JN0-106 Networking Fundamentals Practice Question
You are responsible for a Juniper MX router that connects two customer sites over a Layer 3 VPN. The router is configured with BGP for VPN routes and uses MPLS to forward traffic. Recently, the customer reported that traffic from Site A (10.0.1.0/24) to Site B (10.0.2.0/24) is intermittently failing. You check the routing table on the router and see that both routes are present with valid next-hops. However, when you ping from the router's loopback to the remote site's loopback, the ping succeeds. MPLS labels are being assigned and the LSP is up. You also notice that when the failure occurs, the router's BGP session to the remote PE is still established. The failure seems random and lasts a few seconds before recovering. Which troubleshooting step is most likely to identify the root cause?
⚠ Common exam trap
Many candidates assume a successful ping from the loopback confirms end-to-end forwarding, but the loopback ping uses a different path (e.g., in-band management) and does not test the specific MPLS label path for the customer prefixes, masking the forwarding table discrepancy.
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
✓
Review the forwarding table for the affected prefixes and check for any discrepancies.
The issue is intermittent packet loss with valid routes in the routing table but successful pings from the loopback. This points to a forwarding table (FIB) inconsistency, where the control plane (routing table) has the correct next-hop, but the forwarding plane (PFE) may have a stale or incorrect entry for the specific prefixes. Checking the forwarding table with 'show route forwarding-table' will reveal if the next-hop or label information differs from the routing table, which is a classic symptom of a hardware programming issue or a transient PFE problem.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Check the interface error counters for CRC errors or drops.
Why it's wrong here
Interface errors would likely cause persistent failures, not intermittent.
- ✓
Review the forwarding table for the affected prefixes and check for any discrepancies.
Why this is correct
The forwarding table may have stale entries or incorrect label operations causing intermittent forwarding failures.
- ✗
Verify the BGP session state and check for route flapping.
Why it's wrong here
The BGP session is established and routes are present, so route flapping may not be the cause.
- ✗
Check the MPLS label switching table to ensure labels are correctly assigned.
Why it's wrong here
The LSP is up and labels are assigned, so this is likely not the issue.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This JN0-106 practice question is part of Courseiva's free Juniper Networks certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the JN0-106 exam.