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:
Theoretically, the range is 0 through (220-1). Label values 0-15 are reserved, and values 4-15 are reserved for future use. Values 0-3 are defined as:
A value of 0 represents the "IPv4 Explicit NULL Label". This label indicates that the label stack must be popped, and the packet forwarding must be based on the IPv4 header. This helps to keep Exp bits safe until the egress router. It is used in MPLS based QoS.
A value of 1 represents the "Router Alert Label". When a received packet contains this label value at the top of the label stack, it is delivered to a local software module for processing. The actual packet forwarding is determined by the label beneath it in the stack. However, if the packet is forwarded further, the Router Alert Label should be pushed back onto the label stack before forwarding. The use of this label is analogous to the use of the "Router Alert Option" in IP packets (for example, ping with record route option)
A value of 2 represents the "IPv6 Explicit NULL Label". It indicates that the label stack must be popped, and the packet forwarding must be based on the IPv6 header.
A value of 3 represents the "Implicit NULL Label". This is a label that an LSR can assign and distribute. However, it never actually appears in the encapsulation. It indicates that the LSR pops the top label from the stack and forwards the rest of the packet (labeled or unlabeled) through the outgoing interface (as per the entry in Lfib). Although this value might never appear in the encapsulation, it needs to be specified in the Label Distribution Protocol, so a value is reserved.