The correct troubleshooting step is to review the forwarding table for the affected prefixes and check for any discrepancies. This is because the symptom of intermittent packet loss with valid routes in the routing table and successful pings from the router’s loopback points directly to a forwarding table inconsistency, where the control plane (routing table) has the correct next-hop but the forwarding plane (PFE) holds a stale or incorrect entry. On the JNCIA-Junos exam, this scenario tests your understanding of the separation between the Routing Engine and the Packet Forwarding Engine, a core Junos concept. A common trap is to assume that a valid BGP session or MPLS LSP guarantees traffic flow, but the forwarding table must be verified with the command show route forwarding-table. Remember the mnemonic: “Routes in the brain, forwarding in the veins” – if the brain (routing table) is correct but the veins (forwarding table) are clogged, traffic still fails.
JNCIA-JUNOS Networking Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of networking fundamentals. 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.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
Option B is correct because 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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that 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.
Detailed technical explanation
How to think about this question
In Junos, the routing table (inet.0) stores routes from protocols, while the forwarding table (inet.0 in the PFE) is a separate copy used for actual packet forwarding. Discrepancies can occur due to a 'next-hop resolution' failure where the routing table has a valid next-hop but the PFE cannot resolve it to an outgoing interface, or due to a 'kernel route add' failure during high CPU. This is often seen in scaled environments where the PFE runs out of hardware resources, causing random prefix drops that recover when resources are freed.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the JNCIA-JUNOS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking Fundamentals — This question tests Networking Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Review the forwarding table for the affected prefixes and check for any discrepancies. — Option B is correct because 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.
What should I do if I get this JNCIA-JUNOS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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