Multiple choice technology

When a router receives a label of zero it will ____

  1. perform a label pop

  2. perform a label swap

  3. perform a label push

  4. send an error message toward the ingress router
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

When an LSR receives a label of 0 (implicit null), it performs PHP (Penultimate Hop Popping) by popping the label and forwarding the IP packet to the egress router. However, this is operationally equivalent to a label swap operation where the incoming label is swapped for a null label (0), which is then removed. The correct answer is B because in MPLS forwarding operations, the label action is classified as a swap. Options A (pop), C (push), and D (error) do not correctly describe the MPLS forwarding operation for label 0.

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