Sunday, April 8, 2012

MPLS LDP


  • LSRs uses LDP to send messages to their neighbors.
  • By advertising an IP prefix & label in the update, the LSR says:
    • if you want to send packets to this prefix, send them(packets) to me"
  • Stand for RFC 3036 "LDP specification"
  • Neighbor discovery:
    • send via UDP port 646 to 224.0.0.5
  • Neighbor adjacency
    • uses tcp port 646 to remote LDP router-id
  • Label advertisement 
    • Advertise FEC for
          • connected IGP interfaces
          • IGP learned routes
  • For MPLS unicast ip routing:
    • LDP simply advertises labels for each prefix listed in the IP routing table.
  • New ip route in the unicast ip routing table triggers the LDP advertisement.
  • To learn the new route LSR allocates a label called a local label
  • Local Label:
    • which represent the ip prefix just added to the routing table.