Dear all, here is the schema :

modem bridge ==> Mikrotik RB2011 UAS-RM (v6.38.1) client <==> Fortigate

There is a gre tunnel between mikrotik and fortigate. All the trafic is routed to the Fortigate But i have a problem, some websites (HTTPS essentialy) won't open.I have the problem only through GRE, if i try through Eoip tunnel,(with a mikrotik between Mikrotik and Fortigate) all is working fine.

I think that i have a problem with MRU, MTU or TCP MSS. I have tried to modify the MSS regarding this : /ip firewall mangle add out-interface=gre-tunnel2 protocol=tcp tcp-flags=syn tcp-mss=!0-1436 action=change-mss new-mss=1436 chain=postrouting but it is not working.

What is wrong?

Thank you in advance,

Regards,

Ok i have found, here is the correct value :

/ip firewall mangle
add chain=forward protocol=tcp tcp-flags=syn action=change-mss new-mss=1400

Regards,