I have an IPSec encrypted GRE tunnel between two LANs that has worked great for literally years. Recently we started adding Windows Vista and Windows 7 machines to the mix. The new Vista and Win7 machines have TERRIBLE performance across the WAN/tunnel. I have tried using "ip mtu" and "tcp adjust-mss" commands on the routers LAN and tunnel interfaces (and also messed with MTU and MTUDiscovery on the machines themselves) but I haven't managed to get the Vista and Win7 machines to work as well as the XP and 2003 machines. Anybody have any ideas?