hardMultiple Choice
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?
⚠ Common exam trap
The trap here is assuming that because the CE has a route back to the source, uRPF should pass; candidates forget that strict uRPF checks the PE's own return path interface, not the CE's routing table.
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.
Strict uRPF requires that the source IP of an incoming packet be reachable via the exact same interface on which the packet arrived. In an MPLS L3VPN, the PE router performs the uRPF lookup in the VRF routing table, and if the return path to the source uses a different interface (asymmetric routing), the strict check fails and the packet is dropped. The CE having a route back to the source is irrelevant because uRPF is enforced on the PE's ingress interface, not on the CE.
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
Strict uRPF verifies the packet's source is reachable via the same interface it arrived on. With asymmetric routing the return path differs, so the check fails even though the CE holds a valid route, dropping legitimate traffic.
- ✗
The uRPF 'allow-default' option is not configured, so default routes are not considered.
Why it's wrong here
The allow-default option concerns whether the default route can satisfy the reverse lookup; the stem gives a specific route back on the CE, so a missing default is irrelevant. It is tempting because allow-default is a genuine uRPF parameter, but it applies when only a default route exists.
- ✗
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
Loose mode checks only that a route to the source exists in any table, so it drops less than strict mode, not all traffic. It is tempting because loose mode is a real uRPF variant, but the stem already states strict mode is configured, making this a contradiction rather than the explanation.
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
Advanced EIGRP Features (Stub, Load Balancing, Named Mode)
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 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
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.