300-420 Question 203
Single answerA network engineer is tasked with connecting two remote branch offices over the internet using Generic Routing Encapsulation (GRE). The engineer notices that traffic is not passing through the GRE tunnel. Upon investigation, the engineer observes that the tunnel interface status is 'up', but the line protocol is 'down'. What is the MOST likely cause of this issue?
- A
The GRE tunnel source or destination IP address is unreachable.
- B
The GRE tunnel is missing an IP address on the tunnel interface.
- C
The internet service provider (ISP) is blocking GRE traffic.
- D
The MTU size on the tunnel interface is set incorrectly.
Show answer and explanation
Correct answer: A
Explanation
In GRE tunnels, the 'up' status of the tunnel interface indicates that the interface is configured correctly. However, the line protocol being 'down' typically indicates that the GRE tunnel source or destination IP address is not reachable. GRE relies on the underlying connectivity between the source and destination IP addresses to establish the tunnel. Ensuring IP reachability resolves the issue in most cases.
- A. Correct.
The GRE tunnel requires the source and destination IP addresses to be reachable for the tunnel to function. If either address is unreachable, the line protocol will remain 'down', even if the tunnel interface itself is 'up'.
- B. Incorrect.
While assigning an IP address to the GRE tunnel interface is necessary for proper routing, the absence of an IP address would not cause the line protocol to show 'down'. Instead, routing would fail, but the tunnel's protocol state would remain 'up'.
- C. Incorrect.
ISPs typically do not block GRE traffic by default. While this could theoretically cause issues, it is not the most likely cause in this scenario.
- D. Incorrect.
An incorrect MTU size could lead to packet fragmentation or drops, but it would not cause the line protocol to show 'down'. The tunnel would still establish connectivity.