intermittent problem with RDP over IPSEC Lan2LAN VPN


I think somebody should have encountered this but somehow i can't find a relative thread or document regarding our problem.

Let start with a network layout. In our company network we have 3 gateways. Default gateway is a ISA 2006 server. We don't use the isa client so we're all Secure NAT clients. Next to this ISA we have 2 Zyxel gateways (Zywall 1050 and Zywall 70).

We use the ISA as gateway for standard internet stuff (http(s), ftp, pptp, cisco ipsec client etc etc). For our customer support we started with IPSEC vpn connections between our network and the customer network. Eventually we ended up with a Zyxel Zywall 1050 with 400+ IPSEC vpn's and a Zywall 70 with some left overs from the start of this setup.

We're creating static routes on the ISA server to point to the correct 2nd gateway.

This setup is running for over 5 years with minor issues (mostly ISA filter related).

Since we (the IT Staf :D) has upgraded the laptop's to Windows 7 we encounter some issues mostly with RDP sessions over the IPSEC tunnels. These sessions drop and reconnects every 20 seconds or so.
When we use Windows Server 2003 or Windows XP (any SP) as RDP client we don't encounter the issue and we can be connected for hours without a drop.
When we use our Windows 7 laptop and we add a static route to the correct gateway the RDP session is also stable.

Normally a client request info from the default gateway. If the default gateway has info about the next step (the static route) it return this info to the client and the client starts to connect thru the learned route. This all seems to work fine with Windows Xp but it looks like Windows 7 is refreshing his routing table even if the route is being used.

Has somebody encountered these kind of issues? We've reinstalled our laptops multiple times and encounter this issue since Windows 7 Beta but in the beta the was a legacy networking problem. One of my colleagues was running windows Vista before he installed W7 and didn't encountered this issue with Windows Vsita. If we're running a continues ping -t to a remote server we get replies even if the RDP session reconnects. It rare that we receive a timeout during the ping and it's never at the same time we lose RDP connection.

We've tested different MTU settings but as the VPN itself works fine it must be a W7 issue.

I hope somebody can point us into the right direction.

Rgds. evdzon

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