IS-IS Adjacencies

AC

IS-IS (Intermediate System to Intermediate System) is a routing protocol that is used for communication between routers within a single autonomous system (AS) or across multiple autonomous systems. In IS-IS, routers form adjacencies with other routers to exchange information and build the routing tables.

Regarding IS-IS adjacencies, the following statements are true:

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

This statement is true. IS-IS divides a network into logical areas to facilitate routing and reduce the size of the routing tables. Level 1 routers are routers that are only aware of their own area, while level 2 routers are aware of the entire network. Level 1 adjacencies are formed between routers that are in the same area, and these adjacencies exchange information about routes within the area.

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

This statement is false. Level 2 adjacencies are formed between level 2 routers, which are responsible for routing between areas. These adjacencies exchange information about routes between areas. However, level 2 routers can also form level 2 adjacencies with other level 2 routers within the same area.

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

This statement is true. As mentioned above, level 2 adjacencies are formed between level 2 routers and can be used to exchange information about routes between areas. However, level 2 routers can also form level 2 adjacencies with other level 2 routers within the same area.

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

This statement is false. Level 2 adjacencies can be formed both within the same area and between different areas. However, level 2 routers that are in the same area and form a level 2 adjacency will not exchange information about routes within that area, as that is the responsibility of level 1 routers.

In summary, the correct statements are A and C.