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300-410 Practice Question: An engineer configures unicast Reverse Path…
An engineer configures unicast Reverse Path Forwarding (uRPF) in strict mode on an interface connected to a service provider. The router has a default route pointing to the ISP. Traffic from the ISP is being dropped by uRPF. Which is the most likely explanation?
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
✓
Strict mode uRPF does not use the default route for verification unless the 'allow-default' option is enabled.
Strict mode uRPF checks that the source IP of incoming packets has a matching route in the routing table that points back to the same interface. If the router has a default route, it may not match the specific source IP, causing drops. However, the edge case is that the default route is not considered by strict mode unless the 'allow-default' option is configured. Without 'allow-default', strict mode requires a specific route back to the interface.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Strict mode uRPF does not use the default route for verification unless the 'allow-default' option is enabled.
Why this is correct
By default, strict mode ignores default routes; 'allow-default' includes them.
- ✗
The interface is configured with the wrong IP address, causing uRPF to fail.
Why it's wrong here
IP address mismatch would cause connectivity issues, not uRPF drops.
- ✗
uRPF should be configured in loose mode to work with default routes.
Why it's wrong here
Loose mode checks only that a route exists, not the interface; but strict mode with 'allow-default' can work.
- ✗
The router has multiple default routes, causing uRPF to fail.
Why it's wrong here
Multiple default routes do not cause uRPF to fail; the issue is the lack of 'allow-default'.
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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.