OSPF Area Types for Area 2

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An OSPF stub area has no external routes in it, so you cannot redistribute routes from another protocol into a stub area. A not-so-stubby-area (NSSA) allows external routes to be flooded within the area. These routes are then leaked into other areas. However, external routes from other areas still do not enter the NSSA.

(ABR does not flood LSA types 4 and 5 into an attached NSSA.)

In this scenario, the OSPF network design already extensively uses stub areas due to the presence of 5000 external routes within the backbone and resource constraints in nonbackbone areas. Now, there is a device in Area 2 that must act as an Autonomous System Boundary Router (ASBR), which redistributes external routes into OSPF.

To choose the appropriate OSPF area type for Area 2, we need to consider the impact of each area type on the distribution of routing information within the OSPF domain. Here's a brief explanation of each area type:

1. Stub area: This area type is used to reduce the amount of LSAs flooding in the OSPF domain by filtering out external routes. In a stub area, the ABR summarizes the external routes and advertises them as a default route to the internal routers. A stub area can have only one exit point to the external world, which is the ABR.

2. Non-backbone area: This is any OSPF area that is not the backbone area (Area 0). Non-backbone areas can have multiple exit points to the external world, but they cannot contain Type 5 LSAs (external LSAs) and can only receive a default route from the ABR.

3. Not-so-stubby-area (NSSA): This area type is used in situations where external routes must be redistributed into OSPF, but the area is not a backbone area. NSSAs have the same restrictions as stub areas (i.e., no Type 5 LSAs), but they allow the ASBR to inject external routes as Type 7 LSAs. The ABR then translates these Type 7 LSAs into Type 5 LSAs and distributes them throughout the OSPF domain.

4. Totally stubby area: This area type is similar to a stub area, but it also blocks the propagation of Type 3 LSAs (summary LSAs) from the ABR. Instead, the ABR advertises a single Type 3 LSA for the default route. Totally stubby areas can have only one exit point to the external world, which is the ABR.

In this scenario, we have a device in Area 2 that must act as an ASBR, which means it needs to redistribute external routes into OSPF. This requirement rules out the use of stub areas and totally stubby areas since they both block the propagation of Type 5 LSAs.

A non-backbone area could be used, but it would result in the distribution of 5000 external routes throughout the OSPF domain, which could cause resource constraints on the non-backbone routers.

Therefore, the best OSPF area type for Area 2 in this scenario is the NSSA. It allows the ASBR to inject external routes as Type 7 LSAs, which are translated into Type 5 LSAs by the ABR and distributed throughout the OSPF domain. This approach minimizes the distribution of external routes and avoids resource constraints on non-backbone routers while still allowing the ASBR to redistribute external routes into OSPF.