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300-410 Practice Question: Is troubleshooting MPLS LDP where the LDP session…
A network engineer is troubleshooting MPLS LDP where the LDP session between two directly connected routers is not forming. The engineer runs show mpls ldp discovery and sees that LDP hellos are being sent and received on the link. However, show mpls ldp neighbor shows no neighbors. What is the most likely cause?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The LDP router-id is not reachable via the IGP.
LDP hellos are exchanged, but the session does not form, indicating a problem with the transport connection. The most common cause is that the routers cannot establish a TCP connection to the LDP transport address, often due to a missing route to the LDP router-id or an ACL blocking TCP port 646.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The LDP router-id is not reachable via the IGP.
Why this is correct
Correct because LDP uses TCP to establish the session, and the router-id must be reachable; if the IGP does not have a route to the peer's LDP router-id, the TCP connection fails.
- ✗
The mpls label protocol ldp command is missing globally.
Why it's wrong here
Incorrect because LDP hellos are being sent, indicating LDP is enabled globally; the issue is with session establishment.
- ✗
The interface is configured with mpls ldp igp sync.
Why it's wrong here
Incorrect because LDP-IGP synchronization affects label advertisement and IGP metric, not LDP session formation.
- ✗
The LDP session is using a non-default transport address.
Why it's wrong here
Incorrect because if the transport address is non-default, it must be reachable; the issue is reachability, not the address itself.
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Last reviewed: Jun 18, 2026
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