hardMultiple Choice
300-410 Practice Question: An engineer configures unicast Reverse Path…
An engineer configures unicast Reverse Path Forwarding (uRPF) in strict mode on an interface facing the Internet. Legitimate traffic from a customer network is being dropped. The traffic has a source IP that belongs to the customer's prefix, which is reachable via a different interface on the router. Which is the most likely explanation?
⚠ Common exam trap
Candidates often confuse strict and loose uRPF modes, or assuming that uRPF automatically adapts to asymmetric routing; candidates may overlook that strict mode requires the return path to be via the same 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 customer traffic is arriving on an interface where the return path to the source is via a different interface, violating strict uRPF.
Strict uRPF requires that the source IP of an incoming packet be reachable via the same interface on which the packet arrived. In this scenario, the customer's source prefix is reachable via a different interface, so strict uRPF drops the legitimate traffic. This is the expected behavior of strict mode when routing is asymmetric.
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 'ip verify unicast source reachable-via any' command was used instead of 'rx'.
Why it's wrong here
'reachable-via any' is loose mode, which accepts any source with a route in the RIB, so it would forward this traffic rather than drop it. Loose mode is correct where asymmetric routing is expected and spoofing protection is relaxed.
- ✗
The router has a default route pointing out the same interface, causing uRPF to pass all traffic.
Why it's wrong here
A default route out the same interface makes strict uRPF pass traffic, not drop it, so this contradicts the symptom. Default-route handling is relevant when deciding whether unknown sources should be permitted on an Internet-facing interface.
- ✓
The customer traffic is arriving on an interface where the return path to the source is via a different interface, violating strict uRPF.
Why this is correct
Strict uRPF checks that the source is reachable via the same interface the packet arrived on. Because the customer prefix routes back through a different interface, the check fails and legitimate traffic is dropped, exactly as the stem describes.
- ✗
The 'ip urpf allow-default' command is missing, causing default routes to be ignored.
Why it's wrong here
Strict mode ignores the default route by design; the missing allow-default setting would only matter if the engineer wanted default-routed sources accepted. Allow-default is correct when traffic arrives from sources known only via a default route.
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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.