Segment Routing

Segment routing control plane

Segment routing control plane | Segment routing control plane consist of routing protocols to distribute segment id in the network. SR control plane is specified for IGP protocols IS-IS and for Border gateway protocol (BGP).

Segments distributed by IGP are called IGP segments and those distributed by BGP are called BGP segments.  There are two types of IGP segments ,

1. IGP Prefix Segment – It is distributed by either ISIS or OSPF . IGP prefix segment is associated with prefix advertised by IGP. It signify steer traffic along ECMP aware shortest path to the prefix associated with this segment. Prefix segment is fundamental building block of segment routing .Typically, Prefix segment is identified by Node-SID which is Global  unique in the network , assigned to the loopback address of the node.

2. IGP Adjacency Segment – This type of segment is associated with unidirectional adjacency of IS-IS or OSPF.  It signify  to steer traffic  out to link of the adjacency (ISIS or OSPF) of the segment . Traffic is always steered on the link regardless of shortest path routing. Adjacency segment is local significant to the node of the segment.

And there are two types of BGP segments,

3. BGP Prefix Segment – BGP prefix segment is associated with BGP prefixes , similar to IGP prefix segment associated with IGP prefixes. It signify steer traffic along  the ECMP aware BGP multi-path to the prefix associated with this segment.

4. BGP Peer Segment – BGP peer segments are associated with BGP Peering session to a specific  neighbour . BGP peer segments are applicable  to both IBGP and EBGP.


Why Segment Routing

Lets understand why segment routing is required aka problem statement with existing protocol stack.

We have  been using MPLS (LDP based MPLS) for more than decade now, it is the de-facto forwarding mechanism in almost all telco grade networks. There is stack of protocols used as given below,

  1. IGP , either OSPF or ISIS for prefix exchange between neighbours 
  2. LDP, for label exchange
  3. RSVP for traffic engineering and FRR for link and node failure
  4. BGP , MP-BGP for L3VPN BGP-LU for connecting different IGP domains 
  5. Targeted LDP for layer-2 connectivity , pseudowire etc 

As can see, there is large protocols stack required for MPLS based network along with L2/L3 services on the top of that. Large protocol stack makes management and troubleshooting very difficult , some times, IGP and LDP tables gives synchronisation issues and result into Network downtime. 

Now, with Segment routing , the  protocol stack reduces drastically helps management and troubleshooting easier . Below is reduced protocol stack,

  1. IGP/SR
  2. BGP

What it means is that no more LDP, RSVP , T-LDP. All the functions done by IGP itself. With SR, IGP either ISIS or OSPF, is not only responsible to exchange IP prefix information but also responsible to exchange label information. Label is SR world is called Segment id. 

Moreover, all the limitations of RSVP are eliminated , fast re-route is no more dependent  of RSVP , it is in-built into IGP now, which is called TI-LFA (topology in-dependent loop free alternate), not only that , Segment routing revolutionize traffic engineering completely which is the foundation of Software defined networking . We will discuss all these topics in length in the  subsequent Posts .

I will continue Segment Routing  basics in next post , will introduce working principle, control plane and data plane.


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