These labels are attached to the IP packets, enabling the routers to forward the traffic by looking at the label and not the destination IP address. The packets are forwarded by label switching instead of by IP switching.
The MPLS label also called a shim header is 32bit (4 octets). Below is MPLS label looks like..
A MPLS label contains the following parts.
20-bit label value
3-bit experimental field
1-bit bottom-of-stack indicator
8-bit time-to-live filed
First 20 bit defines the original length of the actual label. Next 3-bit is experimental field which is used to copy QoS value from IP header like IP precedence or DSCP values. After that we have Bottom-of-stack value which defines weather the label is a last label. If it is set to 0 then its mean there are more labels and set as 1 defines that it’s a last label in a packet. Last field contains the 8-bit Time-to-live field which is same as IP Header time-to-live value and copied from IP header at PE router. We can also disable copying of TTL value from IP header to MPLS label.
20-bit label value
3-bit experimental field
1-bit bottom-of-stack indicator
8-bit time-to-live filed
First 20 bit defines the original length of the actual label. Next 3-bit is experimental field which is used to copy QoS value from IP header like IP precedence or DSCP values. After that we have Bottom-of-stack value which defines weather the label is a last label. If it is set to 0 then its mean there are more labels and set as 1 defines that it’s a last label in a packet. Last field contains the 8-bit Time-to-live field which is same as IP Header time-to-live value and copied from IP header at PE router. We can also disable copying of TTL value from IP header to MPLS label.
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