300-410 VPN Technologies Practice Question
A network engineer is configuring a GRE tunnel between two Cisco routers. The tunnel source is a physical interface, and the tunnel destination is a loopback interface on the remote router. The engineer 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 mismatched tunnel key or reversed endpoints cause the line protocol to drop, when in fact reachability of the tunnel destination is the key factor.
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 show up/up only if the tunnel source is valid and the tunnel destination is reachable. If the destination is unreachable, the interface remains up but the line protocol goes down. The fix is to ensure that the underlay network has a route to the tunnel destination, often by advertising the loopback interface into the routing protocol or configuring a static route.
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 key is mismatched.
Why it's wrong here
A mismatched tunnel key would prevent the tunnel from passing traffic, but it would not cause the line protocol to go down. The tunnel key is used for identification and security, but the line protocol state is determined by the reachability of the tunnel destination. If the key is mismatched, the tunnel may still come up but packets will be dropped.
- ✓
The tunnel destination is not reachable.
Why this is correct
For a GRE tunnel to come up, the tunnel destination must be reachable via the underlay network. If the destination is not reachable, the tunnel interface will show up, but line protocol will be down. The engineer should verify routing to the tunnel destination and ensure that the loopback interface is advertised and reachable. This is a common issue when the underlay routing is misconfigured.
- ✗
The tunnel mode is set to GRE multipoint.
Why it's wrong here
Setting the tunnel mode to GRE multipoint (mGRE) changes the tunnel type, but it does not inherently cause the line protocol to go down. In fact, mGRE tunnels do not have a tunnel destination, so the concept of line protocol down due to unreachable destination does not apply. The symptom described is typical of a point-to-point GRE tunnel with an unreachable destination.
- ✗
The tunnel source and destination are reversed.
Why it's wrong here
If the tunnel source and destination were reversed, the tunnel would likely not establish at all, or would establish but with incorrect endpoints. However, the tunnel interface being up but line protocol down specifically indicates that the destination is not reachable. Reversing source and destination would typically cause the tunnel to be down/down or up/down depending on reachability, but it is not the primary cause of this symptom.
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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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
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