OSPF: Type 3 LSA

OSPF does not forward Type 1 or 2 LSAs from one area to another.  Those LSAs stay in the area of which they are propagated in.

ABRs advertise type 3 LSAs into one area to represent the subnets in Type 1 and Type 2 LSAs in another area.  Each type 3 LSA defines a simple inter-area destination that includes the subnet, mask, and the ABRs cost to reach that subnet.


Routers that receive these type 3 LSAs are able to calculate the cost for a route to the subnet defined in the LSA by adding the following:

  • The calculated cost to reach the ABR that created the Type 3 LSA
  • The cost as listed in the type 3 LSA

you can see the cost of the LSA with the show ip ospf database summary link-id command, and the cost to reach the advertising ABR with show ip ospf border-routers command.

This efficiency in the processing of an updated route or the learning of a new route via the Type 3 LSAs allows the SPF algorithm to only be run in the area of origin to where the change occurred. All other routers in other areas using the Type 3 LSAs for knowledge of the advertised subnets need only modify their existing costs to updated subnet.

Rules for originating and processing Type 3 LSAs:

  • An ABr only uses Type 3 LSAs that are received over a backbone area in its SPF calculation.  Type 3 LSAs received over non-backbone areas will be skipped during the ABRs SPF calculation, though they are stored in the ABRs LSDB and flooded within the non-backbone area as usual.
  • When an ABR creates and floods type 3 LSAs to advertise networks from one area to another, only intra-area routes from nonbackbone areas are advertised into the backbone, both inter-area and intra-area routes are advertised from the backbone to the non-backbone areas.

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