MPLS introduction

Traditional IP routing

In traditional IP routing world routing protocols are used to distribute L3 routing information. A forwarding decission is made on packet header and local routing table. Routing lookups are indepedently performed at every hop.

Basic MPLS features

MPLS is forwarding mechanism in which packets are forwareded based on labels. MPLS can run on various L2 technologies like FR, PPP, ATM, Ethernet. MPLS leverages both IP routing and CEF switching. CEF must be ON for MPLS to run.

When customer IP packet enters SP network it is label switched, when it leaves the SP network the label is removed from the packet.


In control plane routers run routing protocols and IP routing table (RIB). From these tables FIB is build and push into data plane ASICs.

When you run MPLS you run also in control plane LDP – label distribution protocol. And from this the label forwarding information base (LFIB) is build and pushed to ASICs.

MPLS basic terminology

A sequence of labels to reach a destination is called LSP (label switch path)

LSRs (Label switch routers)

  • forward the pakets based on labels and also swap labels. Last LSR in the path removes the label and forward the IP packet

Edge LSRs

  • labels IP packets and forwards them into MPLS domain
  • forwards IP packet out of MPLS domain

Benefits of MPLS

  • Unicast and Multicast IP routing (multicast MPLS is more advanced thing and it belongs to multicast VPNs group)
  • MPLS decreases forwarding overhead on core routers (routers dont have to look into routing table, find match, find outgoing interface and send out)
  • BGP free core – SP core routers dont run BGP but MPLS. Just PE routers run BGP.
  • MPLS can support forwarding of non-IP protocols
  • VPN
  • TE
  • QoS
  • ATOM – you can run between CE and PE routers PPP, HDLC, Ethernet, whatever – Any Transport over MPLS 😉