Control Word Functions in MPLS AToM - Exam 400-201: Cisco CCIE Service Provider Written Exam

Three Actions of Control Word in MPLS AToM

Question

What are three actions that the control word performs in MPLS AToM? (Choose three.)

Answers

Explanations

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A. B. C. D. E. F.

BCD.

https://ccdewiki.wordpress.com/2013/05/28/control-word-on-atom/

In MPLS AToM (Any Transport over MPLS), the control word is a 4-byte field that is added to the beginning of each labeled packet. The control word provides additional information about the packet and performs several functions that are essential for proper operation of the AToM service. The three actions that the control word performs in MPLS AToM are:

A. Identifying the VC label: The VC label is the label that is assigned to a specific Virtual Circuit (VC) between two endpoints. In MPLS AToM, the control word is used to identify the VC label, which is used by the ingress edge router (IER) to determine the correct path for the packet to take through the MPLS backbone network.

B. Padding small packets: In some cases, small packets may not be large enough to carry a complete MPLS label stack. In this situation, the control word is used to pad the packet, so that it is large enough to accommodate the label stack. This ensures that the packet can be properly switched and forwarded through the MPLS backbone network.

C. Preserving the sequence of transported frames: When packets are transported over an MPLS AToM connection, it is important that they arrive at the destination in the correct order. The control word is used to preserve the sequence of transported frames by adding a sequence number to each packet. This allows the receiving edge router (RER) to detect and correct any out-of-order packets that may have been received.

Therefore, options A, B, and C are the three correct answers. Options D, E, and F are incorrect because they do not accurately describe the functions of the control word in MPLS AToM.