350-501 MPLS and Segment Routing Practice Question
In MPLS forwarding, what label operation does a transit LSR perform on the top label of a labeled packet?
⚠ Common exam trap
Cisco often tests the distinction between the roles of ingress, transit, and egress LSRs, and the trap here is that candidates confuse the transit LSR's swap operation with the penultimate hop's pop operation or the ingress LSR's push operation.
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
✓
Swap
A transit Label Switch Router (LSR) in an MPLS network receives a labeled packet and must forward it toward the egress LSR. The core operation on the top label is a swap: the incoming label is replaced with an outgoing label learned from the downstream LSR via Label Distribution Protocol (LDP) or other label distribution protocols. This maintains the label-switched path (LSP) and ensures the packet continues along the correct path.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Swap
Why this is correct
Swap is the correct operation for a transit LSR.
- ✗
None
Why it's wrong here
A label operation is always performed on the top label.
- ✗
Pop
Why it's wrong here
Pop is performed by the egress LSR or penultimate hop (PHP).
- ✗
Push
Why it's wrong here
Push is performed by the ingress LSR.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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