CCNP Security Practice Question
A network administrator is configuring a Cisco IOS XE router to protect against spoofed source addresses on an internal interface facing user subnets. The requirement is to drop packets whose source address does not match the routing table entry for the incoming interface. Which feature should be enabled?
⚠ Common exam trap
Test-takers frequently confuse Layer 2 anti-spoofing features like IP Source Guard and Dynamic ARP Inspection with Layer 3 uRPF, which uses the routing table to validate sources on routed interfaces.
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
✓
Unicast Reverse Path Forwarding (uRPF) in strict mode on the interface
Unicast RPF in strict mode checks the routing table for the source address of each incoming packet and verifies that the source is reachable via the same interface the packet arrived on. If not, the packet is dropped, which prevents source address spoofing. This is the standard IOS XE feature for validating source addresses on an interface against the routing table.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Access Control List with the log keyword applied to the interface
Why it's wrong here
An ACL can filter traffic but requires manual entries for each subnet and does not dynamically validate source addresses against the routing table. Using the log keyword only records matches and does not scale or adapt to routing changes. Since the requirement is automatic validation of source reachability on the incoming interface, an ACL is not the right solution.
- ✗
IP Source Guard on the interface
Why it's wrong here
IP Source Guard relies on DHCP snooping bindings to permit only known IP-to-MAC pairs on a Layer 2 switch port. It is a switch access-layer feature and does not use the routing table to validate sources on a routed interface. On a router interface facing user subnets, IP Source Guard is not the appropriate mechanism, so it does not meet the stated requirement.
- ✓
Unicast Reverse Path Forwarding (uRPF) in strict mode on the interface
Why this is correct
Strict uRPF checks that the source address of an incoming packet is reachable via the same interface the packet arrived on, using the routing table. If the source is not reachable through that interface, the packet is dropped, which directly blocks spoofed source addresses from user subnets. This matches the requirement to validate source addresses against the routing table for the incoming interface.
- ✗
Dynamic ARP Inspection on the interface
Why it's wrong here
Dynamic ARP Inspection validates ARP packets against DHCP snooping bindings to prevent ARP spoofing. It does not inspect IP source addresses of data packets or consult the routing table. The scenario requires dropping spoofed IP source addresses based on routing information, which DAI does not do, so it is not the correct feature.
Visual reference
About these practice questions
This 350-401 question is part of Courseiva's 1,923-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
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 350-401 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 350-401 exam.