300-420 Question 201
Single answerAn enterprise network consists of two offices connected over the public internet. The network engineer decides to implement a Generic Routing Encapsulation (GRE) tunnel between the two sites to securely exchange data. After configuring the GRE tunnel, the engineer notices that the tunnel interface remains down. What is the most likely cause of the issue?
- A
The source and destination IP addresses for the tunnel are not reachable.
- B
The GRE tunnel is not encrypted, and encryption is mandatory for the tunnel to come up.
- C
The tunnel interface is missing a configured IP address.
- D
The MTU size of the GRE tunnel is incorrectly configured.
Show answer and explanation
Correct answer: A
Explanation
A GRE tunnel depends on the underlying network to route packets between the source and destination IP addresses. If these addresses are not reachable (due to routing issues, ACLs, or other connectivity problems), the tunnel interface will not establish. Ensuring reachability between the tunnel endpoints is a critical step in GRE tunnel configuration.
- A. Correct.
A GRE tunnel requires the source and destination IP addresses to be reachable over the underlying network. If they are not reachable, the tunnel interface will remain down.
- B. Incorrect.
GRE itself does not provide encryption, and encryption is not mandatory for the tunnel to establish. This is incorrect in this context.
- C. Incorrect.
While a tunnel interface requires an IP address for proper routing, the lack of an IP address alone will not cause the tunnel interface to remain down. It would still be administratively up but non-functional.
- D. Incorrect.
An incorrect MTU size might cause packet fragmentation issues, but it will not prevent the tunnel interface from coming up.