hardMultiple ChoiceObjective-mapped
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 mode drops packets if the reverse path does not match the incoming interface, which occurs with asymmetric routing.
- ✗
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
While loose mode would work, the question asks for the explanation of the drop, not the fix.
- ✗
The customer network is using private IP addresses that are not routable.
Why it's wrong here
Private IPs would be dropped at the ISP edge, but the issue is asymmetric routing.
- ✗
The uRPF allow-default option is missing, which is required for default routes.
Why it's wrong here
Allow-default is for default routes, not for asymmetric routing.
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
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