For a GRE tunnel to be up between two routers, which of the following must be configured?
A. Loopback Interface
B. IP reachability between the loopback interfaces
C. Dynamic Routing between routers.
D. Tunnel interfaces must be in the same subnet.
Do you really mean that the GRE tunnel interface goes down if you configure mismatching IP addresses on both side of GRE tunnel? I guess the tunnel is up (“physically”) but no IP communication can be done over the tunnel.
It is like IP configuration of ethernet interface has nothing to do with physical condition of the interface (l ink/protocol up/down). Check this: unnumbered IP can be configured onto a GRE tunnel. Unnumbered IPs are in different subnet (loopback /32) .