Intermediate System to Intermediate System (IS-IS)

Intermediate System to Intermediate System, or IS-IS, is a dynamic link-state routing protocol used to route traffic within an autonomous system. Routers exchange information about their directly connected links and use this data to build a shared view of the network topology. Each router then calculates suitable paths to reachable networks. IS-IS distinguishes between Level 1 routing within an area and Level 2 routing between different areas. This hierarchical structure supports scalability in large networks. The protocol can support both IPv4 and IPv6 and is particularly common in large service provider, carrier and backbone networks. Its scalability and fast convergence make IS-IS suitable for network infrastructures with demanding availability and routing performance requirements.