Segment Routing v6 Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

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Segment Routing v6 Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

SRv6 IS-IS

Updated: September 8, 2026

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Explains SRv6 integration with IS-IS by defining its routing components and describing how IS-IS advertises SRv6 capabilities, locators, and SIDs.

SRv6 IS-IS is an extension to the Intermediate System-to-Intermediate System (IS-IS) protocol that

  • supports Segment Routing with an IPv6 data plane

  • enables the advertisement of SRv6 capabilities of nodes, and

  • enables the advertisement of node and adjacency segments as SRv6 SIDs within the IS-IS routing domain.

Key concepts of SRv6 IS-IS

  • Intermediate System-to-Intermediate System (IS-IS) protocol: A link-state interior gateway protocol (IGP) used for routing within an administrative domain. SRv6 IS-IS extends this protocol to advertise SRv6-specific information.

  • SRv6 SID (Segment Identifier): An IPv6 address that represents a specific network function or instruction. SIDs are the fundamental building blocks of SRv6 paths and are advertised by nodes within the network. See Segment Identifiers.

  • Node segment: An SRv6 SID that identifies a specific node (router) in the network. When a packet is steered to a node SID, that node processes the packet.

  • Adjacency segment: An SRv6 SID that identifies a specific link (adjacency) between two nodes. When a packet is steered to an adjacency SID, it is forwarded over that specific link.

  • SRv6 locator: An IPv6 prefix that identifies an SRv6-enabled node and is advertised by the IS-IS protocol. Locators act as the "home address" for SIDs allocated by that node, providing reachability to the SIDs. See Enable Traffic Steering with SRv6 Locators.

  • IGP domain: The collection of routers and links that operate under a single instance of an Interior Gateway Protocol (IGP), such as IS-IS. SRv6 SIDs and locators are advertised and learned within this domain.

SRv6 IS-IS functionalities

SRv6 IS-IS performs these functionalities:

  • Interacts with SID Manager to learn local locator prefixes and announces the locator prefixes in the IGP domain.

  • Learns remote locator prefixes from other ISIS neighbor routers and installs the learned remote locator IPv6 prefix in RIB or FIB.

  • Allocates or learns prefix SID and adjacency SIDs, creates local SID entries, and advertises them in the IGP domain.