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VPN bandwidth troubleshooting

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QUESTION POSED ON: 30 April 2007

I have set up a remote site that connects to one of our customer's main offices. I have an ADSL in place that can provide eight Mbps bandwidth but the provider advises that we can generally expect to obtain five-to-six Mbps which is sufficient. However, the transfer rates are pitiful, and it is acting like a dial-up modem operating at 56 Kbps. When I do not use the VPN the transfer/download rates are fine. It looks like there is a problem with the VPN connection, which is limiting the bandwidth available. I have numerous VPN connections that connect to a concentrator at the main site. Could the VPN be acting as a limiter in this situation?

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No, but it's hard to know what's wrong without more information. My first inclination when these types of problems occur is to take a network trace with TCPdump, Wireshark, EtherPeek or some other network analyzer. VPNs insert additional headers into each IP datagram, so I would pay particular attention to the final packet size and ensure that it does not exceed the MSS as this can cause a significant slowdown.

Get more VPN troubleshooting tips in our Virtual Private Networking Project Guide for resellers and systems integrators.

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