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,
- IGP , either OSPF or ISIS for prefix exchange between neighbours
- LDP, for label exchange
- RSVP for traffic engineering and FRR for link and node failure
- BGP , MP-BGP for L3VPN BGP-LU for connecting different IGP domains
- 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,
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.