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Troubleshooting S.L.O.W VPN Connection

Doberman76
Level 1
Level 1

Good Morning,

I am troubleshooting a very slow VPN connection that will periodically drop its connection throughout the day.

The problem is occuring at our satellite office across town that VPN's to our main office's servers.  The satellite office has it's own DLS modem with up to  a 6Mbps to accomadate our 4 T1 multiplexed lines at the main office.  After some research on the issue, I believe that adjusting the MTU is a good starting point.  My question is...well first...is that a good starting point?, and second, do I have to download and run Dr.TCP on all machines at the satellite location?  Or should I make the change on the Cisco 800 router itself through telnet?

Any input on this would be greatly appreciated,

Thanks,

Jay

1 Reply 1

Bastien Migette
Cisco Employee
Cisco Employee

Hi Jason. Adjusting the MTU is a good point. You can define the MTU on the router, and just "ip tcp adjust-mss" so that it will implicitely force the clients to lower their MTU.

the mss will be 40B less than IP MTU (as you substract 20B for tcp header, and 20B for the additional IP Header that is used for tunneling).

you can test for example a MTU of 1400 and an MSS of 1360.

The mss is the maximum segment size. if the router adjust the mss, it will rewrite it during the client connexions so that a TCP segment can't be more than 1360, thus implicitely lowering the MTU on the end stations.

Hope this help.

Regards,

Bastien

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