Cisco Systems Cisco Systems Cisco Systems RtBrick Inc. Segment Routing (SR) leverages the source-routing paradigm. A node steers a packet through an ordered list of instructions called "segments". A segment can represent any instruction, topological or service based. The ingress node prepends an SR header to a packet containing a set of segment identifiers (SIDs). Each SID represents a topological or service-based instruction. Per-flow state is maintained only on the ingress node of the SR domain. An "SR domain" is defined as a single administrative domain for global SID assignment. This document defines an optional, transitive BGP attribute for announcing information about BGP Prefix Segment Identifiers (BGP Prefix-SIDs) and the specification for SR-MPLS SIDs.