Sunday, April 8, 2012

MPLS Header & Label


  • The MPLS header is 4-byte header,
    • located immediately before the IP header
    • also referred as MPLS shim header 
  • MPLS label is actually a 20-bit field in the MPLS header.
  • MPLS Label or MPLS Label Stack (specifically)
  • Fields in MPLS Header are:
    • Label:
      • length is 20-bits,with 
      • identifies the potion of a LSP
    • EXP(Experiment):
      • 3-bits in length
      • Used to map the standard IP packet Type Of Service (TOS) into the Experimental field fro MPLS Class Of Service(COS)
      • only used for experimental purpose only
    • S(Stack bit):
      • MPLS labels are stacked one on other label.
      • to indicate last MPLS header before ip header
    • TTL(Time To Live)
      • The TTl field from the IP TTL is decremented by 1 & then copied into the MPLS label TTL field.
      • When exiting from the MPLS network, MPLS label TTL value is copied back to the IP TTL field
      • If this field is set to 0, the packet will be discarded
      • this field length is 8-bits
  • MPLS label stack Placement:
    • It is placed between Layer 2 header & Layer 3 header.
    • For this some times MPLS labels stack referred as shim header
    • Router forward packets based on the MPLS label header because it comes before the Layer 3 header.
    • In MPLS, ip packets are switched instead of routed.
  • "Labels are bound to routes in the routing table"
  • In label stack, the outer label is used to forward the packet along the LSP, inner label is used to identify the VPN site. 
  • This beneath label called as the VPN label
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