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Which two LSA types are only generated by area border routers? (Choose two.)

Which two LSA types are only generated by area border routers? (Choose two.)

PrepAway - Latest Free Exam Questions & Answers

A.
Type 2

B.
Type 3

C.
Type 4

D.
Type 5

Explanation:
Type 3 LSAs provide summary information about Type 1 and 2 LSAs for other areas.

Type 3 LSAs have simple information the subnet, the mask, and the cost as known by the ABR.

When something changes with a Type 3 LSA (cost), only a partial SPF run is needed. Route summarization at the ABR reduces the amount of Type 3 LSAs but is not required for partial SPF benefits.
Type 3 LSAs by themselves provide for partial SPF benefits.

Note: external routes redistributed into an OSPF area do provide partial SPF benefits for that area, whereas internally injected routes via the network command do not provide partial SPF benefits for that area. External routes are treated as Leafs in the SPF, whereas internal routes are treated as Nodes. The loss of a Leaf can be handled with partial SPF, whereas the loss of a Node requires full SPF.

In a nutshell, redistributed routes are not necessarily a bad thing, especially where there are a large number of routes with a lot change (dial-in).

IS-IS, on the other hand, treats all IP networks as Leafs in the SPF.

If a network represented by a Type 3 LSA goes down, the ABR refloods the LSA with the OSPF infinitecost so that the route is quickly removed from all other routers which contain that LSA in theirLSDB. The alternative would be to simply stop sending that LSA, however the route would not beremoved from the other routers until the LSA aged out (MaxAge default 60 mintues).

Type 4 LSAs are generated by ABRs that have an ASBR in an area it touches. The Type 4 LSA iscreated by the ABR to describe the ASBR and its costs to reach it. Type 4 LSAs are used when anASBR is present, which helps routers in other areas calculatethe exact metric to reach the external route.

Type 4 LSAs (Summary ASB) are always generated by an ABR when an ASBR is present in an area it touches. OSPF routers in a different area than the ASBRs area will look at the Type 5 and Type 4 LSAs to calculate the path and metric to reach the external route. OSPF routers in the same area as the ASBR can look at just the Type 5 LSA to calculate the path. However, the Type 5 LSA by itself does not have enough information for OSPF routers outside the area*, hence the need for the Type 4 LSA.


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