Which OSPF Areas Allow External Routes?

AB

OSPF (Open Shortest Path First) is a popular link-state routing protocol used in computer networks. In OSPF, networks are divided into areas, and routers within an area share topology information with each other. There are several types of OSPF areas, and each has its own characteristics and rules for routing.

Among the four types of OSPF areas, two types allow external routes, which are:

A. Backbone Area: Also known as Area 0, the backbone area is the core of an OSPF network. It interconnects all other areas in the network, and all traffic passing between areas must pass through it. Because of its central role, the backbone area must allow external routes to be propagated through it to other areas.

B. Not-So-Stubby Area (NSSA): A not-so-stubby area is a type of OSPF area that is similar to a stub area but allows for some external routes to be imported into the network. Specifically, an NSSA can import external routes that are redistributed into OSPF using a special type of OSPF route called a Type 7 external route. An NSSA is useful when an organization has a few external routes that need to be imported into the network, but the network is mostly self-contained.

On the other hand, two types of OSPF areas do not allow external routes:

C. Stub Area: A stub area is a type of OSPF area that does not allow external routes to be imported into the network. This makes the stub area more secure and less vulnerable to attacks from outside the network, but it also means that the area cannot be used to connect to other networks.

D. Totally Stubby Area: A totally stubby area is a variant of the stub area that also blocks external routes but takes it a step further by blocking Type 3 Summary LSA from the backbone. This means that routers within the totally stubby area will only learn default route from the ABR (Area Border Router) connecting to the backbone.