The answer is a swap operation from label 17 to label 18. This is correct because in an MPLS VPN core, when a Provider Edge (PE) router receives a labeled packet, it consults its Label Forwarding Information Base (LFIB) to determine the action. Here, the incoming label 17 is associated with the VPNv4 route for 10.2.2.1/32, and the LFIB instructs PE1 to pop label 17 and push label 18, which is the transport label used to forward the packet across the core toward the next-hop PE. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of the MPLS label swap operation on PE routers, specifically how VPN labels are swapped for transport labels in the core. A common trap is confusing a PHP (Penultimate Hop Popping) action with a swap; remember that a swap occurs when the router is not the penultimate hop and must forward the packet deeper into the MPLS domain. Memory tip: "Swap sends it on, pop ends the run"—if the router is not the egress, it swaps the label.
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. 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.
Exhibit
Refer to the exhibit.
PE1# show mpls forwarding-table
Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
16 Pop 10.1.1.0/24 0 Gi0/0/0/0 point2point
17 18 10.2.2.0/24 0 Gi0/0/0/1 192.168.1.2
18 Untagged 10.3.3.0/24 0 Gi0/0/0/2 192.168.2.2
Based on the exhibit, which label operation will occur when a packet destined to 10.2.2.1 enters PE1 with label 17?
Refer to the exhibit.
PE1# show mpls forwarding-table
Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
16 Pop 10.1.1.0/24 0 Gi0/0/0/0 point2point
17 18 10.2.2.0/24 0 Gi0/0/0/1 192.168.1.2
18 Untagged 10.3.3.0/24 0 Gi0/0/0/2 192.168.2.2
A
Forward with no label (Untagged)
Why wrong: Untagged is for prefix 10.3.3.0/24.
B
Pop the label and forward as IP
Why wrong: Pop operation is for prefix 10.1.1.0/24, not 10.2.2.0/24.
C
Push label 17 onto the packet
Why wrong: Label 17 is the incoming label, not pushed.
D
Swap label 17 to label 18
The forwarding table shows outgoing label 18 for this prefix.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Swap label 17 to label 18
The correct answer is D because the exhibit shows an MPLS VPN scenario where PE1 receives a labeled packet destined for 10.2.2.1. The incoming label 17 corresponds to the VPNv4 route for 10.2.2.1/32, and the LFIB on PE1 indicates a swap operation to label 18, which is the transport label used to forward the packet across the MPLS core toward the next-hop PE.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Forward with no label (Untagged)
Why it's wrong here
Untagged is for prefix 10.3.3.0/24.
✗
Pop the label and forward as IP
Why it's wrong here
Pop operation is for prefix 10.1.1.0/24, not 10.2.2.0/24.
✗
Push label 17 onto the packet
Why it's wrong here
Label 17 is the incoming label, not pushed.
✓
Swap label 17 to label 18
Why this is correct
The forwarding table shows outgoing label 18 for this prefix.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between label operations (push, swap, pop) in MPLS VPN scenarios, and the trap here is that candidates mistakenly think the incoming label is a VPN label that must be popped or pushed, rather than recognizing that the LFIB dictates a swap when the packet is transiting the MPLS core.
Detailed technical explanation
How to think about this question
In MPLS VPNs, the ingress PE (PE1) performs a label lookup in the LFIB based on the incoming label. The swap operation replaces the top label (17) with a new label (18) that represents the LSP toward the egress PE. The egress PE then pops the label and forwards the original IP packet to the CE. This mechanism ensures that the VPN label is preserved end-to-end while the transport label changes at each hop.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 350-501 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Architecture — This question tests Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Swap label 17 to label 18 — The correct answer is D because the exhibit shows an MPLS VPN scenario where PE1 receives a labeled packet destined for 10.2.2.1. The incoming label 17 corresponds to the VPNv4 route for 10.2.2.1/32, and the LFIB on PE1 indicates a swap operation to label 18, which is the transport label used to forward the packet across the MPLS core toward the next-hop PE.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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