What are two functions of an NSSA in an OSPF network design? (Choose two.)
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A. B. C. D. E.BC.
OSPF (Open Shortest Path First) is a routing protocol that is widely used to distribute routing information within a single autonomous system (AS). In some cases, an OSPF network may have areas that are not fully autonomous, known as Not-So-Stubby Areas (NSSAs).
Two functions of an NSSA in an OSPF network design are:
B. It allows ASBRs to inject external routing information into the area. An NSSA is a type of OSPF area that can contain external routes, which are routes that are learned from outside the OSPF domain. However, unlike a regular OSPF area, an NSSA cannot import external routes directly from other areas. Instead, external routes are imported into the NSSA by an Autonomous System Boundary Router (ASBR) that is connected to both the OSPF domain and the external network. The ASBR injects the external routes into the NSSA as Type 7 LSAs (Link State Advertisements).
C. An ASBR advertises Type 7 LSAs into the area. As mentioned earlier, an ASBR connected to an external network injects external routes into an NSSA as Type 7 LSAs. These LSAs are not flooded beyond the NSSA boundary and are translated into Type 5 LSAs (external LSAs) by the NSSA ABR (Area Border Router) before they are flooded into the OSPF domain. The Type 5 LSAs are then used by other OSPF routers to determine the best path to the external networks.
Therefore, options B and C are correct answers, as they correctly identify the functions of an NSSA in an OSPF network design. Option A is incorrect because suboptimal routing is not an issue addressed by an NSSA. Option D is incorrect because ABRs do not advertise Type 7 LSAs into an NSSA. Option E is also incorrect because opaque LSAs are not unique to NSSAs and can be used in any OSPF area.