hardMultiple Choice
300-410 Practice Question: Runs the following command to debug MPLS LDP…
A network engineer runs the following command to debug MPLS LDP errors:
R1# debug mpls ldp errors
Output:
*Mar 1 00:01:23.456: LDP: Received malformed hello from 10.0.0.2 *Mar 1 00:01:23.789: LDP: Received malformed initialization from 10.0.0.2 *Mar 1 00:01:24.012: LDP: Session with 10.0.0.2:0 (0x1234) is DOWN
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
✓
The LDP session with 10.0.0.2 failed due to malformed messages from the neighbor
The debug output shows LDP errors. R1 received malformed hello and initialization messages from neighbor 10.0.0.2, causing the LDP session to go down. This indicates a configuration mismatch or software bug on the neighbor.
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 session with 10.0.0.2 failed due to malformed messages from the neighbor
Why this is correct
Malformed hello and initialisation messages from 10.0.0.2 prevent LDP session establishment, so the session goes DOWN. The debug output attributes the failure directly to those malformed messages received from the neighbour, confirming the session failed for that reason.
- ✗
The LDP session with 10.0.0.2 is up and stable
Why it's wrong here
The debug output shows malformed hello and initialisation messages followed by the session going DOWN, so the adjacency is failing, not stable. It is tempting because the session identifier appears in the log, but that line records session teardown, indicating a parameter or transport mismatch between the LDP peers rather than a healthy session.
- ✗
R1 is sending malformed messages to 10.0.0.2
Why it's wrong here
The log states R1 *received* malformed hello and initialisation messages from 10.0.0.2, so the fault lies with the peer's transmitted TLVs, not R1's outbound messages. This option would fit if R1's own debugs showed it generating or sending malformed PDUs.
- ✗
The LDP session is using incorrect transport address
Why it's wrong here
The debug output names malformed hello and initialisation messages, which point to a version or parameter mismatch in the LDP messages themselves, not to transport address selection. Transport address configuration is the culprit when sessions fail after hellos succeed, with TCP connection errors to the wrong loopback address.
Go deeper
Related to this question
Learn chapter
MPLS L3VPN Basics
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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