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300-410 Practice Question: A router is configured with uRPF (Unicast Reverse…
A router is configured with uRPF (Unicast Reverse Path Forwarding) in strict mode on an interface that belongs to a VRF. The network uses asymmetric routing for load balancing. The engineer notices that legitimate traffic from a customer is being dropped. 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 address be reachable via the same interface, but asymmetric routing causes the return path to use a different interface.
Strict uRPF checks that the source IP address of incoming packets has a route in the routing table that points back to the same interface. In asymmetric routing, the return path may be different (e.g., out another interface), so the source IP may not have a route back to the incoming interface. This causes legitimate traffic to be dropped. The edge case is that uRPF strict mode does not account for asymmetric routing, while loose mode does.
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 address be reachable via the same interface, but asymmetric routing causes the return path to use a different interface.
Why this is correct
Strict uRPF fails if the best route to the source is not via the incoming interface. Asymmetric routing violates this assumption.
- ✗
The uRPF loose mode is configured instead of strict mode, which only checks that a route exists for the source IP, not the interface.
Why it's wrong here
Loose mode would allow the traffic, but the question states strict mode is configured.
- ✗
The VRF has a default route that points to the incoming interface, causing uRPF to always succeed.
Why it's wrong here
A default route would make uRPF succeed, not fail.
- ✗
The 'ip verify unicast source reachable-via any' command is used, which is the loose mode, not strict.
Why it's wrong here
This is loose mode, not strict.
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 |
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
MPLS Layer 3 VPN
A technology that uses Multiprotocol Label Switching to create secure, scalable virtual private networks that connect multiple sites at the network layer, where the service provider manages routing between customer sites.
Key term
uRPF
Unicast Reverse Path Forwarding is a network security feature that verifies the source address of incoming packets to prevent IP spoofing attacks.
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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.