300-410 Infrastructure Services Practice Question
A network technician is configuring a GRE tunnel between two routers. The tunnel source is a physical interface, and the tunnel destination is a loopback interface on the remote router. The technician notices that the tunnel interface is up, but line protocol is down. What is the most likely cause?
⚠ Common exam trap
The trap here is assuming that a shutdown source interface would cause up/down, but that would cause down/down; the up/down state specifically points to destination unreachability.
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.
A GRE tunnel interface will be in up/down state if the tunnel source is operational but the destination is unreachable. The router brings up the tunnel interface because the source is up, but the line protocol remains down until a route to the destination is available. Ensuring reachability to the tunnel destination resolves the issue.
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 mode is set to GRE multipoint.
Why it's wrong here
GRE multipoint (mGRE) is used in DMVPN and does not require a tunnel destination. If mGRE were configured, the tunnel destination command would not be present, and the line protocol would be up if NHRP was functioning. The symptom of up/down is more indicative of a reachability issue, not the mode.
- ✗
The tunnel source interface is shutdown.
Why it's wrong here
If the tunnel source interface were shutdown, the tunnel interface would be down/down, not up/down. The physical interface being down would cause the tunnel interface to go down entirely. In this scenario, the tunnel interface is up, indicating the source is operational.
- ✓
The tunnel destination is not reachable.
Why this is correct
If the tunnel destination IP address is not reachable, the tunnel interface will show up/down. The tunnel source is up, so the interface state is up, but without a route to the destination, the line protocol remains down. This is a common issue when the destination is a loopback that is not advertised or when there is no route to it.
- ✗
The tunnel keepalive is misconfigured.
Why it's wrong here
Keepalives are used to detect remote end failures. If keepalives are misconfigured, the tunnel might flap, but the initial state would not be up/down due to keepalives alone. The line protocol would come up if the destination is reachable. Keepalives are not the primary cause for up/down.
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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.