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300-410 Practice Question: Runs the following command on Router R8: R8# show…
A network engineer runs the following command on Router R8:
R8# show logging | include %LDP-5-NBRCHG *Mar 1 00:01:10.123: %LDP-5-NBRCHG: LDP Neighbor 10.0.0.2:0 (1) is UP *Mar 1 00:02:20.456: %LDP-5-NBRCHG: LDP Neighbor 10.0.0.2:0 (1) is DOWN *Mar 1 00:03:30.789: %LDP-5-NBRCHG: LDP Neighbor 10.0.0.2:0 (1) is UP *Mar 1 00:04:40.012: %LDP-5-NBRCHG: LDP Neighbor 10.0.0.2:0 (1) is DOWN
Based on this output, what is the most likely problem?
⚠ Common exam trap
300-410 often tests the misconception that LDP session flapping is caused by LDP configuration issues, but it is frequently due to underlying IGP instability.
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 is flapping due to an unstable IGP route to 10.0.0.2.
The log shows the LDP neighbor 10.0.0.2 repeatedly going UP and DOWN, which indicates a flapping LDP session. The most likely cause is an unstable IGP route to the neighbor's loopback address (used for LDP router ID), causing the LDP session to reset. LDP relies on the underlying IGP for reachability; if the route flaps, the LDP session will flap.
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 is flapping due to an unstable IGP route to 10.0.0.2.
Why this is correct
LDP neighbour adjacencies depend on a stable route to the peer's transport address. Repeated UP/DOWN transitions for 10.0.0.2 indicate the underlying IGP route is flapping, tearing down and rebuilding the TCP session each time the route changes.
- ✗
The MPLS label space is exhausted.
Why it's wrong here
Label space exhaustion would stop new label bindings being advertised while the session stayed up, not cause the neighbour adjacency itself to cycle. Tempting because exhaustion is a genuine MPLS fault, but it manifests as binding failures, whereas the log shows the LDP session flapping, pointing to an unstable underlying link.
- ✗
The router has a mismatched LDP router ID.
Why it's wrong here
A mismatched LDP router ID would prevent the session forming at all, producing no UP/DOWN flapping; the log shows the neighbour repeatedly reaching UP then DOWN. Tempting because router ID conflicts do break LDP adjacencies, but that scenario yields a stuck non-operational state rather than the cyclic transitions shown.
- ✗
The LDP hello interval is set too high, causing slow detection.
Why it's wrong here
A high hello interval slows failure detection but cannot produce repeated UP then DOWN transitions; the adjacency would simply persist longer before dropping. Tempting because hello/hold timers do govern adjacency stability, yet the log's regular flapping indicates the transport is repeatedly failing, not delayed detection.
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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
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