• Forwards based on labels instead of L3 destinations.
  • MPLS not much faster than today’s traditional IP forwarding, but MPLS does create less overhead.
  • Can forward any L3 protocols, not just IP.


  • Label Switching Routers
    • Routers that run MPLS and can receive/transmit labeled packets.


  • Label Switched Path
    • Entire labeled path through an MPLS Domain.
    • Sequence of routers.
    • Unidirectional.
      • Source to destination and vice versa can be completely different paths.
      • Often return is similar due to routing protocols.

MPLS Header

  • Shim between layer 2 frame and layer 3 packet – hence MPLS L2.5
  • 4 bytes/32 bits.
    • 1st 20 bits – label number
    • 2nd 3 bits – experimental, used for QoS, similar to DSCP.
    • 3rd 1 bit – S field, used to show whether last label in stack.
    • 4th 8 bits – TTL, similar to IP TTL used to discard if reaches 0.


  • Label Distribution Protocol
    • Uses Multicast to send hellos to other LDP routers, create neighbor adjacency.
    • UDP to discover neighbors. Adjacency built via TCP.
    • Transport Address is what’s used to build the TCP connection for adjacency.
      • Will choose an IP from neighbor router, need to make sure its actually routable in the IGP.
    • LDP generates a label locally for each prefix found in the RIB. Label information is then added to the Label Information Base (LIB).
    • LIB is used to generate info in the LFIB.
    • show mpls ldp bindings
      • shows LIB of MPLS router
    • show mpls forwarding-table
      • shows LFIB of MPLS router.
    • The Label Switched Path (LSP) can be seen through a traceroute on MPLS router.

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