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2. Design of the YANG Module for OSPF MPLS Segment Routing

The OSPF MPLS Segment Routing YANG module is an augmentation of the OSPF YANG model [RFC9129].

Module Structure Overview​

This YANG module defines the data model for configuring and managing OSPF Segment Routing extensions, including:

Core Configuration Elements​

Segment Routing Global Block (SRGB):

  • Global configuration for OSPFv2/OSPFv3 instances
  • Defines the global Segment Identifier (SID) range
  • Includes starting label value and range size
  • Example: SRGB [16000-23999] represents 8000 available global SIDs

Segment Routing Local Block (SRLB):

  • Defines the local Segment Identifier range
  • Used for node-specific local SID allocation
  • Not propagated within the AS

Maximum SID Depth (MSD):

  • Indicates the maximum SID stack depth a node can process
  • Advertised to other nodes via OSPF LSAs
  • Used for path computation and traffic engineering

Segment Routing Algorithm Support​

The module supports configuration of:

  • Prefix Segment Identifier (Prefix-SID): SID associated with a prefix
  • Adjacency Segment Identifier (Adjacency-SID): SID associated with a link
  • Algorithm Types: Shortest Path First (SPF), Strict SPF, etc.

OSPFv2 and OSPFv3 Support​

The module supports both:

  • OSPFv2 [RFC2328] [RFC8665]: IPv4 networks
  • OSPFv3 [RFC5340] [RFC8666]: IPv6 networks

Both protocol versions share the same Segment Routing configuration model structure.

Configuration Hierarchy​

routing-instance
  └── routing-protocol (OSPF)
      └── ospf
          ├── segment-routing
          │   ├── enabled (true/false)
          │   ├── srgb
          │   │   ├── lower-bound
          │   │   └── upper-bound
          │   ├── srlb
          │   │   ├── lower-bound
          │   │   └── upper-bound
          │   └── msd
          └── areas
              └── area
                  └── interfaces
                      └── interface
                          └── segment-routing
                              └── adjacency-sid

State Data​

The module also provides operational state data, including:

  • Currently active SRGB/SRLB ranges
  • Allocated Prefix SIDs
  • Adjacency SID states
  • MSD capabilities

Integration with OSPF Base Model​

This module extends the OSPF base YANG model [RFC9129] through "augment" statements:

  • Adds Segment Routing-specific configuration nodes
  • Extends interface configuration to support Adjacency SIDs
  • Adds Segment Routing-related LSA state information

Compatibility Considerations​

The module design ensures:

  • Backward compatibility with OSPF implementations that do not support Segment Routing
  • Segment Routing functionality is optionally configurable
  • Coexistence with traditional OSPF traffic engineering
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