Suppressing LSAs in OSPF Stub Area
Question
R3 is an OSPF router connect to both Area 0 and Area 1. Area 1 has been configured as a stub area. Which two LSA types does R3 suppress from Area 1?
(Choose two.)
Answers
Explanations
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A. B. C. D.BD
Stub areas are areas through which, or into which, AS external advertisements are not flooded (LSA types 4 and 5).
Sure, I'll explain the answer to this question in detail.
OSPF (Open Shortest Path First) is a link-state routing protocol that operates within an autonomous system (AS). OSPF routers exchange LSAs (Link State Advertisements) to build and maintain the topology of the network.
In OSPF, there are several types of LSAs that can be advertised by a router, including:
Router LSA (Type 1): Advertises information about the router's interfaces and their IP addresses within the same area.
Network LSA (Type 2): Advertises information about the routers and network segments that are directly connected to a multi-access network (e.g., Ethernet).
Summary LSA (Type 3): Advertises summarized information about networks that are outside the local area, but within the same AS.
ASBR Summary LSA (Type 4): Advertises summarized information about external routes that are originated by an ASBR (Autonomous System Boundary Router).
External LSA (Type 5): Advertises information about external routes that are learned from other ASes.
Now, coming back to the question, R3 is an OSPF router connected to both Area 0 (the backbone area) and Area 1. Area 1 has been configured as a stub area. A stub area is an OSPF area that does not accept external routes (Type 5 LSAs) into the area, but instead, it replaces them with a default route.
When an OSPF router is configured to operate in a stub area, it will suppress certain types of LSAs that are not necessary for the proper functioning of the area. Specifically, a stub area will suppress the following LSAs:
Type 5 LSAs (External LSAs): External routes learned from other ASes are suppressed within a stub area because stub areas are not meant to carry traffic to other ASes.
Type 4 LSAs (ASBR Summary LSAs): These LSAs are suppressed in a stub area because they are used to advertise external routes from an ASBR. Since stub areas do not accept external routes, there is no need to advertise these LSAs within the area.
Therefore, the correct answers to this question are B. External LSA and D. ASBR Summary LSA. R3 will suppress these types of LSAs from Area 1 because it has been configured as a stub area. Note that Router LSAs and Summary LSAs are not suppressed in a stub area.
