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300-410 Practice Question: Is troubleshooting a scenario where a router is…
A network engineer is troubleshooting a scenario where a router is dropping IPv6 packets that are destined for a server on a directly connected network. The engineer checks the interface and finds that uRPF is enabled in loose mode. The router has a default route pointing to an upstream router. The source address of the packets is 2001:db8:100::1, which is not in the routing table (the router has no route to that prefix). What is the most likely cause of the packet drops?
⚠ Common exam trap
Cisco often tests the misconception that a default route satisfies uRPF loose mode, but in reality, uRPF requires a route to the specific source prefix, not a default route.
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 uRPF loose mode check fails because there is no route to the source address in the routing table.
With uRPF loose mode enabled, the router checks whether a route to the source address exists in the routing table, but it does not verify that the incoming interface matches the reverse path. Since the source address 2001:db8:100::1 is not present in the routing table, the loose mode check fails and the packets are dropped. The presence of a default route does not help because uRPF requires an exact match for the source prefix, not a default route.
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 uRPF loose mode check fails because there is no route to the source address in the routing table.
Why this is correct
Correct because loose mode requires at least one route to the source address in the FIB; if no route exists, the packet is dropped.
- ✗
The uRPF loose mode check fails because the source address is not reachable via the same interface.
Why it's wrong here
Incorrect because loose mode does not require the same interface; it only requires any route.
- ✗
The router has an ACL that blocks traffic from that source.
Why it's wrong here
Incorrect because the symptom is specifically related to uRPF, not an ACL.
- ✗
The uRPF mode should be strict mode to allow the traffic.
Why it's wrong here
Incorrect because strict mode would also drop the packet if the route does not point back to the same interface.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 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 300-410 exam.