Segment Routing is a new technology that will increase the benefit to IP and MPLS networks. It is an alternate to LDP and RSVP which means without using the LDP and RSVP we still can generate and distribute the transport labels and steer the traffic without using the RSVP signalling. Segment routing is based on label switching but for labels generation and distribution LDP and RSVP is not used. Segment Routing is an extension to an IGP (OSPF/ISIS). Labels are called segments in Segment Routing.
As per IETF draft “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.Segment routing uses IP control plane but MPLS and IPv6 as data plane for its operation. There are two main components of Segment Routing:- 1. Control Plane:- Generation and distribution of transport labels across the Segment Routing domain by using IGP (OSPF/ISIS) 2. Data Plane:- Add the labels (Segments) on the packet header Different Type of Data Plane Operation supported by Segment Routing 1. Continue:- Forwarding action based on active segment 2. Push:- Add a segment to the SR header of the packet and set that as Active Segment 3. Next:- Mark the next segment as the active segment and execute the instruction encoded by the new active segment Comparison between MPLS and Segment Routing Operation
| Segment Routing Operation | MPLS Operation |
|---|---|
| Segment Routing Header/Segment List | Label Stack Header |
| Active Segment | Topmost Label |
| Push | Label Push |
| Next | PoP |
| Continue | Swap |
| Segment Id | MPLS Label |
2. Adjacency Segment:- Each router will assign a locally significant segment ID for each of its IGP adjacencies and it is not globally unique like Node/Prefix SID. Segment Routing enabled routers allocate Adjacency segment ID for their all attached interfaces automatically when the segment routing is enabled on the router.
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