hardMultiple Choice
300-410 Practice Question: An engineer enables unicast RPF (uRPF) in strict…
An engineer enables unicast RPF (uRPF) in strict mode on an interface. Afterward, some legitimate traffic from a BGP neighbor is dropped. The neighbor has two paths to the router, and traffic may arrive on a different interface than the return path. What is the most likely explanation?
⚠ Common exam trap
The trap is conflating strict uRPF with a simple reachability check; candidates forget that strict mode demands interface symmetry, which breaks in asymmetric routing topologies even when the source is perfectly reachable.
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 uRPF drops packets if the source IP is not reachable via the receiving interface, which fails in asymmetric routing scenarios.
Strict unicast RPF checks that the source IP of each incoming packet is reachable through the same interface on which the packet arrived, using the FIB. In an asymmetric routing scenario where the BGP neighbor has two paths and return traffic leaves via a different interface than the one receiving the packet, the reverse lookup fails and strict uRPF drops the legitimate traffic. This is the classic failure mode of strict mode in multihomed or load-balanced designs.
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 uRPF drops packets if the source IP is not reachable via the receiving interface, which fails in asymmetric routing scenarios.
Why this is correct
Strict uRPF checks that the packet's source is reachable through the receiving interface's routing table. With two paths and asymmetric routing, return traffic may use a different interface, so legitimate BGP-sourced packets fail the check and are dropped.
- ✗
The uRPF 'allow-default' option was not configured, so default routes are ignored.
Why it's wrong here
The allow-default option only affects whether the default route can satisfy the uRPF lookup; it does not explain drops when a specific route to the source exists but points out a different interface. It is tempting because default-route handling is a genuine uRPF nuance, but it is irrelevant to this asymmetric-path scenario.
- ✗
The neighbor's BGP updates have a source IP that is not in the routing table.
Why it's wrong here
Strict uRPF checks that the source is reachable via the same interface the packet arrived on; asymmetric arrival on the other path fails that check even though the source prefix is present in the routing table. The option is tempting because missing routes do cause drops, but here the route exists via a different interface.
- ✗
Loose mode should be used instead, but strict mode was configured by mistake.
Why it's wrong here
Strict uRPF drops traffic arriving on an interface other than the one the routing table uses to reach the source; switching to loose mode would accept any source present in the table regardless of arrival interface. Loose mode is tempting for multihomed asymmetric designs, but the stem asks for the explanation of the drops, not a remedy.
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.