We have four sites joined by permanent VPN links between Draytek 2820 routers, over ADSL. The VPNs function fine for Windows file sharing and printing, however one application (which uses terminal emulation software, using telnet) is troublesome over the VPN links. Connections frequently drop and / or become slow to respond, despite other services (e.g. Remote Desktop) being fine.
The software vendor says that they recommend an MTU size of 1540 or above - yet as far as I'm aware (and please correct me if I'm wrong) ADSL doesn't support an MTU above 1492. In this case 1472 is the maximum ping response we can get, both over the VPN and to other hosts on the local network (and this is the recommended setting by the ISP for the ADSL connections). The only host I can get a response from above 1472 is "localhost" (i.e. an XP PC pinging itself) which then goes only as high as 1492.
Is it possible to increase the MTU size over the VPN links? For that matter, is it possible over the local ethernet? How would we make the change - on the routers and switches? What would it actually achieve?