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300-410 Practice Question: Is troubleshooting a connectivity issue where a…
A network engineer is troubleshooting a connectivity issue where a host on VLAN 10 cannot reach a server on VLAN 20. Both VLANs are on the same switch, which is running IPv6. The engineer checks the switch and finds that uRPF (unicast Reverse Path Forwarding) is enabled in strict mode on the VLAN 20 interface. The host's IPv6 address is 2001:db8:10::100/64, and the server's address is 2001:db8:20::200/64. The switch has a default route pointing to a next-hop router. The host sends traffic to the server, but the switch drops the packets. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between strict and loose uRPF modes, and the trap here is that candidates assume a default route is sufficient for strict mode, not realizing that strict mode requires a specific route pointing back to the source's ingress interface.
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 strict mode check fails because the switch does not have a specific route to the host's subnet pointing back to the VLAN 10 interface.
With uRPF strict mode enabled on the VLAN 20 interface, the switch checks that the source IP address of incoming packets (2001:db8:10::100) is reachable via the same interface on which the packet arrived. Since the switch only has a default route pointing to a next-hop router and no specific route for the 2001:db8:10::/64 subnet pointing back to the VLAN 10 interface, the FIB lookup for the source address fails the strict uRPF check, causing the switch to drop the packet.
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 strict mode check fails because the switch does not have a specific route to the host's subnet pointing back to the VLAN 10 interface.
Why this is correct
Correct because uRPF strict mode requires a matching route in the FIB that points to the same interface on which the packet was received; a default route does not satisfy this requirement.
- ✗
The uRPF mode should be loose mode to allow traffic from any source as long as there is a route in the FIB.
Why it's wrong here
Incorrect because loose mode would still require a route to the source, but a default route would satisfy loose mode; however, the scenario indicates strict mode is the issue.
- ✗
The host's IPv6 address is not in the switch's neighbor cache.
Why it's wrong here
Incorrect because uRPF checks the routing table, not the neighbor cache.
- ✗
The switch has an ACL that blocks traffic between VLANs.
Why it's wrong here
Incorrect because the symptom is specifically related to uRPF, not an ACL.
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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.