IS-IS Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

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IS-IS Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

IS-IS protocol

Updated: June 9, 2026

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Outlines IS-IS protocol fundamentals, provides details on protocol features in Cisco IOS XR, and lists supported standards and RFCs for comprehensive understanding and compliance.

An Intermediate System-to-Intermediate System (IS-IS) protocol is a link-state interior gateway protocol that

  • supports hierarchical routing with Level 1 and Level 2 areas,

  • synchronizes network topology information using Link State Packets (LSPs), and

  • calculates optimal routing paths using the Shortest Path First (SPF) algorithm.

Background and evolution

Originally developed for the OSI protocol suite, IS-IS has evolved to support both IPv4 and IPv6 routing. It is widely used in enterprise and service provider networks because it provides scalability, fast convergence, and operational stability in large and complex topologies.

Key characteristics of IS-IS

IS-IS provides these key capabilities:

  • Support for single-topology and multitopology routing for IPv4, IPv6, and multicast

  • Hierarchical routing through Level 1 and Level 2 areas

  • Integration with advanced networking features such as traffic engineering, fast reroute, and routing security

  • Efficient operation in large-scale network environments with minimal manual route management

IS-IS protocol features in Cisco IOS XR

Cisco IOS XR implements IS-IS as a standards-based routing protocol for IPv4 and IPv6 networks. The implementation supports a broad set of features for flexible deployment and interoperability in modern network environments.

Cisco IOS XR supports these IS-IS capabilities:

  • Single-topology and multitopology IS-IS for IPv4 and IPv6

  • Flexible configuration of Level 1 and Level 2 routing

  • Support for multiple areas and multiple IS-IS instances

  • Control over SPF calculations and LSP flooding behavior

  • Route-policy integration for route control and redistribution

  • Security features, including HMAC-MD5 authentication and keychain management

  • Integration with MPLS, fast reroute, traffic engineering, and multicast routing

Standards and RFCs supported

Cisco’s IS-IS implementation adheres to these key standards and RFCs, ensuring interoperability and reliable operation across different network devices:

  • ISO/IEC 10589: Specifies the IS-IS routing protocol for OSI environments, defining the original packet structure and protocol operation.

  • RFC 1195: Extends IS-IS to support routing of IP (IPv4) traffic, enabling IS-IS for use in IP-based networks.

  • IETF IS-IS Working Group drafts: Include ongoing protocol enhancements such as IPv6 support, traffic engineering, and multitopology routing. Cisco IOS XR supports these advanced features for modern networking.

  • RFC 8405: Standardizes algorithms for shortest-path-first (SPF) interval tuning, improving efficiency and resiliency during network convergence.

  • RFC 4444: Defines Management Information Base (MIB) objects for IS-IS, enabling effective SNMP-based network monitoring and management.

Compliance with these standards allows Cisco IOS XR’s IS-IS to interoperate seamlessly across heterogeneous network environments.