300-410 VPN Technologies Practice Question
A network engineer is configuring a GRE tunnel between two Cisco routers. The tunnel source is GigabitEthernet0/0 on Router A with IP 192.168.1.1, and the tunnel destination is 192.168.2.1 on Router B. After configuration, the tunnel interface is up, but no traffic passes through. What is the most likely cause?
⚠ Common exam trap
The trap here is assuming that a tunnel interface being up means the tunnel is fully operational, when in fact it can be up even if the underlay destination is unreachable.
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 tunnel destination is not reachable via the underlay network.
A GRE tunnel requires the tunnel destination to be reachable via the underlay network. If the destination is not reachable, the tunnel interface may appear up, but encapsulated packets cannot be delivered, resulting in no traffic flow. Other options are either incorrect or would cause different symptoms. Ensuring underlay reachability and proper routing is essential for GRE tunnel operation.
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 are in different subnets.
Why it's wrong here
The tunnel source and destination addresses do not need to be in the same subnet; they just need to be reachable via the underlay network. GRE tunnels can be established between routers with tunnel endpoints in different subnets as long as there is IP connectivity. Therefore, this is not the cause of the issue.
- ✗
The tunnel mode is set to GRE/IPv4, which is not supported.
Why it's wrong here
GRE/IPv4 is a common and supported tunnel mode on Cisco IOS routers. It is the default mode for GRE tunnels. Therefore, this is not a valid cause for traffic not passing. The issue is more likely related to underlay reachability or routing.
- ✗
The tunnel interface is missing an IP address.
Why it's wrong here
If the tunnel interface is missing an IP address, it would typically not be up/up, or it would not be able to route traffic. However, the scenario states the tunnel interface is up, implying an IP address is configured. Without an IP address, the interface would be in an up/down state or not functional. Thus, this is less likely than underlay reachability.
- ✓
The tunnel destination is not reachable via the underlay network.
Why this is correct
For a GRE tunnel to pass traffic, the tunnel destination IP address must be reachable through the underlay network. If the destination is not reachable, the tunnel interface may still show up if keepalives are not configured, but packets will be dropped because they cannot be encapsulated and sent. Ensuring reachability via a route or directly connected network is essential.
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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.