300-410 Infrastructure Services Practice Question
A network engineer is troubleshooting a Cisco IOS XE router that is configured for IPv6 First Hop Security on a user VLAN. Hosts report intermittent connectivity, and the engineer suspects that IPv6 Router Advertisement (RA) messages from an unauthorized device are being accepted. Which feature should be enabled to ensure that only RAs from the legitimate router are processed by hosts?
⚠ Common exam trap
Test-takers frequently confuse RA Guard with DHCPv6 Guard or Source Guard, but only RA Guard specifically inspects and filters Router Advertisement messages to prevent rogue default router advertisements.
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
✓
IPv6 RA Guard
RA Guard is the IPv6 First Hop Security feature that filters Router Advertisement and Redirect messages on a per-port basis. When configured to block RAs on host-facing ports, it ensures hosts only accept RAs from the authorized router, mitigating rogue RA attacks. The other features address different threats such as rogue DHCPv6 servers or spoofed source addresses.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
IPv6 Source Guard
Why it's wrong here
IPv6 Source Guard validates the source address of IPv6 packets against the binding table. It does not prevent a rogue device from sending Router Advertisement messages with a valid source address. RA spoofing requires a dedicated RA Guard policy to filter at Layer 2.
- ✗
IPv6 Destination Guard
Why it's wrong here
Destination Guard enforces address reachability by dropping traffic to destinations not present in the binding table. It does not filter Router Advertisement messages, so it cannot stop an unauthorized device from advertising itself as a default router. The symptom points to RA spoofing, not destination reachability.
- ✓
IPv6 RA Guard
Why this is correct
RA Guard examines incoming Router Advertisement and Redirect messages on a port and can block or allow them based on a policy. By configuring RA Guard on host-facing ports to block RAs, only the legitimate router's RAs are accepted, preventing rogue RA attacks and restoring stable connectivity.
- ✗
IPv6 DHCPv6 Guard
Why it's wrong here
DHCPv6 Guard filters DHCPv6 server and client messages on a port, preventing rogue DHCPv6 servers. It does not inspect or filter Router Advertisement messages, so an unauthorized device sending RAs would still be able to influence hosts. The issue described involves RA spoofing, not DHCPv6.
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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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