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MPLS notes

32 bit header per label

  • 20 bit - label
  • 3 bit - exp (for QoS)
  • 1 bit (bottom of stack), the label that is closest to IP header will be marked as 1. others are 0 (multi-layered mpls)
  • 8 bit TTL

A label per FEC (Forward equivalent classes, how FEC is determined may be different from vendors)

label distribution

  • LDP
  • RSVP-TE
  • BGP-MP

label 3 - implicit-null, indicates PHP

label 0 - explicit-null

Loop prevention

  • based on IGP,
  • TTL (discard on expiring)
    • copy IP TTL to MPLS TTL on ingress, and copy back on egress (will expose intermediate routers)
    • do not copy, hard set label TTL as 255