a distance
vector protocol, the filter affects not only the router the filter refers to, but also all
the routers that receive advertisements from the filtered router. This is the result of
the distance vector using their routing tables as the basis for the network
announcements. When filtering is employed with a link state protocol, however, the
filter will only affect the local routing table. Link state protocols use link state
announcements to construct their routing tables, and these are unaffected by route
filters.
Routing filters are configured in two parts, starting with the filter list, which is a
standard access list. Access list entries are entered using only the network address:
asbr-a1(config)#access-list 2 permit 129.0.0.0
asbr-a1(config)#access-list 2 permit 192.168.0.0
asbr-a1(config)#access-list 2 permit 192.168.10.0
After the list is created, it is applied to the routing protocol configuration with the
routing configuration subcommand . Inbound filter lists are applied to interfaces. They
permit or deny the flow of route announcements through a router interface.
Inbound filters are used for suppressing route propagation with distance vector
protocols or the addition of route announcements on a local router when used with
a link state protocol. They are often used to correct a side effect of route
redistribution known as route feedback. Route feedback occurs when routing
information is advertised in the wrong direction across a redistributing router.
Inbound filters are also useful from a security perspective, if you are only routing to