The correct answer is that traffic to 10.1.1.0/24 will be label-switched with an implicit null label. This is because the VRF CUSTOMER route for 10.1.1.0/24 shows a next hop of 192.168.1.2 and is associated with label 3, which is the reserved implicit null label. In MPLS L3VPN, label 3 instructs the upstream router to pop the top label and forward the packet using IP routing directly to the connected CE, avoiding an unnecessary label lookup at the penultimate hop. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of MPLS label operations and the distinction between implicit null (label 3) and explicit null (label 0), where implicit null is used when the downstream router is directly connected and can forward based on the IP header alone. A common trap is confusing implicit null with explicit null—remember that implicit null means “pop before sending,” while explicit null means “send with a zero label for QoS.” A useful memory tip: think of label 3 as the “pop and drop” instruction for the directly connected next hop.
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 Labels Label Next Hop
16 Pop 10.1.1.0/24
No Label 10.2.2.0/24
PE1# show ip bgp vpnv4 vrf CUSTOMER
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 100:1
10.1.1.0/24 192.168.1.1 0 100 0 65001 i
10.2.2.0/24 192.168.2.2 0 100 0 65002 i
Refer to the exhibit. A PE router in an MPLS L3VPN network shows the above output. The VRF CUSTOMER contains two routes. Which statement about forwarding for these routes is true?
Refer to the exhibit.
PE1# show mpls forwarding-table
Local Outgoing Prefix Labels Label Next Hop
16 Pop 10.1.1.0/24
No Label 10.2.2.0/24
PE1# show ip bgp vpnv4 vrf CUSTOMER
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 100:1
10.1.1.0/24 192.168.1.1 0 100 0 65001 i
10.2.2.0/24 192.168.2.2 0 100 0 65002 i
A
Both routes are reachable via MPLS.
Why wrong: 10.2.2.0/24 has no label, so it is not reachable via MPLS.
B
Traffic to 10.1.1.0/24 will be label-switched with an implicit null label.
'Pop' means implicit-null label (label 3), which is popped by the penultimate hop.
C
Traffic to 10.2.2.0/24 will be forwarded using the BGP next hop label.
Why wrong: The BGP next hop (192.168.2.2) does not have a label in the LFIB, so no label is available.
D
Traffic to 10.2.2.0/24 will be forwarded using IP routing.
Why wrong: 'No Label' indicates no label is available; the router will drop the packet because it cannot forward in MPLS core without a label.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Traffic to 10.1.1.0/24 will be label-switched with an implicit null label.
The output shows that for the VRF CUSTOMER, the route to 10.1.1.0/24 has a next hop of 192.168.1.2 and is associated with label 3 (implicit null). In MPLS L3VPN, label 3 instructs the upstream router to pop the MPLS label and forward the packet using IP routing to the directly connected next hop. Therefore, traffic to 10.1.1.0/24 will be label-switched with an implicit null label, meaning the label is removed before forwarding to the CE router.
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.
✗
Both routes are reachable via MPLS.
Why it's wrong here
10.2.2.0/24 has no label, so it is not reachable via MPLS.
✓
Traffic to 10.1.1.0/24 will be label-switched with an implicit null label.
Why this is correct
'Pop' means implicit-null label (label 3), which is popped by the penultimate hop.
Related concept
Read the scenario before looking for a memorised answer.
✗
Traffic to 10.2.2.0/24 will be forwarded using the BGP next hop label.
Why it's wrong here
The BGP next hop (192.168.2.2) does not have a label in the LFIB, so no label is available.
✗
Traffic to 10.2.2.0/24 will be forwarded using IP routing.
Why it's wrong here
'No Label' indicates no label is available; the router will drop the packet because it cannot forward in MPLS core without a label.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between implicit null (label 3) and explicit null (label 0), and the trap here is that candidates assume any route with a label in the VRF table is fully MPLS-switched end-to-end, not realizing that label 3 means the label is popped before the final hop.
Detailed technical explanation
How to think about this question
In MPLS L3VPN, the VRF table stores the per-prefix label assigned by the remote PE. Label 3 (implicit null) is a special label that signals the upstream LSR to perform penultimate hop popping (PHP), removing the label before sending the packet to the directly connected CE. This avoids an unnecessary label lookup on the PE for directly attached subnets. The route to 10.2.2.0/24 with label 16000 will be label-switched across the MPLS core, and the PHP router will pop the label before delivery to the CE.
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.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Traffic to 10.1.1.0/24 will be label-switched with an implicit null label. — The output shows that for the VRF CUSTOMER, the route to 10.1.1.0/24 has a next hop of 192.168.1.2 and is associated with label 3 (implicit null). In MPLS L3VPN, label 3 instructs the upstream router to pop the MPLS label and forward the packet using IP routing to the directly connected next hop. Therefore, traffic to 10.1.1.0/24 will be label-switched with an implicit null label, meaning the label is removed before forwarding to the CE router.
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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Question Discussion
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