When using a gre tunnel, we identify the source as one of the physical interfaces on the device.1)What is the significance if the source interface being used is the lan interface or if it is Wan interface.Would there be any problems in this.
2)If the tunnel source is undefined, but destination is defined, what could be the result & would the traffic still pass through? if no, why.
If yes, would it take the same path or go via another path?
When I first read your post I interpreted it as advocating the WAN interface, especially since it does not mention using any other interface. But I agree that your post makes the point very clearly that what is required is that the destination address and the source address must be reachable.