How does OSPF filter received routes

As a link state-based routing protocol, OSPF is different from a distance-vector (D-V) algorithm routing protocol and ensures consistent topological relationship. This is an important condition for loop-free OSPF routing. You can run the filter-policy import command to filter routes in OSPF configuration. The link-state advertisement (LSA) of a route filtered out exists in the OSPF database. OSPF simply does not add the route to the routing table of the route manager. In addition, the LSA of the route is still advertised. That is, the neighbors still have this route. Note that the filter-policy export command can be used to filter routes when external routes are redistributed for advertisement.

OSPF provides another feature: OSPF Area Border Router (ABR) Type 3 LSA filtering. This feature extends the ability of an ABR that is running the OSPF protocol to filter type 3 LSAs that are sent between different OSPF areas. This feature allows only packets with specified prefixes to be sent from one area to another area and restricts all packets with other prefixes. This type of area filtering can be applied out of a specific OSPF area, into a specific OSPF area, or into and out of the same OSPF areas at the same time.

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