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200-901 Practice Question: An engineer is troubleshooting packet loss…
An engineer is troubleshooting packet loss between two hosts on different subnets. The traceroute shows that packets reach the first hop router but then stop. The router's ARP table shows an incomplete entry for the next-hop IP address. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between Layer 2 failures (incomplete ARP) and Layer 3 failures (routing or ACL issues), tempting candidates to blame routing convergence or ACLs when the real problem is a missing ARP reply due to a physical or data-link problem.
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 next-hop device is powered off or has a Layer 1 issue.
An incomplete ARP entry for the next-hop IP address indicates that the router sent an ARP request but received no reply. This typically means the next-hop device is unreachable at Layer 2, often because it is powered off, has a failed interface, or there is a cabling issue. Since traceroute shows packets reaching the first hop but not beyond, the Layer 2 failure at the next hop is the most direct cause.
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 MTU is misconfigured on the outgoing interface.
Why it's wrong here
MTU issues cause packet drops but not ARP incompleteness.
- ✗
The routing protocol has not converged yet.
Why it's wrong here
Routing convergence affects the route table, not ARP.
- ✓
The next-hop device is powered off or has a Layer 1 issue.
Why this is correct
If the next-hop device is offline, the router cannot complete ARP.
- ✗
An ACL is blocking traffic on the outgoing interface.
Why it's wrong here
ACLs drop packets after routing, they don't prevent ARP resolution.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-901 practice question is part of Courseiva's free Cisco 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 200-901 exam.