SCayr

So another BT Router (en5861) blew up! It was replaced with a Netgear DG384 and everything was looking fine. Apart from one thing - a Netgear FVS318 sits on the network and creates a VPN Tunnel to a sub-office, allowing them to connect to a terminal server. The VPN was up, but a ping showed packet loss of 40% and noone at the sub-office could connect to the Terminal Server.

After much head scratching and googling I found the solution and thought I’d share it. (Or at least keep a record for when it happens again and I don’t remember what the fix was )

The MTU (Maximum Transmission Unit) setting on the router was set to 1492, and was set to 1500 on the VPN Endpoints. I found a couple of articles suggesting packet fragmentation was causing the problem because the MTU on the firewall was bigger than that on the router.

I set the 318s MTU to 1400 at both ends and suddenly 0% packet loss! Great, got the sub-office to try Remote Desktop, but this still didn’t work. A bit of trial and error later, and the magic number for MTU we found was 1472.

With the reliability of the EN5861 I’m sure this is something that’s happened before and will happen again. Has anyone else had similar problems or different cures?

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