Free 642-883 Sample Question 11
Refer to the topology diagram in the exhibit it.
Which IS-IS feature could be implemented so that the return path for the packets from router Y in area 49.00AA to router X in area 49.00BB will use the more optimal path?
A. Enable route leaking to pass Level 2 information into the Level 1 routers.
B. Change the area 49.00AA type from a stub area to a regular area.
C. Change the IS-IS administrative distance on router Y in area 49.00AA.
D. Change the IS-IS metric type from narrow to wide on all IS-IS routers.
Correct Answer: A
The IS-IS routing protocol allows for a two-level hierarchy of routing information. There can be multiple Level 1 areas interconnected by a contiguous Level 2 backbone. A router can belong to Level 1, Level 2, or both. The Level 1 link-state database contains information about that area only. The Level 2 link-state database contains information about that level as well as each of the Level 1 areas. An L1/L2 router contains both Level 1 and Level 2 databases. It advertises information about the L1 area to which it belongs into L2. Each L1 area is essentially a stub area. Packets destined for an address that is outside of the L1 area are routed to the closest L1/L2 router to be forwarded on to the destination area. Routing to the closest L1/L2 router can lead to sub-optimal routing when the shortest path to the destination is through a different L1/L2 router.
Route leaking helps reduce sub-optimal routing by providing a mechanism for leaking, or redistributing, L2 information into L1 areas. By having more detail about inter area routes, an L1 router is able to make a better choice with regard to which L1/L2 router to forward the packet.