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300-410 Practice Question: An engineer is troubleshooting an ISATAP tunnel…
An engineer is troubleshooting an ISATAP tunnel between a Windows host and a Cisco router. The host can ping the router's IPv6 address configured on the tunnel interface, but cannot reach any other IPv6 networks beyond the router. The router has a default route pointing to an upstream IPv6 router. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between simply configuring a tunnel interface (which allows direct communication) and enabling the ISATAP router functionality (which is required for prefix advertisement and routing beyond the tunnel).
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
✓
The router is not configured as an ISATAP router; it only has the tunnel interface but lacks the 'ipv6 isatap' command.
For ISATAP to function, the router must be explicitly configured as an ISATAP router using the 'ipv6 isatap' command under the tunnel interface. Without this command, the router will not advertise the ISATAP prefix or respond to Router Solicitations from the host, so the host can only communicate with the router's own tunnel IPv6 address but cannot learn a default route or reach other IPv6 networks.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The router is not configured as an ISATAP router; it only has the tunnel interface but lacks the 'ipv6 isatap' command.
Why this is correct
Correct because without 'ipv6 isatap', the router does not respond to ISATAP router solicitations, and the host will not use it as a default gateway for off-link traffic.
- ✗
The host's ISATAP interface has an incorrect IPv4 address for the router's tunnel source.
Why it's wrong here
Incorrect because the host can ping the router's IPv6 address, indicating the tunnel is working for direct communication.
- ✗
The router's tunnel interface is missing the 'ipv6 enable' command.
Why it's wrong here
Incorrect because the tunnel interface is up and reachable, so IPv6 is enabled on it.
- ✗
The upstream router does not have a route back to the ISATAP prefix.
Why it's wrong here
Incorrect because the issue is that the host is not sending traffic to the router for off-link destinations, not that the upstream router lacks a return route.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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