Segment routing adds the ability to perform shortest path routing and source routing using the concept of abstract segments to IS-IS and OSPF routing protocols. A segment can represent a local prefix of a node, a specific adjacency of the node (interface/next hop), a service context, or a specific explicit path over the network. For each segment, the IGP advertises an identifier referred to as a Segment ID (SID).
When segment routing is used together with the MPLS data plane, the SID is a standard MPLS label. A router forwarding a packet using segment routing will therefore push one or more MPLS labels.
Segment routing using MPLS labels can be used in both shortest path routing applications (see the 7705 SAR Routing Protocols Guide for information) and in traffic engineering (TE) applications, as described in this section.
The following are the objectives and applications of segment routing:
ability for a node to specify a unicast shortest-route or source-routed forwarding path with the same mechanism; IGP can be reused to minimize the number of control plane protocols
ability to use IGP-based MPLS tunnels without the addition of any other signaling protocol
ability to tunnel services from ingress PE to egress PE with or without an explicit path and without requiring forwarding plane or control plane state in intermediate nodes
ability to use Layer 3 spoke SDP interfaces to support multicast for segment routing. See the 7705 SAR Routing Protocols Guide, ‟Multicast for Segment Routing”.
FRR: ability to expand coverage of basic LFA to any topology with the use of a source-routed backup path; precomputation and setup of backup path without additional signaling
support for LFA policies with shared-risk constraints, admin-groups, and link/node protection
support for SR-TE entropy labels
support for TE that includes loose/strict options, distributed and centralized TE, path disjointness, ECMP awareness, and limited or no per-service state on midpoint and tail-end routers
support for fine-grained flow steering and service chaining via a centralized stateful Path Computation Element (PCE) such as the one provided by the Nokia Network Services Platform (NSP)