CCNA Network Infrastructure and Connectivity Practice Question
A technician is troubleshooting a dual-stack network where an IPv6-only host cannot reach an IPv4 resource. The technician issues the show ipv6 interface brief command on the local router and notices the interface facing the host has a link-local address but no global unicast address. The technician then checks the running configuration and finds that the command ipv6 unicast-routing is missing. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the misconception that configuring an IPv6 address on an interface is sufficient for IPv6 routing, when in fact the global `ipv6 unicast-routing` command is required to enable the router to forward IPv6 packets and send 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 unicast routing has not been enabled on the router.
The missing `ipv6 unicast-routing` command means the router is not acting as an IPv6 router, so it does not send Router Advertisement (RA) messages. Without RAs, the host cannot autoconfigure a global unicast address or learn a default gateway, breaking IPv6 connectivity to any IPv4 resource even if NAT64 is present.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
An IPv6 access list on the router is blocking Router Advertisement messages.
Why it's wrong here
An access list could block ICMPv6 Router Advertisements, preventing hosts from obtaining prefixes and global unicast addresses. However, this does not explain why the router's own interface lacks a global unicast address when statically configured or when the router itself should be generating RAs. The missing ipv6 unicast-routing command is a more fundamental cause.
- ✓
IPv6 unicast routing has not been enabled on the router.
Why this is correct
The ipv6 unicast-routing command is required to enable IPv6 forwarding on Cisco routers. Without it, the router does not participate in IPv6 routing, does not generate Router Advertisements, and interfaces will not obtain global unicast addresses through SLAAC or DHCPv6 relay. The show ipv6 interface output displaying only a link-local address, combined with the absence of ipv6 unicast-routing in the configuration, confirms this root cause.
- ✗
The IPv4 resource is not configured for NAT64 translation.
Why it's wrong here
While NAT64 is a common method for IPv6-only hosts to access IPv4 resources, the symptom explicitly shows a missing global unicast address on the router interface and a missing ipv6 unicast-routing command. This indicates a basic IPv6 routing failure, not a translation service configuration issue.
- ✗
The host has an incorrect default gateway for IPv6.
Why it's wrong here
A misconfigured default gateway on the host would prevent it from reaching off-link resources. However, the technician observed a missing global unicast address on the router interface and the absence of ipv6 unicast-routing, indicating that the router itself is not properly configured for IPv6 routing. The host's gateway issue would be a consequence, not the root cause.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓IPv6 unicast routing has not been enabled on the router.Correct answer▾
Why this is correct
The ipv6 unicast-routing command is required to enable IPv6 forwarding on Cisco routers. Without it, the router does not participate in IPv6 routing, does not generate Router Advertisements, and interfaces will not obtain global unicast addresses through SLAAC or DHCPv6 relay. The show ipv6 interface output displaying only a link-local address, combined with the absence of ipv6 unicast-routing in the configuration, confirms this root cause.
✗An IPv6 access list on the router is blocking Router Advertisement messages.Wrong answer — click to see why▾
Why this is wrong here
This option focuses on a filtering issue, not the disabled routing engine, and would not cause the router’s own interface to lack a global unicast address.
✗The IPv4 resource is not configured for NAT64 translation.Wrong answer — click to see why▾
Why this is wrong here
NAT64 configuration would affect translation, but the root cause visible in the output is the lack of IPv6 routing capability on the router.
✗The host has an incorrect default gateway for IPv6.Wrong answer — click to see why▾
Why this is wrong here
This shifts the blame to the host, but the router-side evidence (missing command and missing global unicast) clearly indicates a router configuration problem.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Go deeper
Related to this question
Learn chapter
Troubleshoot: Inter-VLAN Routing Not Working
Key term
Router
A router is a networking device that connects different networks together and directs data traffic between them by choosing the best path for data to travel.
Key term
IPv6
IPv6 is the most recent version of the Internet Protocol, designed to replace IPv4 by providing a vastly larger number of unique addresses and improved network features.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 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 200-301 exam.