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 connected to a network with asymmetric routing. Users report intermittent connectivity issues. Which is the most likely explanation?
⚠ Common exam trap
Cisco often tests the misconception that uRPF drops packets only when there is no route for the source IP, but the real trap is that strict mode requires the reverse path to use the exact same interface, which fails under asymmetric routing even when a valid route exists.
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 packets to arrive on an interface that is not the best return path, so uRPF drops them.
Unicast Reverse Path Forwarding (uRPF) in strict mode checks that the source IP address of an incoming packet has a route in the routing table pointing back out the same interface on which the packet arrived. In an asymmetric routing scenario, packets may arrive on an interface that is not the best return path, causing the uRPF check to fail and the packet to be dropped. This explains the intermittent connectivity issues reported by users.
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 packets to arrive on an interface that is not the best return path, so uRPF drops them.
Why this is correct
Strict uRPF checks that the source address is reachable via the same interface the packet arrived on. With asymmetric routing, return traffic uses a different path, so the incoming interface fails the reverse-path lookup and legitimate packets are dropped, causing intermittent connectivity.
- ✗
The uRPF configuration is missing the 'allow-default' option.
Why it's wrong here
The allow-default option permits the default route to satisfy the reverse lookup, which is a loose-mode behaviour; strict mode deliberately omits it. Adding it is tempting because it appears to relax the check. Intermittent failures stem from asymmetric routing, where the reverse path exits a different interface than the one receiving traffic.
- ✗
The routing table does not have a route for the source IP.
Why it's wrong here
A missing route for the source address would drop traffic consistently, not intermittently, and strict uRPF already requires a matching route. It is tempting because uRPF is fundamentally a source-address lookup. The actual cause is asymmetric routing: the reverse path points out a different interface, so legitimate return packets fail the check.
- ✗
The interface has multiple IP addresses.
Why it's wrong here
Strict uRPF checks only the primary address of the receiving interface against the routing table; additional secondary addresses do not alter that lookup, so they cannot explain intermittent drops. Multiple addresses are tempting because they complicate interface configuration. Asymmetric routing itself causes the failure, since return traffic arrives on an interface the FIB would not select.
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 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.