The MPLS label is a fixed 4 byte identifier added to the packet by the ingress router between the data-link layer (Layer2) and the network layer (Layer3) and is used by all middle routers to switch the packet to its destination without the need for any routing table (Layer3) look-ups. MPLS is considered a layer 2.5technology and the MPLS header is called the shim header.
Simply MPLS inserts a 32-bit Label field between the layer 2 frame header and layer 3 packet headers (frame Mode) for this reason it is considered layer 2.5 technology.
MPLS Label Structure
- Label: label value, 20 bits.
- EXP: Experimental bits, Name are currently changed to Traffic class, 3 bits.
- S: bottom of stack, 1 bit.
- TTL: Time to live, 8 bits.
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