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2. Design of the YANG Module for IS-IS MPLS Segment Routing

The IS-IS SR MPLS YANG module requires support for the base SR module [RFC9020], which defines the global SR management independent of any specific routing protocol configuration, and support of the IS-IS base model [RFC9130], which defines the basic IS-IS configuration and state.

The "ietf-isis-sr-mpls" data model defines both the data nodes to configure IS-IS SR MPLS extensions and the additions to the IS-IS Link State Protocol Data Units (LSPs) necessary to support MPLS SR.

2.1. Segment Routing Activation​

Activation of IS-IS SR MPLS is done by setting the "enable" leaf to true. This triggers advertisement of SR MPLS extensions based on the configuration parameters that have been set up using the base SR module.

Configuration Parameters​

The module provides the following configuration parameters:

Segment Routing Global Block (SRGB):

  • Global configuration for the IS-IS instance
  • Defines the range of global Segment Identifiers (SIDs) that can be used in the network
  • Includes start value and range size

Segment Routing Local Block (SRLB):

  • Defines local segment identifier range
  • Used for SIDs with local significance

Maximum SID Depth (MSD):

  • Indicates the maximum SID stack depth the node can process
  • Helps in path computation to determine feasible segment routing paths

State Information​

The IS-IS MPLS Segment Routing module augments the IS-IS module to provide:

  • Segment routing capability state
  • SRGB/SRLB state
  • MSD values

2.2. Advertising Mapping Server Policy​

The base SR module defines mapping server policies. By default, IS-IS will not advertise or process any mapping server entry. The IS-IS SR MPLS module allows the advertisement of one or multiple mapping server policies through the "bindings/advertise/policies" leaf-list. The "bindings/receive" leaf controls the reception and process of mapping server entries.

Mapping Configuration​

Each mapping associates:

  • IP Prefix: IPv4 or IPv6 prefix
  • SID Value: Segment identifier assigned to that prefix
  • SID Flags: Flags controlling SID behavior
    • For example: local/global, persistent/ephemeral, etc.

Use Cases​

The mapping server is primarily used for:

  • Interoperability: Help nodes that do not support segment routing
  • Transition Scenarios: Provide smooth transition while SR is being gradually deployed in the network
  • Centralized Management: Centrally configure SID allocation policies

2.3. IP Fast Reroute​

The IS-IS SR MPLS module augments the Fast Reroute (FRR) container under interface. It brings the ability to activate Topology Independent Loop-Free Alternate (TI-LFA) and also enhances Remote LFA (RLFA) to use SR tunneling instead of LDP.

Augmenting IS-IS FRR​

The IS-IS MPLS Segment Routing module augments the IS-IS Fast Reroute configuration defined in the IS-IS model, adding:

Loop-Free Alternate (LFA):

  • TI-LFA (Topology Independent LFA): Loop-free backup paths implemented using segment routing
  • Can be configured for specific levels and interfaces

TI-LFA Advantages​

Compared to traditional LFA, TI-LFA provides:

  1. Higher Protection Coverage: Can provide protection for more failure scenarios
  2. Topology Independent: Does not depend on specific network topology
  3. Deterministic: Pre-computed paths ensure fast switchover

Configuration Parameters​

  • Enable/Disable TI-LFA: Per level or per interface
  • Backup Path Computation Method:
    • Node protection
    • Link protection
    • Downstream paths
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