Understanding MPLS Labels?

--> An MPLS label is a 4-byte identifier that is used by network devices to make forwarding decisions for a packet.

--> Each and every label define the destination and services for each packet and identify a forwarding equivalence class (FEC).

-->  The label put on a particular packet represents the FEC to which the packet is assigned.

--> Each LSR informs its neighbors of the label bindings that it has made.

--> The FEC is a group of IP packets that are forwarded in the same manner, over the same path, and with the same forwarding treatment.

--> An FEC might correspond to a destination IP subnetwork or any traffic class that the edge LSR considers significant.

--> MPLS 32 bit label identifier is divided into four parts.

i) Actual Label (20 bit label )

ii) Experimental bit ( 3 bit label for QOS)

iii) Bottom of Stack bit (1 bit for indicating whether the label is the last label in packet)

iv) TTL field ( 8 bits for loop prevention)

--> MPLS works in two modes,

i) Frame Mode : In this mode, the label inserted between layer 2 and layer 3.

ii) Cell Mode: In this mode, VPI/VCI fields are used as labels.