Mutual redistribution between protocols should be done very carefully
and in a controlled manner. Incorrect configuration could lead to
potential looping of routing information.
A rule of thumb for mutual
redistribution is not to allow information learned from a protocol to be
injected back into the same protocol. Passive interfaces and distribute
lists should be applied on the redistributing routers. Filtering
information with link-state protocols such as OSPF is a tricky business.
Distribute-list out works on the ASBR to filter redistributed routes into other protocols. Distribute-list in
works on any router to prevent routes from being put in the routing
table, but it does not prevent link-state packets from being propagated,
downstream routers would still have the routes. It is better to avoid
OSPF filtering as much as possible if filters can be applied on the
other protocols to prevent loops.
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