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MPLS : What is MPLS , Why we need MPLS, is it still Used?

This article is just a Jump start to MPLS and overview, like why MPLS came into picture, is it still into used. We will have more articles following with all the tech terms and terminology about this technology.

What is MPLS?

  • MPLS is multiprotocol label switching technology, was an anew forwarding mechanism which replaced how are traditional IP packet was forwarded. As we know when ever router receives IP packet, it will look up in the Control Plane (Brain of router) for forwarding and data plane will process based on the lookup and exit the packet from the outgoing interface.
  • The traditional L3 Device/Router/Switch with L3 capabilities not the latest one which we used now, had to perform control lookups for each and every packet, it used to give the packet to control plane for lookup and then data plane used to process that. PLANE: Allocation of resources CPU, Memory, Bandwidth.
  • What I mean by Control plane is MAC, ARP, RIB, LIB tables. So, an IP packet used to go through multiple lookups in CP and then DP used to forward packet. What MPLS did was it brought new forwarding mechanism, where in instead of doing multiple lookups and referring CP, which used more CPU cycles, it built a forwarding table in data plane itself.
  • That forwarding table is called the Label table, which it keeps handy by using LDP ( label distribution protocol which is also dynamic and IGP based).
  • The table that MPLS refer is also called as LFIB, or label forwarding information based.
  • We already had an article for MPLS label , you can click on the below link to refer that:

MPLS LABEL : https://ipinbits.com/mpls-label/

Just Remember that MPLS is not competing IP it is complimenting IP

Enough of theory lets try to understand the above theory with an example, consider the below topology.

Let’s see without MPLS, how it will work, here the devices in the cloud is not running mpls, it would be having tradition IP forwarding, so each and every devices needs to perform route lookup in CP and then Data forwarding in DP, All the routers R-1, R-2 , R-3, R-4 would require to refer RIB in CP and then forward via data plane.

But when you enable MPLS in the cloud i.e. in ISP, Between R-1, R-2, R-3, R-4, what it does it when an IP packet comes on R-1 ingress port, it would add an label to it and forward to R-2, R-2 will swap that label (using LFIB), forward it to R-3, will pop the label (PHP), then give the naked IP packet to R-4.

So this is just and example of 4 routers, imagine 100’s of routers in an ISP, so how much control plane processing is saved and lookups are limited to data plane itself.

Don’t worry about the terminology, push, pop ,swap , PHP we will add more articles with this fundamentals of MPLS, which explain MPLS in more details.


Now the Question is why MPLS is it still relevant in the world where the Hardware delivered by the Vendors like Cisco, Juniper have wire speed transfer be it Control plane or Data plane?

Answer is Yes, it still pretty much relevant and will be there for long time, because of the following

Applications that it brings along with its fast-forwarding mechanisms.

  • Fast Forwarding Mechanism in Data plane, no need of Control plane lookups.
  • VPN applications (L3 and L2), we will have separate entire articles on this.
  • Traffic Engineering (RSVP-TE), again separate articles.

This was just an overview of why MPLS came into existence, is it still relevant and why are we still using it.

We will bring more articles in this technology soon !!!! Stay tuned at Ipinbits.com

RFC for MPLS : https://tools.ietf.org/html/rfc3031

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