OSPF Areas Flashcards
Stubby Area
OSPF stubby areas are identified by the area flag in the OSPF hello packet. Every router within an OSPF stubby area needs to be configured as a stub so that the routers can establish/maintain OSPF adjacencies.
OSPF stub areas prohibit Type 5 LSAs (external routes) and Type 4 LSAs (ASBR summary LSAs) from entering the area at the ABR. RFC 2328 states that when a Type 5 LSA reaches the ABR of a stub area, the ABR generates a default route for the stub via a Type 3 LSA.
When the Type 5 LSA (172.16.1.0/24) reaches the ABR (R3), the ABR generates a default route by using a Type 3 LSA. While R4 does not see the route to the 172.16.1.0/24 network in its routing table, it has connectivity to that network through the default route. Inter-area routes are allowed.
Stubby Area
Default route
Default routes are Type 3 summary LSAs (interarea summaries) because stub areas do not allow Type 5 LSAs within them. A benefit to using a Type 3 LSA is that it cannot be advertised from a nonbackbone area into a backbone area.
Totally Stubby Area
Totally stubby areas prohibit Type 3 LSAs (interarea), Type 4 LSAs (ASBR summary LSAs), and Type 5 LSAs (external routes) from entering the area at the ABR. When an ABR of a totally stubby area receives a Type 3 or Type 5 LSA, the ABR generates a default route for the totally stubby area.
Only intra-area and default routes should exist within a totally stubby area.
Member routers (non-ABRs) of a totally stubby area are configured the same as those in a stub area. ABRs of a totally stubby area have no-summary appended to the configuration. The keyword no-summary does exactly what it states: It blocks all Type 3 (summary) LSAs going into the stub area, making it a totally stubby area.
Not-So-Stubby Area
OSPF stub areas prohibit Type 5 LSAs (external routes) and Type 4 LSAs (ASBR summary LSAs) from entering the area at the ABR, and they prohibit redistribution of external routes into the stub area, too. The not-so-stubby-area (NSSA) prohibits Type 5 LSAs from entering at the ABR but allows for redistribution of external routes into the NSSA.
As the ASBR redistributes the network into OSPF in the NSSA, the ASBR advertises the network with a Type 7 LSA instead of a Type 5 LSA. When the Type 7 LSA reaches the ABR, the ABR converts the Type 7 LSA to a Type 5 LSA.
The ABR does not automatically advertise a default route when a Type 5 or Type 7 LSA is blocked.
NSSA Configuration
The command area area-id nssa [default-information-originate] is placed under the OSPF process on the ABR. All routers in an NSSA must be configured with the nssa option, or they do not become adjacent because the area type flags must match in the OSPF hello packet in order to become adjacent. A default route is not injected on the ABRs automatically for NSSAs, but the optional command default-information-originate can be appended to the configuration if a default route is needed in the NSSA.
Totally NSSA
Totally stubby areas prohibit Type 3 LSAs (interarea), Type 4 LSAs (ASBR summary LSAs), and Type 5 LSAs (external routes) from entering the area at the ABR, and they prohibit routes from being redistributed within that area. OSPF areas that need to block Type 3 and Type 5 LSAs and still provide the capability of redistributing external networks into OSPF should use the totally NSSA.
When the ASBR redistributes the network into OSPF, the ASBR advertises the network with a Type 7 LSA. As the Type 7 LSA reaches the ABR, the ABR converts the Type 7 LSA to a Type 5 LSA. When an ABR for a totally NSSA receives a Type 3 LSA from the backbone, the ABR generates a default route for the totally NSSA. When an interface on the ABR is assigned to Area 0, it acts as the trigger for the Type 3 LSA that leads to the default route generation within the totally NSSA.
Totally NSSA Configuration
Member routers of a totally NSSA use the same configuration as members of an NSSA. ABRs of a totally stubby area have no-summary appended to the configuration. The command area area-id nssa no-summary is configured under the OSPF process.
