Why would my VPN connection work better than my direct connection?

Question

I have a new Windows 7 64bit laptop, which connects wirelessly to a ASDL router/modem.

With my regular connection, page requests often time out - usually with form submissions.

When I use the same connection to connect to a VPN (using OpenVPN) the requests seem to work fine. These are not requests to sites that are only available via the VPN - just regular websites

I think this is specific to this new laptop (provided and set up by my employer) - other machines work fine.

Many thanks for any advice, James

Answer 1

It could be an MTU problem. Your VPN might be using a smaller MTU that works fine, but your native connection is trying to use a full 1500 byte MTU that's too large for some network paths that have Path MTU Discovery black holes, resulting in timeouts.

Try setting your Windows 7 laptop's wireless interface's MTU to 1400 or even 1000 and see if the problem goes away. Then try progressively higher values until you find the highest value that doesn't cause the timeouts. The most common non-1500 MTU value is 1492 for DSL networks that use PPPoE.