IS-IS

A Protocol Designed for the Core

Intermediate System to Intermediate System (IS-IS) is a link-state routing protocol originally designed for ISO's CLNS (Connectionless Network Service) and adapted for IP routing (Integrated IS-IS, RFC 1195). Like OSPFOpen Shortest Path First. An interior gateway routing protocol that uses link-state advertisements and Dijkstra's algorithm to compute the shortest path within an autonomous system., IS-IS builds a complete topology map of the network using LSPs (Link State PDUs) and runs Dijkstra's Shortest Path First algorithm to compute loop-free paths. It is the dominant IGP in large service provider and internet backbone networks.

Why ISPs Prefer IS-IS Over OSPF

IS-IS has several architectural advantages for large-scale deployments. It runs directly over Layer 2 (not over IP like OSPF), making it slightly more resilient to IP-layer issues. IS-IS uses a two-level hierarchy (Level 1 for intra-area, Level 2 for inter-area backbone) without the complex area types of OSPF (backbone/stub/NSSA). Critically, IS-IS was extended to support both IPv4Internet Protocol version 4. The fourth revision of IP using 32-bit addresses (e.g., 192.168.1.1), providing approximately 4.3 billion unique addresses. Still the most widely used internet protocol despite address exhaustion. and IPv6Internet Protocol version 6. The successor to IPv4 using 128-bit addresses (e.g., 2001:0db8::1), providing a virtually unlimited address space of 3.4 x 10^38 addresses. Designed to solve IPv4 address exhaustion. with relatively simple additions, while OSPF required a separate protocol (OSPFv3) for IPv6.

IS-IS in Modern Networks

Major internet exchange participants and tier-1 carriers predominantly use IS-IS internally alongside BGPBorder Gateway Protocol. The routing protocol that exchanges reachability information between autonomous systems, effectively determining how data traverses the internet. Often called the "postal service of the internet." for external routing. IS-IS's proven stability, minimal resource overhead compared to OSPF in very large topologies, and strong vendor support (Juniper, Cisco, Nokia) have kept it dominant in provider cores despite OSPF's greater market share in enterprise environments. The Routing TableA data structure stored in a router that lists known network destinations and the next hop or interface for reaching each one. Routing tables are populated by static configuration or dynamic routing protocols like BGP and OSPF. computed by IS-IS carries an administrative distance of 115 on Cisco devices, slightly preferred over OSPF's 110 in some redistribution scenarios.