Quick Answer
The correct answer is swap, as a transit Label Switch Router (LSR) must perform a label swap operation on the top label of a labeled packet to forward it along the label-switched path. This is because the transit LSR sits between the ingress and egress routers, and it replaces the incoming label with a new outgoing label learned from the downstream neighbor via LDP or other label distribution protocols, effectively maintaining the MPLS forwarding chain without inspecting the IP header. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of MPLS data-plane operations, often appearing in questions that distinguish between push (ingress), swap (transit), and pop (penultimate hop popping or egress). A common trap is confusing the transit LSR’s role with PHP—remember, a transit LSR never pops the label unless it is the penultimate hop. Memory tip: “Swap keeps the train on the tracks—the label changes, but the path stays.”
This 350-501 practice question tests your understanding of mpls and segment routing. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
In MPLS forwarding, what label operation does a transit LSR perform on the top label of a labeled packet?
Swap
Swap is the correct operation for a transit LSR.
None
Why wrong: A label operation is always performed on the top label.
Pop
Why wrong: Pop is performed by the egress LSR or penultimate hop (PHP).
Push
Why wrong: Push is performed by the ingress LSR.
Answer choices
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
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.
Related concept
Read the scenario before looking for a memorised answer.
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.
Common exam traps
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.
Detailed technical explanation
Under the hood, the swap operation updates the label in the MPLS shim header and decrements the TTL (unless the pipe model is used). In real-world deployments, the swap operation is implemented in hardware using a Label Forwarding Information Base (LFIB) that maps incoming label to outgoing label, interface, and next-hop. A subtle behavior is that if the transit LSR is also the penultimate hop, it may perform a pop instead of a swap, but the default role of a pure transit LSR is swap.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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MPLS and Segment Routing — study guide chapter
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FAQ
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
The correct answer is: 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
This 350-501 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-501 exam.
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