Contents of an MPLS Label

A.

MPLS (Multiprotocol Label Switching) is a technique used in high-performance telecommunications networks to direct and prioritize network traffic. MPLS assigns a label to each packet to identify its path through the network and enable quick switching between paths. The label is inserted between the Layer 2 and Layer 3 headers of the packet.

The contents of an MPLS label include:

• Label: The label is a 20-bit or 32-bit number that identifies a specific forwarding equivalence class (FEC) for the packet. An FEC is a group of packets that are forwarded in the same way. The label is used by the routers to quickly determine the path that the packet should take through the network.

• Traffic class: The traffic class field is a 3-bit field that indicates the priority of the packet. It is used to support quality of service (QoS) in the network.

• Bottom of stack (BoS): The BoS bit is a 1-bit flag that indicates whether this is the last label in the stack of MPLS labels.

• Time to live (TTL): The TTL field is an 8-bit field that is decremented by one at each hop along the path of the packet. It is used to limit the time that a packet can stay in the network and prevent packets from looping indefinitely.

Based on the above explanation, the correct answer is A. The contents of an MPLS label include a 20-bit label, 3-bit traffic class, 1-bit bottom of stack, and 8-bit TTL. Option B and D are incorrect because the label is either 20 bits or 32 bits, not always 32 bits. Option C is incorrect because it includes a 3-bit flow label, which is not part of the contents of an MPLS label.