Wednesday, April 11, 2012

MPLS TTL field & It propagation


  • MPLS TTL is similar to IP header's TTL
  • IP header's TTL used for:
    • identifying loops
    • traceroute command to find the ip address of each router in a particular end to end route.
  • MPLS TTL used for same above ip TTL functions.
  • From this we confirmed that, presence or absence of MPLS in a network has no effect on the TTL related processes.
  • When switching LSR will decrement the MPLS TTL but not the IP TTL.
  • TTL in MPLS network:
    • At Ingress E-LSR:
      • It decrements the IP TTL field in unlabeled packet
      • then push a label in unlabeled packet
      • & copy the decremented IP TTL into the new MPLS TTL.
    • At LSR:
      • When LSR swaps a label, MPLS TTL will be decremented 
      • & doesn't effect the IP TTL
    • At Egress E-LSR:
      • After an egress E-LSR decrements the MPLS TTL field, it pops the MPLS label (header)
      • & then copies the MPLS TTL to the IP TTL.
  • A looping packet would decrements to TTL 0 and discarded.
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