Why Type 3 LSAs Are Still Seen in Stub Area Configuration
Question
You must configure an OSPF area that does not accept type 3 link-state advertisements (LSAs) from area 0 (other than the default route) You have configured your area to be stub, but you still see type 3 LSAs.
What will account for this behavior?
Answers
Explanations
Click on the arrows to vote for the correct answer
A. B. C. D.A
OSPF (Open Shortest Path First) is a link-state routing protocol used to exchange routing information between routers in a network. OSPF areas are used to divide a large network into smaller logical areas to simplify network management and reduce routing overhead.
In OSPF, type 3 LSAs are summary LSAs that are used to advertise routes between different areas. By default, all OSPF areas will accept type 3 LSAs from other areas. However, in some cases, it may be necessary to restrict the propagation of type 3 LSAs.
In this scenario, the requirement is to configure an OSPF area that does not accept type 3 LSAs from area 0, except for the default route. This can be achieved by configuring the area as a stub area. Stub areas are areas that do not allow external routes (type 5 LSAs) and summarize all their internal routes to a single default route.
If type 3 LSAs are still being seen in the configured stub area, then it is likely that the stub area is not configured correctly. The two types of stub areas in OSPF are:
Totally stubby area: This type of area will not accept any type 3 LSAs except for the default route. To configure a totally stubby area, the command 'area x stub no-summary' should be used, where x is the area number.
Not-so-stubby area (NSSA): This type of area allows external routes (type 5 LSAs) to be injected into the area, but still suppresses type 3 LSAs from other areas. To configure an NSSA, the command 'area x nssa' should be used.
Based on the given scenario, it seems like the correct configuration should be a totally stubby area, as this type of area does not allow any type 3 LSAs except for the default route. Therefore, option A, "You should have configured your area as totally stubby," is the correct answer.
Option B, "You should have used a not-so-stubby-area," is incorrect because an NSSA still allows external routes and is not suitable for this scenario.
Option C, "You are experiencing a routing loop," is unlikely to be the cause of this behavior since it is not related to the configuration of the OSPF area.
Option D, "You have exported routes into OSPF," may be a possible cause of the behavior, but it is not related to the requirement of not accepting type 3 LSAs from area 0. If routes are being exported into OSPF, it may be necessary to adjust the OSPF configuration to prevent unwanted routes from being advertised.
