350-501 Architecture Practice Question
Exhibit
Refer to the exhibit. Router# show mpls forwarding-table Local Outgoing Prefix Bytes Label Outgoing Next Hop Label Label or Tunnel Id Switched Interface 16 Pop *********** 0 Gi0/0/0 192.168.1.2 17 18 10.1.1.0/24 1500 Gi0/0/1 192.168.2.2 18 20 10.2.2.0/24 3000 Gi0/0/1 192.168.2.2 19 Untagged 10.3.3.0/24 0 Gi0/0/2 192.168.3.2
Refer to the exhibit. The router has an outgoing label of 'Pop' for prefix ***********. What does this imply?
⚠ Common exam trap
Cisco often tests the distinction between 'Pop' (PHP) and 'Untagged' (no label assigned), so candidates mistakenly think 'Pop' means the router is dropping the label or not forwarding traffic, when in fact it is a deliberate MPLS optimization.
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
✓
The router is the penultimate hop and will remove the label before forwarding
When a router has an outgoing label of 'Pop' for a prefix in MPLS, it indicates that the router is the penultimate hop (PHP) in the LSP. PHP means the router will remove (pop) the top label before forwarding the packet to the next hop, reducing the burden on the egress LSR. This is standard MPLS behavior as per RFC 3031, where the egress router signals an implicit null label (label 3) to its upstream neighbor.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The router has not learned a label for that prefix
Why it's wrong here
Untagged indicates no label; Pop indicates a label is removed.
- ✗
The router will not forward traffic for that prefix
Why it's wrong here
It forwards traffic after removing the label.
- ✓
The router is the penultimate hop and will remove the label before forwarding
Why this is correct
Penultimate hop popping (PHP) removes the outermost label.
- ✗
The router will push a new label for that prefix
Why it's wrong here
Push happens at ingress, not when outgoing label is Pop.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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