300-410 Layer 3 Technologies Practice Question
An engineer is configuring a GRE tunnel between two Cisco routers. The tunnel source is a physical interface, and the tunnel destination is the remote router's physical interface. After configuration, the tunnel interface is up, but no traffic passes through it. The engineer verifies that the physical interfaces are up and IP connectivity exists between the tunnel endpoints. What is the most likely cause?
⚠ Common exam trap
The trap here is assuming that a tunnel being up automatically means traffic will flow, overlooking the need for routing to direct traffic into 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
✓
There is no route to the remote network through the tunnel.
A GRE tunnel can be up, but without a route directing traffic into the tunnel, no data will traverse it. The tunnel interface's up state only indicates that the tunnel source and destination are reachable and the tunnel is operational. Routing is still required to forward traffic. Therefore, the absence of a route to the remote network via the tunnel is the most likely cause.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
There is no route to the remote network through the tunnel.
Why this is correct
Even if the tunnel interface is up, traffic will not pass unless there is a route directing traffic destined for the remote network out the tunnel interface. Without such a route, packets will follow the default route or be dropped. This is a common oversight: the tunnel is operational, but the routing table lacks an entry pointing to the tunnel for the remote subnets. Thus, no traffic passes.
- ✗
The tunnel destination is not reachable.
Why it's wrong here
If the tunnel destination were unreachable, the tunnel interface would typically be in a down state (unless keepalives are disabled and the line protocol remains up). The question states that IP connectivity exists between the tunnel endpoints, so the destination is reachable. Therefore, this is not the cause. The tunnel being up indicates that the router believes the destination is reachable.
- ✗
The tunnel keepalive is misconfigured.
Why it's wrong here
Keepalives are used to track the reachability of the tunnel destination. If keepalives are misconfigured, the tunnel might flap or go down, but the question states the tunnel is up. Misconfigured keepalives would not cause the tunnel to be up but not pass traffic; they would likely cause the tunnel to go down if the destination is unreachable. Since connectivity exists, keepalives are not the issue.
- ✗
The tunnel interface is missing an IP address.
Why it's wrong here
If the tunnel interface lacks an IP address, it would not be able to route traffic, but the question states that the tunnel interface is up. A tunnel interface can be up without an IP address, but it would not be usable for traffic. However, the more common issue when the tunnel is up but traffic doesn't pass is a routing problem, not a missing IP address. Also, without an IP address, the tunnel would not be able to source packets, but the interface would still show 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.