Segment Routing: Alternate of LDP and RSVP

A typical service provider network is having a customer edge router (CE), provider edge (PE), provider (P) and route reflector (RR) routers. Protocols used by Service Provider are OSPF/ISIS for IGP, MP-BGP for VPN, LDP for Labels and RSVP-TE for steering the particular traffic or reserving bandwidth for specific application or destination.

Problems with the existing network:-
1. Forwarding is happening on the basis of destination not as per the requirement of source.
2. Source can never dynamically demand the bandwidth and delay for reaching out particular application. (This can be done by using RSVP-TE but this is more a static task rather than dynamic task)
3. All the signaling is happening hop by hop.
4. In case of any link congestion, traffic can’t be steered automatically.
5. To provide guaranteed bandwidth, full mesh RSVP-TE need to be implemented either in the core or edge or in the full SP network. This requires lot of resources.
6. As we are moving towards the application word but don’t see any kind of linkage between the network and application world.
7. Online or Proactive Capacity Planning
8. In the current topology, SP can’t define the dual-plane networks.

Can Segment Routing help to overcome the existing issues?
Segment Routing (SR - IETF Draft) enables any node to select any explicit path for each of its traffic classes. This explicit 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 link-state routing protocol (IS-IS/OSPF). These segments act as topological sub-paths that can be combined together to form the desired explicit path. There are two forms of segments: nodal and adjacency. A nodal segment represents a path to a node. An adjacency segment represents a specific adjacency to a node. A nodal segment is typically a multi-hop path while an adjacency segment is a one-hop path.

The utmost Advantage of using Segment Routing helps service provider to reduce the complexity of network by reducing the number of protocols. As we are moving towards the application world, this can be easily integrated with applications world to take dynamic decisions. Analytics seems to be a big factor behind this because that is node which helps to take dynamic decision. Along with this Segment Routing provides automatic traffic protection without implementing FRR.


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