Understanding OSPF Type 7 LSAs

B

OSPF (Open Shortest Path First) is a popular link-state routing protocol used for routing in IP networks. OSPF uses various types of Link State Advertisements (LSAs) to exchange information about the network topology. Each LSA has a unique type and purpose.

OSPF Type 7 LSAs are used in OSPFv2 when OSPF is used as the routing protocol between an area border router (ABR) and an autonomous system boundary router (ASBR) that is redistributing external routes into OSPF. These LSAs are generated by the ASBR and flooded into the OSPF domain. They carry information about external routes that are injected into OSPF by the ASBR.

The correct answer to the question is option B: They are converted into Type 5 LSAs by the ABR.

When an ABR receives a Type 7 LSA, it converts it into a Type 5 LSA before flooding it into other areas. Type 5 LSAs are used to describe external routes in OSPF. The conversion from Type 7 to Type 5 is necessary because Type 7 LSAs are specific to the NSSA (Not-So-Stubby Area) and cannot be propagated outside the NSSA. In contrast, Type 5 LSAs can be propagated throughout the OSPF domain.

It's worth noting that OSPFv3 does not use Type 7 LSAs. Instead, it uses Type 5 LSAs with a new bit, the E-bit, to indicate whether the route is external or not. In OSPFv3, the ABR converts external LSAs into Type 5 LSAs and sets the E-bit to 1.

In summary, OSPF Type 7 LSAs are converted into Type 5 LSAs by the ABR when they are received from an ASBR. Type 5 LSAs are then flooded throughout the OSPF domain to provide information about external routes.