OSPF: Filtering

Link state protocols such as OSPF do not advertise routes, they advertise topology information.  SPF loop prevention relies on all routers to have an identical copy of the topology information to work effectively.  Only routers originating LSAs can make changes to LSAs, any changes in its contents by other routers could cause irregularities which would in turn cause loops or connectivity issues to begin failing.

IOS supports three variations of filtering for OSPF:

  • Filtering routes, not LSAs, using the distribute-list command routers can filter the routes that its SPF process is attempting to add to the routing table without affecting the LSDB.
  • ABR type 3 LSA filtering is the process by which an ABR prevents the creation of specific type 3 summary LSAs
  • Using the are range no-advertise option, this is another process to prevent ABRs from creating certain type 3 LSAs
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