Cisco Systems Cisco Systems Cisco Systems RtBrick Inc. The Segment Routing (SR) architecture allows a node to steer a packet flow through any topological path and service chain by leveraging source routing. The ingress node prepends an SR header to a packet containing a set of segment identifiers (SID). Each SID represents a topological or a service-based instruction. Per-flow state is maintained only on the ingress node of the SR domain. Ah SR domain is defined as a single administrative domain for global SID assignment. This document defines an optional, transitive BGP attribute for announcing BGP Prefix Segment Identifiers (BGP Prefix-SID) information.