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A brief about MPLS Header & Label

November 23, 2018

A brief about MPLS Header & Label

MPLS stands for Multi Protocol Label Switching which actually using switching ( Label) in the core and routing on the edges. The important part of understanding here is the MPLS label for which we will have a discussion point today.

Fig 1.1- MPLS Label

Note⭐ MPLS header is of 32 bits and MPLS header consists of 4 fields and these fields  

Label: Label is of 20 bits and Label provides the MPLS Label value. The purpose of the MPLS label is to decide on the forwarding Path in the MPLS environment. As you know that multiple labels can be used for the MPLS packet encapsulation. We will talk number of the labels used in the MPLS network.

EXP: EXP stands for Experimental bits and is of 3 bits. These are used for QoS, normally the IP precedence value of the IP packet will 

S bit: S stands for "bottom of stack" and is 1 bit in size. The important thing you need to know here is When this bit is set to one, it’s the last MPLS label. When it’s set to zero then there is one or more MPLS labels left.

TTL: TTL stands for time to live and is 8 bits of size. Each hop decrements the TTL by one.

Note⭐ MPLS header is added in between the L2 and L3 header of the packet

QoS Marking in MPLS Labels
MPLS label has 3 Traffic Class bits and is used for packet classification and prioritization. The DSCP values of IP packet are mapped into TC bits of MPLS label

Points to remember for MPLS labels:

  • MPLS Label has local significance
  • MPLS uses two labels while normal operations and three labels when using MPLS traffic engineering
  • Outer label always used for switching MPLS packets in network
  • Inner labels usually used for services (e.g. L2/L3 VPN)
  • MPLS Reserved label range is: 0-15
  • 16 to 1023 Label Range: label space shared by static LSPs and static constraint-based routed LSPs (CR-LSPs)
  • 1024 or above Label Range: label space for dynamic signaling protocols, such as Label Distribution Protocol (LDP), Resource Reservation Protocol-Traffic Engineering (RSVP-TE), and Multiprotocol Border Gateway Protocol (MP-iBGP)
  • One router assigns the MPLS label independently
  • 20 bits for the label gives label range of 0-1048575