mediumMultiple ChoiceObjective-mapped
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?
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 LDP transport connections between two routers. The messages indicate that a TCP connection from 10.0.0.2 (LDP port 646) to 10.0.0.1 (ephemeral port 1025) was accepted, established, and is now up. This confirms LDP adjacency is forming at the transport layer.
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 output shows the TCP connection for LDP is accepted, established, and up, indicating a successful LDP session setup.
- ✗
LDP session is being torn down between 10.0.0.1 and 10.0.0.2
Why it's wrong here
The output shows connection establishment, not teardown.
- ✗
LDP is using UDP for transport
Why it's wrong here
LDP uses TCP (port 646) for reliable transport, not UDP.
- ✗
LDP label bindings are being exchanged
Why it's wrong here
This debug only shows transport layer events; label exchange is a separate process.
Visual reference
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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