hardMultiple Choice
300-410 Practice Question: Runs the following command to troubleshoot an…
A network engineer runs the following command to troubleshoot an MPLS LDP issue:
R1# debug mpls ldp transport
LDP: Transport connection to 2.2.2.2:0 via TCP (passive) LDP: Connection from 2.2.2.2:0 to 1.1.1.1:646 LDP: Transport connection to 2.2.2.2:0 via TCP (active) LDP: Connection from 1.1.1.1:646 to 2.2.2.2:0 LDP: Hold timer expired for peer 2.2.2.2:0 LDP: Closing transport connection to 2.2.2.2:0
What does this output indicate?
⚠ Common exam trap
Cisco often tests the distinction between session flapping due to hold timer expiration versus session failure due to authentication or address mismatch, and the trap here is that candidates may see 'transport connection' messages and incorrectly assume the session is up, missing the critical 'Hold timer expired' line.
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 flapping due to hold timer expiration.
The output shows the LDP hold timer expiring for peer 2.2.2.2, followed by the transport connection being closed. This indicates the LDP session is repeatedly going up and down (flapping) because the hold timer expires before the session can stabilize, often due to network congestion, high CPU, or mismatched hello/hold timers. Option B correctly identifies this flapping behavior caused by hold timer expiration.
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 established and stable.
Why it's wrong here
The hold timer expiring and the transport connection closing prove the session has torn down, not stabilised. It is tempting because the log shows both active and passive TCP connections forming, which resembles successful establishment, but the final two lines show the peer relationship failing after that initial exchange.
- ✓
LDP session is flapping due to hold timer expiration.
Why this is correct
The log shows the TCP transport connection being opened, then closed after the hold timer expires, so the LDP session repeatedly re-establishes and drops. This confirms flapping caused by hold timer expiry rather than a configuration mismatch.
- ✗
LDP session is down because of authentication failure.
Why it's wrong here
Authentication failures produce TCP resets or MD5 signature errors during session establishment, not a hold-timer expiry after the connection is already up. The log shows the session established, then the hold timer expiring, indicating missed keepalives. Authentication is configured with 'password' under the LDP session, which would fail before reaching this state.
- ✗
LDP session is down because of a transport address mismatch.
Why it's wrong here
A transport address mismatch prevents the TCP connection from forming at all, since LDP validates the peer's advertised transport address before session establishment. Here the TCP connection succeeded in both passive and active directions, so addresses matched. The hold timer expiring afterwards points to keepalive loss, not address validation.
Go deeper
Related to this question
Learn chapter
Device Management and Network Monitoring (SNMP, Syslog, NetFlow)
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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