mediumMultiple Choice
300-410 Practice Question: Runs the following command to troubleshoot an…
A network engineer runs the following command to troubleshoot an MPLS L3VPN issue:
R1# debug mpls ldp transport
Output:
*Mar 1 00:01:23.456: mpls_ldp_transport: LDP transport connection from 10.0.0.2:646 to 10.0.0.1:1025 *Mar 1 00:01:23.456: mpls_ldp_transport: LDP transport connection from 10.0.0.2:646 to 10.0.0.1:1025 is accepted *Mar 1 00:01:23.456: mpls_ldp_transport: LDP transport connection from 10.0.0.2:646 to 10.0.0.1:1025 is established *Mar 1 00:01:23.456: mpls_ldp_transport: LDP transport connection from 10.0.0.2:646 to 10.0.0.1:1025 is up
What does this output indicate?
⚠ Common exam trap
Many candidates confuse LDP transport (TCP) with LDP discovery (UDP), or misinterpreting the debug output as a teardown when it actually shows establishment. Candidates might also think label bindings are exchanged immediately upon TCP connection, but they occur after session initialization.
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
✓
LDP session is being established between 10.0.0.1 and 10.0.0.2
The debug output shows the LDP transport connection progressing through states: received, accepted, established, and up. This sequence confirms that a TCP-based LDP session is being successfully established between the LSRs at 10.0.0.1 and 10.0.0.2. The port numbers (646 for LDP, 1025 as the ephemeral source port) and the 'up' state indicate the session is operational.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
LDP session is being established between 10.0.0.1 and 10.0.0.2
Why this is correct
The debug output shows the LDP transport connection progressing through accepted, established and up states between 10.0.0.1 and 10.0.0.2, confirming the TCP session underpinning the LDP session is being established successfully between the two routers.
- ✗
LDP session is being torn down between 10.0.0.1 and 10.0.0.2
Why it's wrong here
The output shows a connection accepted, established and up, which is session formation, not teardown. Teardown messages would show 'closing' or 'down' states. This option is tempting because debug output can reveal failures, but here every line confirms successful establishment between the peers.
- ✗
LDP is using UDP for transport
Why it's wrong here
The debug output shows TCP port 646, the well-known LDP transport port, with a connection accepted, established and up — LDP transport runs over TCP, not UDP. UDP appears only in LDP discovery hellos on port 646. UDP would be relevant if you were troubleshooting basic neighbour discovery rather than session establishment.
- ✗
LDP label bindings are being exchanged
Why it's wrong here
This debug covers the LDP transport (TCP) session only; label binding exchange is separate and appears under debug mpls ldp advertisements or bindings. The output confirms the TCP connection reached the up state, which is a prerequisite for, but not evidence of, binding exchange. Binding debug is correct when troubleshooting label distribution.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
MPLS Label Distribution
MPLS Label Distribution is the process by which routers exchange labels that tell them how to forward packets across a network without looking at the IP address each time.
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.