MPLS Label Value 1: Purposes and Significance

The Purpose of MPLS Label Value 1 Explained

Question

Which are the two purposes of the MPLS label value 1? (Choose two.)

Answers

Explanations

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

CF.

http://www.cisco.com/c/en/us/support/docs/multiprotocol-label-switching-mpls/mpls/4649-mpls-faq-4649.html#qa3 http://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtml

The MPLS label value 1 serves two purposes, which are as follows:

  1. Indicating an Implicit Null Label: The first purpose of the MPLS label value 1 is to indicate an implicit null label. When the label value 1 is used in the label stack, it indicates that the label stack has been popped and that the packet should be forwarded based on the IP header. In other words, the implicit null label signifies the end of the label stack and the start of the IP forwarding process. The implicit null label is useful for reducing the label stack depth and improving forwarding performance.

  2. Indicating a Router Alert Label: The second purpose of the MPLS label value 1 is to indicate a router alert label. When the label value 1 is used in the label stack and the Router Alert Option is set in the IP header, it signals that the packet should be sent to the router's CPU for processing instead of being forwarded based on the IP header. The router alert label is useful for sending control packets to the router for specific purposes, such as monitoring and troubleshooting.

Therefore, the correct answers are B (indicating an implicit null label) and F (indicating a router alert label). The other options, A (used for MPLS OAM packets), C (used for VPNv6 packets), and D (used to carry the QoS value in the label stack), are not associated with the MPLS label value 1. Option E (indicating an explicit null label) is a valid label value, but it is not one of the purposes of the MPLS label value 1.