350-501 Penultimate Hop Popping (PHP) Practice Question
Exhibit
show mpls forwarding-table 10.0.1.0/24 Local tag Outgoing tag Prefix Bytes tag switched Outgoing interface Next Hop 17 Pop tag 10.0.1.0/24 0 Gi0/1 192.168.2.1
Refer to the exhibit. What is the role of this router for prefix 10.0.1.0/24?
⚠ Common exam trap
A common trap is assuming that any router performing a label pop is the egress LSR. In MPLS with implicit null, the penultimate hop performs the pop, and the egress LSR receives an unlabeled IP packet.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Penultimate hop.
The router is the penultimate hop for prefix 10.0.1.0/24 because it performs a 'Pop tag' operation (implicit null label). In MPLS, the penultimate hop pops the label before forwarding the unlabeled IP packet to the egress LSR. Therefore, this router is not the egress; it is the penultimate Label Switching Router (LSR).
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Transit LSR.
Why it's wrong here
Incorrect. A transit LSR forwards labeled packets without changing the label; this router pops the label, so it is not a transit LSR.
- ✗
Egress LSR.
Why it's wrong here
Incorrect. An egress LSR removes the label and forwards the IP packet; but with implicit null, the egress LSR receives an unlabeled packet. Since this router performs a pop, it is the penultimate hop, not the egress.
- ✓
Penultimate hop.
Why this is correct
Correct. The 'Pop tag' indicates this router is the penultimate hop, which removes the label before forwarding to the egress LSR.
- ✗
Ingress LSR.
Why it's wrong here
Incorrect. An ingress LSR imposes the initial label; this router is near the destination, not the source.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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