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VPN connection problems

By james ·
I am trying to connect a stable VPN between my office and another network I'm connected to through an sDSL bridge and some other intervening networking (described below).

The problem:
I can establish a connection over which traffic initially flows quite happily. However, whilst the VPN starts off with fairly reasonable data rates through it, it soon slows down to such a point that it's essentially unusable and programs connected through the VPN eventually time out - but windows still thinks the VPN is connected. The amount of time this takes to occur seems quite random, but is usually less than 5 minutes - usually a lot less.

In terms of some hopefully useful details about the networking set-up, I'm connecting from a Windows XP box, using the built in VPN client, through an sDSL modem [which connects at ~1.5Mb/s] which goes to a building over the road. From there, it goes into a switch and then over fibre to another organisation's data centre a few hundred meters away. There there is a dedicated router (running mikrotik router software) which does most of the more "complicated" networking stuff (BGP, bandwidth throttling [we share their internet connection] and so on); from there, the VPN would be routed through a small internet exchange housed in that datacenter (http://ginx.org.za/) to the other organisation's Nortel Contivity VPN solution, also in that datacenter.

Interestingly, I get more or less exactly the same symptoms if I connect to a MS RAS server hosted at the same institution but not in the main datacenter.

I have tried turning pretty much every single setting in the VPN client on and off in various configurations to no avail. Apparently, many other windows clients are quite happy to connect to the same VPN and remain connected pretty much indefinitely without significant problems, but not from this location.

Does anyone have any idea what the problem might be, and what a fix might entail? This has been frustrating me for over a week now!

All the rest of my networking seems quite happy, it's just the VPN that dies.

Many thanks for any assistance.

Best Regards,
James.

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DSL connection settings set to 'dial on demand' or always connected?

by robo_dev In reply to VPN connection problems

If the DSL modem is having to reconnect, that would wreak havoc, obviously. Look for data errors in the logs of the DSL modem.

It sometimes helps to force the Windows VPN client to use TCP for authentication.

By default, Kerberos uses UDP port 88, and packet fragmentation or out-of-order packets will kill the VPN connection.

support.microsoft.com/kb/244474

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