Interior Gateway Protocol – IS-IS (Intermediate System - Intermediate System)

IS-IS is an interior gateway link-state routing protocol used in large enterprise and service provider networks. It operates at the data link layer (Layer 2) and uses the Link State Protocol Data Units (LSPDUs) to exchange information between directly connected routers. The protocol is mainly utilized to build and maintain a consistent view of the network topology, allowing routers to make efficient routing decisions.

The original ISO IS-IS protocol was adapted to additionally carry IP prefixes and is known as Integrated IS-IS or Dual IS-IS, hereafter referred to as IS-IS.

Here’s a summary of how IS-IS operates:

• Neighbor Discovery: When a router running IS-IS is initialized or connected to the network, it begins the process of discovering its neighboring routers. IS-IS uses a specialized Hello protocol to exchange Hello packets with other routers on the same broadcast or point...