In addition to the backbone (Area 0) and other non-backbone areas described and used in the examples in previous sections of this module, the OSPF specification also defines several “special” types of areas. The configuration of these areas is used primarily to reduce the size of the Link State Database on routers residing within those areas by preventing the injection of different types of LSAs (primarily Type 5 LSAs) into certain areas, which include the following:
- Not-so-stubby Areas
- Totally Not-so-stubby Areas
- Stub Areas
- Totally Stubby Areas
Not-so-stubby Areas (NSSAs)
Not-so-stubby Areas (NSSAs) are a type of OSPF Stub Area that allows the injection of external routing information by an ASBR using an NSSA External LSA (Type 7). As stated in the previous section, Type 4, Type 5, and Type 7 LSAs are used for external route calculation. We will not examine Type 7 LSAs in detail or how they are used in NSSAs.
Totally Not-so-stubby Areas (TNSSAs)
Totally Not-so-stubby Areas (TNSSAs) are an extension of NSSAs. Like NSSAs, Type 5 LSAs are not allowed into a TNSSA; unlike NSSAs, Summary LSAs are also not allowed into a TNSSA. In addition, when a TNSSA is configured, the default route is injected into the area as a Type 7 LSA. TNSSAs have the following characteristics:
- Type 7 LSAs are converted into Type 5 LSAs at the NSSA ABR
- They do not allow Network Summary LSAs
- They do not allow External LSAs
- The default route is injected as a Summary LSA
Stub Areas
Stub areas are somewhat similar to NSSAs, with the major exception being that external routes (Type 5 or Type 7) are not allowed into Stub Areas. It is important to understand that Stub functionality in OSPF and EIGRP is not at all similar. In OSPF, the configuration of an area as a Stub Area reduces the size of the routing table and the OSPF database for the routers within the Stub Area by preventing external LSAs from being advertised into such areas without any further configuration. Stub Areas have the following characteristics:
- The default route is injected into the Stub Area by the ABR as a Type 3 LSA
- Type 3 LSAs from other areas are permitted into these areas
- External route LSAs (i.e., Type 4 and Type 5 LSAs) are not allowed
Totally Stubby Areas
Totally Stubby Areas (TSAs) are an extension of Stub Areas. However, unlike Stub Areas, TSAs further reduce the size of the LSDB on routers in the TSA by restricting Type 3 LSAs, in addition to the external LSAs. TSAs are typically configured on routers that have a single ingress and egress point into the network, for example in a traditional hub-and-spoke network. The area routers forward all external traffic to the ABR. The ABR is also the exit point for all backbone and inter-area traffic to the TSA, which has the following characteristics:
- The default route is injected into Stub Areas as a Type 3 Network Summary LSA
- Type 3, Type 4, and Type 5 LSAs from other areas are not permitted into these areas