The answer is to check the MPLS LDP interface configuration and enable LDP on all core interfaces. This is correct because while the LDP session between the PEs may be up, the missing transport label for the remote PE’s loopback indicates that LDP label bindings are not being generated or advertised for that loopback, typically because LDP is not enabled on the underlying physical or sub-interfaces that carry the core traffic. In an MPLS L3VPN, the transport label is assigned via LDP for the BGP next-hop (the remote PE loopback), and without it, packets are dropped even though the VPN label is present. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of the MPLS forwarding plane and the distinction between LDP session state and label binding propagation—a common trap is assuming a session up means all bindings are exchanged. Remember the memory tip: “Session up, binding missing? Check the interfaces, not the session.”
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A service provider is troubleshooting an MPLS L3VPN scenario where a customer in VRF Blue cannot reach a server in VRF Blue at a remote site. The PEs are running MPLS with LDP. The VRF on both PEs shows the remote prefix. The PE at the local site shows the label from the remote PE for the prefix in the BGP table. However, when pinging from the CE, the packets are dropped. A packet capture on the core shows MPLS packets with the correct VPN label, but the transport label is missing. Further investigation shows that the LDP session between the two PEs is up, but the LDP label binding for the remote PE's loopback is not present. What is the most likely cause and correct action?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check MPLS LDP interface configuration and enable LDP on all core interfaces
The LDP session is up, but the label binding for the remote loopback is missing. This could be because LDP is not enabled on the underlying interfaces between the PEs, or because the label space is configured incorrectly. Typically, if LDP is enabled on all core interfaces, label bindings for loopbacks are automatically exchanged. The missing binding suggests that either LDP is not configured on some interfaces, or an access-list is blocking LDP. The most direct action is to verify that LDP is enabled on all core interfaces using the command 'show mpls ldp interface'.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Increase the TTL on the MPLS packets
Why it's wrong here
TTL issues would not cause missing label binding.
✗
Change the transport address in LDP to the loopback IP
Why it's wrong here
Transport address is already the loopback by default; this is not a missing binding issue.
✗
Configure the remote PE's loopback to be advertised via BGP
Why it's wrong here
The loopback is already in the IGP; LDP uses IGP to distribute labels.
✓
Check MPLS LDP interface configuration and enable LDP on all core interfaces
Why this is correct
LDP must be enabled on each core interface to exchange label bindings for loopbacks.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
Got this wrong? Here's your next step.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 350-501 ACL questions on filtering logic and placement.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Check MPLS LDP interface configuration and enable LDP on all core interfaces — The LDP session is up, but the label binding for the remote loopback is missing. This could be because LDP is not enabled on the underlying interfaces between the PEs, or because the label space is configured incorrectly. Typically, if LDP is enabled on all core interfaces, label bindings for loopbacks are automatically exchanged. The missing binding suggests that either LDP is not configured on some interfaces, or an access-list is blocking LDP. The most direct action is to verify that LDP is enabled on all core interfaces using the command 'show mpls ldp interface'.
What should I do if I get this 350-501 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 350-501 ACL questions on filtering logic and placement.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Standard ACLs match source addresses.
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