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.