300-410 Infrastructure Security Practice Question
A network administrator is configuring Unicast Reverse Path Forwarding (uRPF) on a Cisco IOS router to mitigate IP spoofing. The router has two interfaces: GigabitEthernet0/0 connects to the internet, and GigabitEthernet0/1 connects to the internal network. The administrator wants to ensure that packets arriving on GigabitEthernet0/0 are dropped if their source address is not reachable via that interface. However, the administrator also wants to allow asymmetric routing where return traffic may use a different path. Which uRPF mode should be configured on GigabitEthernet0/0?
⚠ Common exam trap
Many candidates confuse strict and loose uRPF modes, or using outdated syntax, when the requirement to allow asymmetric routing points to loose mode.
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
✓
ip verify unicast source reachable-via any
Loose uRPF, configured with ip verify unicast source reachable-via any, checks the routing table for the source address but does not require the source to be reachable via the incoming interface. This allows asymmetric routing while still providing spoofing mitigation. Strict uRPF (rx) would drop packets in asymmetric scenarios, and the other options are either invalid or equivalent to strict mode.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
ip verify unicast source reachable-via rx
Why it's wrong here
The rx keyword enables strict uRPF, which requires that the source address be reachable via the same interface the packet arrived on. This would drop packets in asymmetric routing scenarios because the return path may differ. While it provides strong spoofing protection, it does not meet the requirement to allow asymmetric routing.
- ✓
ip verify unicast source reachable-via any
Why this is correct
The any keyword enables loose uRPF, which checks that the source address is reachable via any interface in the routing table. This allows asymmetric routing because the return path can be different, while still dropping packets with spoofed source addresses that are not in the routing table. This meets the requirement.
- ✗
ip verify unicast reverse-path
Why it's wrong here
The ip verify unicast reverse-path command is the older syntax for strict uRPF. It is equivalent to reachable-via rx and would therefore not allow asymmetric routing. This command would drop packets if the source is not reachable via the incoming interface, failing the requirement.
- ✗
ip verify unicast source reachable-via tx
Why it's wrong here
The tx keyword is not a valid option for uRPF. The correct keywords are rx for strict mode and any for loose mode. Using tx would result in a syntax error and not provide any spoofing protection. This option is invalid.
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
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