For which reason can a GRE tunnel have an Up/DOWN status?
A. The tunnel source interface is UP.
B. A tunnel destination is undefined.
C. The tunnel destination address is routable via a route that is separate from the tunnel.
D. The tunnel has been shut down.
option B is a possibility since
Up/down – This implies that, even though the tunnel is administratively up, something causes the line protocol on the interface to be down.
Four Different Tunnel States
There are four possible states in which a GRE tunnel interface can be:
Up/up – This implies that the tunnel is fully functional and passes traffic. It is both administratively up and it’s protocol is up as well.
Administratively down/down – This implies that the interface has been administratively shut down.
Up/down – This implies that, even though the tunnel is administratively up, something causes the line protocol on the interface to be down.
Reset/down – This is usually a transient state when the tunnel is reset by software. This usually happens when the tunnel is misconfigured with a Next Hop Server (NHS) that is it’s own IP address.
When a tunnel interface is first created and no other configuration is applied to it, the interface is not shut by default
https://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation-gre/118361-technote-gre-00.html
A working link:
https://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation-gre/118361-technote-gre-00.html