The correct answer is to verify that the LDP router IDs are reachable via routing, because LDP sessions rely on TCP port 646 for label exchange, and the session cannot be established if the routers cannot route to each other’s LDP router ID. This is a common pitfall: even when interfaces are directly connected and in the same subnet, the LDP Hello messages sent to the multicast address 224.0.0.2 will be received, but the subsequent TCP connection for the session will fail if the router IDs are not reachable through the routing table. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this concept tests your understanding that LDP neighbor discovery is link-local via UDP, but session establishment is unicast TCP. A frequent trap is assuming that directly connected interfaces guarantee session formation; in reality, you must also check that each router has a route to the peer’s LDP router ID, often via a loopback interface. Memory tip: “Hello is local, session is global—route the ID to make the TCP connect.”
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 network engineer is troubleshooting an LDP session failure between two directly connected routers. The routers are configured with the 'mpls ldp' command under the interface. The 'show mpls ldp neighbor' command shows no neighbors. Which two additional pieces of information should the engineer verify? (Choose two.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Verify that the interface IP addresses are in the same subnet.
Option A is correct because LDP sessions are established only between directly connected LSRs when using the default 'mpls ldp' interface configuration. The LDP Hello messages are sent as UDP packets to the multicast address 224.0.0.2, and if the interface IP addresses are not in the same subnet, the Hello messages will not be received, preventing neighbor discovery.
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.
✓
Verify that the interface IP addresses are in the same subnet.
Why this is correct
LDP hello messages are sent to the all-routers multicast address; mismatched subnets prevent discovery.
Related concept
Read the scenario before looking for a memorised answer.
✗
Verify that TCP port 646 is open on the interface ACL.
Why it's wrong here
LDP uses UDP for discovery and TCP for session; but the question asks for verification steps, and this is too specific; not the first step.
✗
Verify that 'mpls ip' is enabled globally.
Why it's wrong here
The 'mpls ip' command is not needed; 'mpls ldp' under interface is sufficient.
✓
Verify that the LDP router IDs are reachable via routing.
Why this is correct
LDP uses TCP to establish session; the transport address must be reachable.
Related concept
Read the scenario before looking for a memorised answer.
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between LDP Hello (UDP multicast) and LDP session (TCP unicast), and candidates mistakenly focus on TCP ACLs or global MPLS commands instead of verifying subnet adjacency and router ID reachability.
Trap categories for this question
Command / output trap
The 'mpls ip' command is not needed; 'mpls ldp' under interface is sufficient.
Detailed technical explanation
How to think about this question
LDP uses a two-phase process: first, UDP Hello messages (port 646) are sent to the all-routers multicast address 224.0.0.2 to discover neighbors; only after a Hello is received does the router attempt a TCP connection (port 646) to the discovered LDP ID. The LDP router ID must be reachable via routing for the TCP session to be established, which is why verifying reachability (Option D) is also critical—if the router ID is not in the routing table, the TCP SYN will fail.
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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
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: Verify that the interface IP addresses are in the same subnet. — Option A is correct because LDP sessions are established only between directly connected LSRs when using the default 'mpls ldp' interface configuration. The LDP Hello messages are sent as UDP packets to the multicast address 224.0.0.2, and if the interface IP addresses are not in the same subnet, the Hello messages will not be received, preventing neighbor discovery.
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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