Q1. IS-IS uses hierarchical levels to route packets in an OSI environment. Which of the following is true about an IS (Intermediate System)?
A. An IS can only be a level 1 router.
B. An IS can be either level 1 or Level 2 router but not both at the same time.
C. An IS can be both Level 1 and Level 2 router at the same time.
D. An IS can neither be Level 1 nor Level 2 router. Only ES (End System) can assume Level 1/ Level 2 function.
Correct Answer: C
An Intermediate System (router) can assume the functionality of Level 1 router or Level 2 router or both. Note that Level 1 routers are used within an area. They can not connect between two different areas. To connect two different areas in IS-IS routing domain, we need Level 2 router. Level 2 routers form the backbone of IS-IS routing domain connecting different areas. A router can be both Level1 and Level 2 at the same time. In this case, two different link state databases are maintained by the router for L1 LSPs and L2 LSPs respectively.
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