IS-IS Adjacencies

AC

IS-IS (Intermediate System to Intermediate System) is a link-state routing protocol used to exchange routing information between network routers in a hierarchical architecture. In IS-IS, routers form adjacencies to share link-state information and build a topology map of the network.

In IS-IS, there are two types of adjacencies: Level 1 adjacencies and Level 2 adjacencies. Let's examine the statements and determine which ones are correct:

A. Level 1 adjacencies can form in the same (intra) area only.

This statement is correct. Level 1 adjacencies are formed between routers in the same area (intra-area). An area is a logical grouping of routers and links within a larger IS-IS domain. All routers within the same area must have the same area ID. Level 1 routers only have information about their own area and do not have information about other areas.

B. Level 2 adjacencies are formed between (inter) areas only.

This statement is incorrect. Level 2 adjacencies are formed between routers in different areas (inter-area) and are used to exchange routing information between areas. Level 2 routers have information about the entire IS-IS domain, including all areas and inter-area routes.

C. Level 2 adjacencies can be established either interarea or intra-area.

This statement is incorrect. Level 2 adjacencies can only be established between routers in different areas (inter-area). Intra-area Level 2 adjacencies do not exist.

D. Level 2 adjacencies can form within the same (intra) area only.

This statement is incorrect. Level 2 adjacencies can only be formed between routers in different areas (inter-area). Intra-area Level 2 adjacencies do not exist.

In summary, the correct statements about IS-IS adjacencies are:

• Level 1 adjacencies can form in the same (intra) area only.
• Level 2 adjacencies are formed between (inter) areas only.