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3. IS-IS Segment Routing over MPLS YANG Module

This section presents the complete YANG module for IS-IS Segment Routing extensions over the MPLS data plane.

Module Structure​

The ietf-isis-sr-mpls YANG module augments the IS-IS base YANG model [RFC9130] and integrates with the segment routing base module [RFC9020].

Module Header​

module ietf-isis-sr-mpls {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-isis-sr-mpls";
  prefix isis-sr-mpls;

  import ietf-routing {
    prefix rt;
    reference
      "RFC 8349: A YANG Data Model for Routing Management
                 (NMDA Version)";
  }

  import ietf-segment-routing-mpls {
    prefix sr-mpls;
    reference
      "RFC 9020: YANG Data Model for Segment Routing";
  }

  import ietf-isis {
    prefix isis;
    reference
      "RFC 9130: YANG Data Model for the IS-IS Protocol";
  }

  organization
    "IETF LSR - Link State Routing Working Group";

  contact
    "WG Web:   `https://datatracker.ietf.org/wg/lsr/`
     WG List:  <mailto:[email protected]>

     Editor:   Stephane Litkowski
               <mailto:[email protected]>
     Editor:   Yingzhen Qu
               <mailto:[email protected]>
     Editor:   Acee Lindem
               <mailto:[email protected]>
     Editor:   Ing-Wher Chen
               <mailto:[email protected]>
     Editor:   Jeff Tantsura
               <mailto:[email protected]>";

  description
    "This YANG module defines the configuration and operational
     state for IS-IS extensions for Segment Routing (SR) over the
     MPLS data plane.

     This YANG module conforms to the Network Management
     Datastore Architecture (NMDA) as described in RFC 8342.

     Copyright (c) 2025 IETF Trust and the persons identified as
     authors of the code.  All rights reserved.

     Redistribution and use in source and binary forms, with or
     without modification, is permitted pursuant to, and subject
     to the license terms contained in, the Revised BSD License
     set forth in Section 4.c of the IETF Trust's Legal Provisions
     Relating to IETF Documents
     (https://trustee.ietf.org/license-info).

     This version of this YANG module is part of RFC 9902;
     see the RFC itself for full legal notices.";

  revision 2025-12 {
    description
      "Initial revision.";
    reference
      "RFC 9902: A YANG Data Model for IS-IS Segment Routing
                 over the MPLS Data Plane";
  }
}

Key Data Nodes​

Segment Routing Activation​

The module provides an enable leaf to activate IS-IS SR MPLS functionality:

Path: /rt:routing/rt:control-plane-protocols/rt:control-plane-protocol/isis:isis/segment-routing/enabled

Type: boolean
Default: false

When enabled, the router advertises SR capabilities based on the SRGB/SRLB configuration from the base SR module.

Mapping Server Configuration​

Advertise Policies:

  • Path: /rt:routing/rt:control-plane-protocols/rt:control-plane-protocol/isis:isis/segment-routing/bindings/advertise/policies
  • Type: leafref to mapping server policy names
  • Description: Controls which mapping server policies are advertised in IS-IS

Receive Bindings:

  • Path: /rt:routing/rt:control-plane-protocols/rt:control-plane-protocol/isis:isis/segment-routing/bindings/receive
  • Type: boolean
  • Description: Controls whether to receive and process mapping server entries

TI-LFA Configuration​

The module augments the IS-IS interface fast-reroute configuration to support Topology Independent Loop-Free Alternate (TI-LFA):

Path: /rt:routing/rt:control-plane-protocols/rt:control-plane-protocol/isis:isis/interfaces/interface/fast-reroute/ti-lfa

Configuration Options:

  • enable: Enable/disable TI-LFA
  • level: Apply to level-1, level-2, or both
  • node-protection: Enable node protection

Operational State Data​

Router Capabilities​

The module augments IS-IS router capabilities to include:

SR Capability:

  • Path: /rt:routing/rt:control-plane-protocols/rt:control-plane-protocol/isis:isis/router-capabilities/sr-capability
  • Contents:
    • SRGB ranges
    • SR algorithms supported
    • MPLS encapsulation capability

SR Algorithms:

  • Path: /rt:routing/rt:control-plane-protocols/rt:control-plane-protocol/isis:isis/router-capabilities/sr-algorithms
  • Type: List of algorithm identifiers
  • Common Values:
    • 0: Shortest Path First (SPF)
    • 1: Strict Shortest Path First

Local Blocks:

  • Path: /rt:routing/rt:control-plane-protocols/rt:control-plane-protocol/isis:isis/router-capabilities/local-blocks
  • Contents: SRLB range information

LSP Database Augmentations​

The module adds SR-specific TLVs to IS-IS LSP database entries:

Prefix-SID Sub-TLV:

  • Algorithm identifier
  • SID/Index/Label value
  • Flags (R-flag, N-flag, P-flag, E-flag, V-flag, L-flag)

Adjacency-SID Sub-TLV:

  • Flags (F-flag, B-flag, V-flag, L-flag, S-flag, P-flag)
  • Weight
  • SID/Label/Index

SID/Label Binding TLV:

  • Prefix
  • Range
  • SID/Label mappings

Usage Example​

See Appendix A for a complete configuration example.

Complete Module Definition​

For the complete YANG module definition including all data nodes, augmentations, and notifications, please refer to:

Note: The complete YANG module code (approximately 500+ lines) includes detailed descriptions, constraints, and references for each data node. Network operators should use YANG validation tools to ensure configuration compliance with the module's constraints.

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