Cisco Systems Cisco Systems Cisco Systems Deutsche Telekom Orange Orange Telekom Srbija British Telecom TDC Oy Alcatel-Lucent Ericsson Segment Routing (SR) enables any node to select any path (explicit or derived from IGPs SPT computations) for each of its traffic classes. The path does not depend on a hop-by-hop signaling technique (neither LDP nor RSVP). It only depends on a set of "segments" that are advertised by the IS-IS routing protocol. These segments act as topological sub-paths that can be combined together to form the desired path. There are two forms of segments: node and adjacency. A node segment represents a path to a node. An adjacency segment represents a specific adjacency to a node. A node segment is typically a multi- hop path while an adjacency segment is a one-hop path. SR's control- plane can be applied to IPv6 and MPLS dataplanes. Segment Routing control-plane can be applied to the MPLS dataplane: a node segment to node N is instantiated in the MPLS dataplane as an LSP along the shortest-path (SPT) to the node. An adjacency segment is instantiated in the MPLS dataplane as a cross-connect entry pointing to a specific egress datalink. This document describes the Segment Routing functions, a set of use cases it addresses and the necessary changes that are required in the IS-IS protocol.