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 the ingress interface of a PE router in an MPLS L3VPN. The router is receiving VPN traffic from a customer edge (CE) router. The engineer notices that some legitimate traffic is being dropped by uRPF. The engineer verifies that the CE router has a route back to the source address in its routing table. What 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
✓
Asymmetric routing is causing the return path to use a different interface, violating the strict uRPF check.
uRPF strict mode checks that the source address of an incoming packet has a route in the routing table that points back to the same interface on which the packet was received. If there is asymmetric routing (i.e., the return path takes a different interface), uRPF strict mode will drop the packet. In an MPLS L3VPN, traffic from the CE to the PE may take one path, but return traffic from the PE to the CE may take a different path (e.g., due to load balancing or different routing policies). This is a common edge case. The solution is to use uRPF loose mode or to ensure symmetric routing.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Asymmetric routing is causing the return path to use a different interface, violating the strict uRPF check.
Why this is correct
Correct. uRPF strict mode requires that the return path uses the same interface; asymmetric routing causes legitimate traffic to be dropped.
- ✗
The uRPF 'allow-default' option is not configured, so default routes are not considered.
Why it's wrong here
Incorrect. The 'allow-default' option allows uRPF to consider default routes, but the issue here is asymmetric routing, not default routes.
- ✗
The CE router is not advertising the source network to the PE via BGP.
Why it's wrong here
Incorrect. If the CE were not advertising the source network, the PE would not have a route to the source, and uRPF would drop the packet. But the question states the CE has a route back.
- ✗
The uRPF mode is set to 'loose' instead of 'strict', causing all traffic to be dropped.
Why it's wrong here
Incorrect. Loose mode only checks that a route exists, not the interface; it would not cause drops due to 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 |
Go deeper
Related to this question
Learn chapter
OSPF Route Summarization and Filtering
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.
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
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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.