hardMultiple Choice
300-410 Practice Question: An engineer configures uRPF (Unicast Reverse Path…
An engineer configures uRPF (Unicast Reverse Path Forwarding) in strict mode on a router interface facing the Internet. After configuration, legitimate traffic from customers is being dropped. The engineer verifies that the routing table has a route back to the source IP address. Which is the most likely explanation?
⚠ Common exam trap
300-410 often tests the assumption that 'route exists = uRPF passes' — the trap is forgetting that strict uRPF requires the route to point out the SAME interface, making asymmetric routing the classic failure cause.
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
✓
Asymmetric routing causes the return path to use a different interface
Strict uRPF checks that the source IP of an incoming packet is reachable via the same interface the packet arrived on. If the return route to the source points out a different interface (asymmetric routing), the check fails and the packet is dropped — even though a route to the source exists in the RIB.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Asymmetric routing causes the return path to use a different interface
Why this is correct
Strict uRPF requires the source's return path to match the ingress interface. With asymmetric routing, the best route back to the source exits a different interface, so the check fails and legitimate packets are dropped despite the route existing in the table.
- ✗
The uRPF allow-default option is not configured
Why it's wrong here
Strict uRPF drops packets unless the source is reachable via the same interface they arrived on; asymmetric routing from customers makes the return path differ, so allow-default (or loose mode) is needed. It is tempting because allow-default is a real uRPF setting, and it is correct when you want to permit traffic lacking any return route.
- ✗
The source IP address is not in the routing table
Why it's wrong here
The stem already confirms a route back to the source exists, so this cannot explain the drops. Strict uRPF checks the incoming interface against the routing table's return path; the route may exist via a different interface, causing the drop.
- ✗
uRPF strict mode requires CEF to be disabled
Why it's wrong here
uRPF strict mode depends on CEF; it is enabled by default and disabling it would break forwarding entirely, not cause selective drops. The drop stems from asymmetric routing, where the return path differs from the ingress interface. CEF is unrelated to this failure.
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.