our network has 5 branch offices and 2 main offices.
To connect them we have tunnels gre that runs inside vpn ipsec tunnels.
The vpn are provided by cisco vpn concentrators 3005.
We changed our cisco vpn 3000 concentrators with new asa (main offices) and cisco routers with ipsec features on it (branch offices).
Since them we have problems on our exchange to deliver mail between our offices.
Our exchange servers run on virtual machines.
The remaining services are running well.
The mtu size on the tunnels gre have not been changed.
Does anyone have an idea what the problem is?