Which list defines the contents of an MPLS label?

Which list defines the contents of an MPLS label?
A . 20-bit label; 3-bit traffic class; 1-bit bottom stack; 8-bit TTL
B. 32-bit label; 3-bit traffic class; 1-bit bottom stack; 8-bit TTL
C. 20-bit label; 3-bit flow label; 1-bit bottom stack; 8-bit hop limit
D. 32-bit label; 3-bit flow label; 1-bit bottom stack; 8-bit hop limit

View Answer

Answer: A

Explanation:

The first 20 bits constitute a label, which can have 2^20 values. Next comes 3 bit value called Traffic Class. It was formerly called as experimental (EXP) field. Now it has been renamed to Traffic Class (TC). This field is used for QoS related functions. Ingress router can classify the packet according to some criterion and assign a 3 bit value to this filed. If an incoming packet is marked with some IP Precedence or DSCP value and the ingress router may use such a field to assign an FEC to the packet. Next bit is Stack bit which is called bottom-of-stack bit. This field is used when more than one label is assigned to a packet, as in the case of MPLS VPNs or MPLS TE. Next byte is MPLS TTL field which serves the same purpose as that of IP TTL byte in the IP header

Reference: https://tools.ietf.org/html/rfc5462