Segment Routing

Segment Routing uses a routing technology or technique known as source packet routing. In source packet routing, the source or ingress router specifies the path a packet will take through the network, rather than the packet being routed hop by hop through the network based upon its destination address.

It is a flexible, scalable way of doing source routing. The source chooses a path and
encodesit in the packet header as an ordered list ofsegments.Segments are identifier for any type of instruction.
However, source packet routing is not a new concept. In fact, source packet routing has existed for over 20 years. As an example, MPLS is one of the most widely adopted forms of source packet routing, which uses labels to direct packets through a network. In an MPLS network, when a packet arrives at an ingress node an MPLS label is prepended to the packet which determines the packet’s path through the network.

Each segment is identified by the segment ID (SID) consisting of a flat unsigned 32-bit integer. Segment instruction can be:

• Go to node N using the shortest path
• Go to node N over the shortest path to node M and then follow links Layer 1, Layer 2, and Layer 3
• Apply service S

With segment routing, the network no longer needs to maintain a per-application and per-flow state. Instead, it obeys the forwarding instructions provided in the packet.
Segment Routing relies on a small number of extensions to Cisco IntermediateSystem-to-Intermediate System (IS-IS) and Open Shortest Path First (OSPF) protocols. It can operate with an MPLS (Multiprotocol Label Switching) or an IPv6 data Plane, and it integrates with the rich multi service capabilities of MPLS, including Layer 3 VPN (L3VPN), Virtual Private Wire Service (VPWS), Virtual Private LAN Service (VPLS), and Ethernet VPN (EVPN).

Segment routing can be directly applied to the Multiprotocol Label Switching (MPLS) architecture with no change in the forwarding plane. Segment routing utilizes the network bandwidth more effectively than traditional MPLS networks and offers lower latency. A segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels. The segment to process is on the top of the stack. The related label is popped from the stack, after the completion of a segment.

How Segment Routing Works
A router in aSegment Routing network is capable ofselecting any path to forward traffic, whether it is explicit or Interior Gateway Protocol (IGP) shortest path. Segments represent subpaths that a router can combine to form a complete route to a network destination. Each segment has an identifier (Segment Identifier) that is distributed throughout the network using new IGP extensions.
The extensions are equally applicable to IPv4 and IPv6 control planes. Unlike the case for traditional MPLSnetworks, routers in aSegment Router network do not require Label Distribution Protocol (LDP) and Resource Reservation Protocol – Traffic Engineering (RSVP-TE) to allocate or signal their segment identifiers and program their forwarding information.

Each router (node) and each link (adjacency) has an associated segment identifier (SID). Node segment identifiers are globally unique and represent the shortest path to a router as determined by the IGP. The network administrator allocates a node ID to each router from a reserved block. On the other hand, an adjacency segment ID islocally significant and represents a specific adjacency,such as egressinterface, to a neighboring router. Routers automatically generate adjacency identifiers outside of the reserved block of node IDs. In an MPLS network, a segment identifier is encoded as an MPLS label stack entry. Segment IDs direct the data along a specified path. There are two kinds of segment IDS:

• Prefix SID— A segment ID that contains an IP address prefix calculated by an IGP in the service provider core network. Prefix SIDs are globally unique. A node SID is a special form of prefix SID that contains the loopback address of the node as the prefix. It is advertised as an index into the node specific SR Global Block or SRGB.
• Adjacency SID— A segment ID that contains an advertising router’s adjacency to a neighbor. An adjacency SID is a link between two routers. Since the adjacency SID is relative to a specific router, it is locally unique.