Segment Routing: Alternate of LDP and RSVP - Test Results

In continuation to my previous post "Segment Routing: Alternate of LDP and RSVP", I would like to bring some testing results which really help me to understand better the behavior of segment routing control plane without using LDP and RSVP. In the testing environment, every router is allocated one node segment per PE.


Test Scenario 1:- Segment Routing Packets are sent across the network using multiple ECMP
This is the simplest one and works as per your IGP metric.

How does it differ from standard MPLS?
The only difference is that segment routing doesn't require any LDP to build its control plane. In MPLS labels are generated for each and every prefix in IGP or only for loopbacks. In segment routing, per PE is having single label which is called node segment.



Test Scenario 2:- Segment Routing Packets are sent across the network using single IGP Path
If destination has two ECMP paths, packets are always sent to both the paths. But this can be changed by tweaking the IGP metric and forcefully we can utilize single path instead of two. By using segment routing we are imposing two labels of different segment nodes and lookup is done according to that. By doing this we are only using a single path even though we have two ECMP paths are available.

How does it differ from standard MPLS and RSVP-TE?
If the same has to be done in MPLS, we need to enable RSVP-TE to play around it. RSVP-TE with LDP requires lot of planning in terms of bandwidth reservation and adding more complexity in terms of operations. But in SR, you don't require anything to get this done.



Test Scenario 3:- Segment Routing Packets are sent across the network not using IGP Path
Labels has been imposed according to the path required to forward the packets.
How does it differ from standard MPLS and RSVP-TE?
To get this done, we require LDP and RSVP-TE. We need to create the TE tunnels. Explicitly define the tunnel path. By doing this we are only adding the complications for the operation team. In segment routing, the all we are adding only node segment IDs in the label stack.




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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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