IS-IS (Intermediate System to Intermediate System) is a routing protocol used in computer networks to facilitate the exchange of routing information between network devices. It operates at the link layer of the OSI model and is primarily used in large-scale networks, such as those found in Internet Service Providers (ISPs) and data centers. IS-IS is known for its scalability and efficiency, allowing it to support a large number of routers and complex network topologies. It uses a link-state routing algorithm, which means that each router maintains a complete map of the network, enabling it to make informed routing decisions based on the current network state.