350-501 MPLS and Segment Routing Practice Question
Which two actions are performed by an MPLS label edge router (LER) when it receives an unlabeled IP packet destined for a prefix with an LSP? (Choose two.)
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
✓
It forwards the packet based on the MPLS label.
The LER (or ingress LSR) imposes a label stack (push operation) on the packet, then forwards it based on the label. It does not swap labels (that's done by LSRs), nor does it pop the label (egress LER does).
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
It performs a recursive lookup in the LFIB.
Why it's wrong here
Recursive lookup is not typical; LFIB is used directly.
- ✗
It performs a label swap operation.
Why it's wrong here
Swap is done by intermediate LSRs.
- ✓
It forwards the packet based on the MPLS label.
Why this is correct
Correct: After pushing, the packet is forwarded using the label.
- ✗
It pops the top label and forwards based on the inner header.
Why it's wrong here
Pop is done by egress LER.
- ✓
It performs a label push operation to add an MPLS label.
Why this is correct
Correct: Ingress LER pushes a label.
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