300-410 Layer 3 Technologies Practice Question
A network administrator is configuring a GRE tunnel between two Cisco routers to transport IPv6 traffic over an IPv4-only core. The administrator enters the following configuration on Router A:
interface Tunnel0
ipv6 address 2001:DB8:100::1/64 tunnel source 10.1.1.1 tunnel destination 10.2.2.2 tunnel mode gre ipv6
However, the tunnel interface remains down. What is the most likely cause?
⚠ Common exam trap
The trap here is focusing on the tunnel mode or IPv6 addressing while overlooking that the tunnel source interface must be up and the destination must be reachable.
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 physical interface providing the tunnel source address is not operational or there is no route to the tunnel destination.
A GRE tunnel interface will only come up if the tunnel source is operational and there is a route to the tunnel destination. In this scenario, the tunnel source is 10.1.1.1, which must be assigned to an up interface, and the router must have a route to 10.2.2.2. Without these conditions, the tunnel remains down regardless of the tunnel mode or IPv6 configuration.
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 tunnel source and destination must be IPv6 addresses, not IPv4.
Why it's wrong here
For a GRE tunnel transporting IPv6 over IPv4, the tunnel source and destination are IPv4 addresses. The tunnel mode gre ipv6 indicates that the payload is IPv6. Using IPv6 addresses for the tunnel endpoints would be incorrect for this scenario.
- ✗
The tunnel mode should be gre ipv6, but it is incorrectly set to gre ipv4.
Why it's wrong here
The configuration already uses tunnel mode gre ipv6, which is correct for transporting IPv6 over IPv4. The tunnel mode is not the issue. The problem is likely the lack of a route to the tunnel destination or the tunnel source not being up.
- ✗
The tunnel interface requires an IPv4 address in addition to the IPv6 address to become operational.
Why it's wrong here
A GRE tunnel interface configured with tunnel mode gre ipv6 does not require an IPv4 address; it only needs an IPv6 address for the payload. The tunnel endpoints are IPv4, but the tunnel interface itself is IPv6. The absence of an IPv4 address on the tunnel interface is not the cause of the tunnel being down.
- ✓
The physical interface providing the tunnel source address is not operational or there is no route to the tunnel destination.
Why this is correct
A GRE tunnel interface remains down if the tunnel source address is not up or if there is no route to the tunnel destination. The administrator must ensure the physical interface with the source address is operational and that a route exists to 10.2.2.2. Without reachability, the tunnel cannot come up.
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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.