Using interface/gre I created unsecured tunnel (point to point). I would like to ask, if it is any recommended MTU to be set? or it is just good to leave it default?
Check the Tx Drops after you set it at default. It also might depend on the MTU's of the interfaces on each end. As an example, setting a GRE tunnel, over the internet, and using IPSec and a PPPoE connection on one end drove it to 1406 but using a cable modem on the other end drove it to 1426.
As it still kept dropping packets, even when I set the cable modem GRE to 1406, so I set them both to 1370.
Seems very stable now. Your experience might vary, but there you go.