hardMultiple Choice
300-410 Practice Question: An engineer enables uRPF (strict mode) on an…
An engineer enables uRPF (strict mode) on an interface facing the Internet. Legitimate traffic from a customer network is being dropped. The customer network uses asymmetric routing where return traffic takes a different path. 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
✓
The uRPF strict mode requires that the source IP's best path is out the same interface; asymmetric routing violates this.
Strict uRPF checks that the source IP of incoming packets has a route in the FIB pointing back to the same interface. With asymmetric routing, the return path may use a different interface, causing the check to fail and the packet to be dropped.
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 uRPF strict mode requires that the source IP's best path is out the same interface; asymmetric routing violates this.
Why this is correct
Strict uRPF checks that the packet's source address is reachable via the same interface it arrived on. Asymmetric routing sends return traffic by a different path, so the reverse-path lookup fails and legitimate customer traffic is dropped.
- ✗
The uRPF loose mode should be used instead, as it only requires a route to the source IP in the FIB.
Why it's wrong here
Loose mode checks only that a route to the source exists in the FIB, which tolerates asymmetric paths; strict mode requires the source be reachable via the receiving interface. Loose mode is tempting because it is the standard fix for asymmetry, but the question asks for the explanation of the drop, not the remedy.
- ✗
The customer network is using private IP addresses that are not routable.
Why it's wrong here
Strict uRPF checks the source against the incoming interface's FIB entry, so asymmetric return paths cause drops regardless of addressing. Private addresses are tempting because they are non-routable, but that would be the cause only if the customer genuinely used RFC 1918 space across the Internet.
- ✗
The uRPF allow-default option is missing, which is required for default routes.
Why it's wrong here
The allow-default option permits strict uRPF to pass traffic when the only matching route is a default route; it is not required merely because a default route exists. It is tempting because default routes are common on Internet-facing interfaces, but the drop stems from the source not being reachable via the receiving interface.
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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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