Which 4 statements regarding MPLS Label Stack Encoding is true?
A.
A value of 4 represents the “Implicit NULL Label.”
B.
A value of 0 represents the “IPv4 Explicit NULL Label.”
C.
A value of 1 represents the “Router Alert Label”.
D.
A value of 2 represents the “IPv6 Explicit NULL Label”
E.
A value of 1 represents the “IPv1 Explicit NULL Label”
F.
A value of 3 represents the “Implicit NULL Label”
Explanation:
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.