IS-IS Designated Intermediate System (DIS)

A

The IS-IS (Intermediate System to Intermediate System) protocol is a link-state routing protocol used in large enterprise networks and service provider networks. It is designed to efficiently route IP packets within a network by using a hierarchical structure of areas and levels.

In IS-IS, designated intermediate system (DIS) is a function used on broadcast or multi-access networks to reduce the amount of traffic generated by the IS-IS protocol. The DIS function is used to select a single router to be responsible for forwarding Link State Protocol Data Units (LSPs) on behalf of all other routers on the network segment. This reduces the amount of traffic generated by the protocol, as each router only sends a copy of its LSP to the DIS.

Therefore, the correct answer is A: The DIS function is used only on broadcast or multi-access networks.

Option B is incorrect because the DIS function is not used on point-to-point link types. On point-to-point links, each router forms an adjacency with its neighbor, and there is no need for a DIS.

Option C is incorrect because adjacencies are formed between all routers on the network segment, including the DIS. The DIS does not reduce adjacency overhead, but it does reduce the amount of LSP traffic generated by the protocol.

Option D is incorrect because IS-IS does not support a backup DIS mechanism. If the DIS fails, a new DIS must be elected, and the network will experience a brief period of increased LSP traffic until the new DIS is fully operational.

In conclusion, the designated intermediate system (DIS) is used to reduce the amount of traffic generated by the IS-IS protocol on broadcast or multi-access networks, and it is not used on point-to-point links.