MTU problems with VPN
Posted on 2003-10-24
I want to connect two offices using VPN on Microsoft Windows 2000. The installation is not the problem; we had this set up and working in just a few hours.
However, when I connect to a POP3 server using a telnet client talking raw POP3 protocol I found that a packet is dropped silently when i send the LIST command. When there are a lot of messages in the inbox the response is not received.
I've tried a router-to-router solution without using VPN. The same problem occured with that solution. After hours and hours of debugging, reading and testing I found that the problem lies in the MTU. I made a few changes in the MTU settings and it worked! :)
The router-to-router solution is not encrypted. As soon as I add encryption OR use VPN the same problem happens again. I guess it is again the MTU, but this time I really can't figure it out.
Here is how the network looks like:
Internet -- InetGateway (192.168.1.100) -- Office1 (192.168.1.*) -- W2kGateway1 (192.168.1.1) --(WIFI Connection)-- W2kGateway2 (192.168.2.1) -- Office2 (192.168.2.*)
One more important thing: Both Windows 2000 gateways have 2 NIC's. One is configured and connected to the office LAN, the other is configured in a seperate subnet (10.0.0.*) and connected to the other gateway by a wireless connection.
Could anyone explain to me which adapter needs which MTU value? And what is "RAS MTU" and do I need to set this? (I found this option with DrTCP)
Help would really be appreciated, I can't figure it out ... Thanks in advance.