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?

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.

-tp

it is set to 1500, rx, tx drops: 0

I know this is a very old thread, but it seems like a good place to share my experience.

I just spent about 15 hours trying to work out why Windows remote desktop and domain authentication stuff wouldn't work across a client's inter site VPN when I changed it from pure IPsec to GRE over IPsec (an attempt to fix RDP disconnects). Two of the four computers worked after some poking about, but the the other two would always hang while trying to connect, or fail authentication. Pinging the server was fine though.

Eventually it occurred to me it may be related to the MTU. I lowered the Windows ethernet interface MTU to 1300, which fixed the issues on that computer.

Not wanting to have to change the MTU for all current and future computers in this location, I tried changing the MTU in the GRE interface on both ends of the VPN connection; I had left it blank when I created them.

Setting the MTU to 1500 fixed it.

I hope this helps someone =)