Which feature describes the adjacency SID?

An Adjacency Segment Identifier (SID) is a fundamental component of Segment Routing (SR) that represents a specific, direct adjacency to a neighboring router. A key characteristic of an Adjacency SID is its local significance. This means the SID value is allocated by and is only meaningful to the local router that advertises it. Another router in the same SR domain can use the identical SID value for one of its own adjacencies without conflict. This contrasts with Prefix SIDs, which must be globally unique within the SR domain to ensure unambiguous routing to a specific node or prefix.

A. It applies only to point-to-point links. This is incorrect. Adjacency SIDs can be assigned to adjacencies on any link type, including point-to-point, broadcast, and non-broadcast multi-access (NBMA) links.
B. It applies only to multipoint links. This is incorrect. Adjacency SIDs are commonly used on point-to-point links and are not restricted to multipoint or broadcast link types.
D. It is globally unique. This is incorrect. Global uniqueness is a defining characteristic of Prefix SIDs, which are allocated from the Segment Routing Global Block (SRGB). Adjacency SIDs are explicitly local.

1. RFC 8402: Segment Routing Architecture.

Section 3.2

"Adjacency Segment": "An Adjacency SID is a local label that identifies an adjacency to a neighbor. An Adjacency SID is locally significant to the node that allocates it." This statement directly confirms the local uniqueness of the Adjacency SID.

2. Cisco Systems

"Segment Routing Configuration Guide

Cisco IOS XE Bengaluru 17.6.x"

Chapter: "Segment Routing for MPLS Data Plane"

Section: "Segment Identifier": "Adjacency SID—An adjacency SID is a local label that represents a specific adjacency

such as a link

to a neighboring router." The term "local label" explicitly denotes its local significance.

3. Cisco Systems

"Segment Routing MPLS Architecture" White Paper.

Section: "Segment Routing Building Blocks"

Subsection: "Adjacency SID": "The Adjacency SID is by nature a local SID. Its value is relevant only to the router that advertises it." This document reinforces that the scope of an Adjacency SID is confined to the originating router.