Question 170 of 2,011
hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer configures unicast Reverse Path…
An engineer configures unicast Reverse Path Forwarding (uRPF) in strict mode on an interface. After the configuration, legitimate traffic from a customer network is being dropped. The engineer verifies that the customer's IP prefix is in the routing table. 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 router has multiple equal-cost paths to the customer network, and the return path uses a different interface, causing strict uRPF to drop the packet.
Strict uRPF checks that the source IP of incoming packets has a route back to the same interface. If there is asymmetric routing, where the return path goes out a different interface, strict uRPF will drop the packets. A common edge case is when the router has multiple equal-cost paths to the source network, and the return traffic uses a different interface than the one the packet arrived on. In such cases, uRPF strict mode will fail because it only checks the best route, not all routes.
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 router has multiple equal-cost paths to the customer network, and the return path uses a different interface, causing strict uRPF to drop the packet.
Why this is correct
Strict uRPF requires that the best route to the source points back to the receiving interface. If there are multiple equal-cost paths, the router may choose a different interface for the return path, causing drops.
- ✗
The customer network uses private IP addresses that are not routable, so uRPF drops them.
Why it's wrong here
uRPF does not care about private vs public IPs; it only checks the routing table.
- ✗
The interface is configured with 'ip verify unicast source reachable-via any', which is loose mode, not strict mode.
Why it's wrong here
The scenario says strict mode is configured, so this is not the issue.
- ✗
The routing table has a default route that points to a different interface, causing uRPF to use the default route for verification.
Why it's wrong here
By default, uRPF does not use default routes for verification unless the 'allow-default' option is configured. If a default route exists, it may cause issues, but the scenario says the customer prefix is in the routing table, so the default route is not used.
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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Last reviewed: Jun 19, 2026
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