MPLS

• 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.

LSR

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

LSP

• 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.

LDP

• 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.