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Introduction to MPLS

What is MPLS?

  • Multi-Protocol Label Switching is a new paradigm in the way routers make forwarding decisions.
  • Multiprotocol: MPLS is capable of transporting many L2 and L3 protocols
  • Label Switching: Packets are switched based on labels, not destination IP!
  • Labels - Ultimately determine the destiny of a packet through an MPLS network

Why MPLS?

  • Unified network infrastructure
  • ATM concepts integrated into an IP world
  • Scalable and secure L3 VPNs using a peer-to-peer model
  • BGP-free core
  • Traffic engineering capabilities
  • WAN Quality of Service (QoS) requirements

MPLS Key Terms

MPLS Labels

  • MPLS is often described as a shim or layer 2.5 protocol
  • An MPLS packet contains a label stack, consisting of one or more MPLS labels

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  • Label - the MPLS label itself, determines destiny of the packet
  • EXP- Experimental (not so much any more), used for QoS markings
  • S - Boolean, if set to 1, it's bottom of the label stack. If 0, there's another label on the stack
  • TTL - Time-to-live for loop prevention and hop count (as it is in IP)

Push vs Pop vs Swap - The swap operation means that the top label in the label stack is replaced with another - The push operation means that the top label is replaced with another and then one or more additional labels are pushed onto the label stack - The pop operation means that the top label is removed

Label Switched Routers

  • A label switched router (LSR) is a router running MPLS
  • LSR can be an ingress edge, intermediate or egress edge LSR
  • A Label Switched Path (LSP) is the series of LSRs that switch a labeled packet

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Forwarding Equivalency Class (FEC)

  • Group of packets that are forwarded the same way through the MPLS network
  • Every packet in the same FEC enters the MPLS network with the same label
  • FEC can theoretically be based on many different things L3 destination prefix, QoS, L2 PVC, BGP next-hop, TE tunnel, link color, etc...