Stub areas in OSPF are a way to improve scaling and reduce the amount of computing required on the routers. For routers in a non-backbone area it might not be necessary to have the information of all AS external routes. When a router in a non-backbone area wants to send traffic to a network that is outside the OSPF domain it most likely has to send the traffic to an ABR first. The router can therefore suffice with a default route to the ABR.
In a stub area AS external LSAs (Type 5) are not flooded. These are blocked by the ABR. Since ASBR summary LSAs (Type 4) are required to find ASBRs these are also blocked since the routers have a default route and need no knowledge of ASBRs.
When using stub areas consider the following:
– All routers in a stub area must be configured as such. If not they will not be able to form adjacencies.
– Virtual links can not be used inside, or traverse a stubby area
– No router in a stub area can be an ASBR
– Since stub areas use a default route sub-optimal routing can occur when the stub area has more than one ABR.
Totally stubby areas
Where a stub area blocks routes external to the OSPF domain, a totally stubby area contains only a default route, even to networks within the OSPF domain, but outside its own area. So a totally stubby area only knows about the networks within the area and has no knowledge of the rest of the network.
The ABR of a totally stubby area will not only block Type 4 and Type 5 LSAs, but will also block Type 3 Network Summary LSAs with the exception of a special Type 3 advertising the default route.
Not So Stubby Areas
One of the considerations of stubby areas is that they can not contain a ASBR. Not So Stubby Areas (NSSAs) allow for ASBRs to be contained in a stubby-like area. NSSAs have the same characteristics as stubby areas, but they allow for ASBRs. The ASBR originates Type 7 NSSA External LSAs within the area. These LSAs have a special flag, the P bit. If this bit is set to one the ABR will translate the Type 7 LSA into a Type 5 AS External LSA and send this into the rest of the OSPF domain. If the P bit is set to zero the external route will only be known in the NSSA.