Sunday, July 12, 2015

Basics Of Segment Routing

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 OperationMPLS Operation
Segment Routing Header/Segment ListLabel Stack Header
Active SegmentTopmost Label
PushLabel Push
Segment IdMPLS Label

There are two main types of segments in Segment Routing:-
1. Node Segment (Node/Prefix SID):- Node segment ID which is also called as Prefix segment ID is used to specify loopback interface of Segment Routing enabled device. The forwarding is associated with Node segment ID. The operator assigns a domain wide unique Node segment ID for each router in the network. This can be done manually or using a centralized controller (SDN Use Case).

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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