The correct answer is that the LDP session is being established between 10.0.0.1 and 10.0.0.2, as the debug mpls ldp transport output clearly shows the TCP transport connection moving through accepted, established, and up states. This debug command reveals the underlying TCP handshake between the two routers on port 646, which is the foundation for the Label Distribution Protocol adjacency; without this transport layer success, no label exchange can occur. On the Cisco CCNP ENARSI 300-410 exam, this question tests your ability to distinguish between the transport (TCP) and the session (UDP hello) phases of LDP—a common trap is confusing the transport debug with the mpls ldp neighbor debug, which shows the full session state. Remember that transport is about the TCP pipe, not the labels themselves. A useful memory tip: “Transport is TCP, session is UDP hello—if transport is up, the pipe is ready to fill.”
300-410 MPLS L3VPN Practice Question
This 300-410 practice question tests your understanding of mpls l3vpn. 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 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
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.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Trap categories for this question
Command / output trap
The output shows connection establishment, not teardown.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
MPLS L3VPN — This question tests MPLS L3VPN — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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