Multi Protocol Label Switching (MPLS) is a data-carrying mechanism that belongs to the family of
packet-switched networks. For an MPLS label, if the stack bit is set to1, which option is true?

A.
The stack bit will only be used when LDP is the label distribution protocol

B.
The label is the last entry in the label stack.

C.
The stack bit is for Cisco implementations exclusively and will only be used when TDP is the
label distribution protocol.

D.
The stack bit is reserved for future use.

Explanation:

MPLS Header Packet Format
LABEL: 20 bits
EXP: Experimental, 3bits – are reserved for experimental use
S: Bottom of stack, 1 bit
TTL: Time to Live, 8bits – same as IP TTL
The bottom-of-stack bit, or “stack bit”, is just used to indicate it is the bottom of the label stack
because it is possible (and common) to have more than one label attached to a packet. The
bottommost label in a stack has the S bit set to 1, other labels have the S bit set to 0. Sometimes it
is useful to know where the bottom of the label stack is and the S bit is the tool to find it.