Cisco VPN - MTU Problems

[e-estevez]

Hi, I have a VPN between three sites, in each site I have a Cisco 831 that establish a VPN tunnel with a VRF (on my ISP) then I have a clear channel in the Headquarter to my ISP, the problem I have is that I have to manually modify the MTU size from 1500 (default) to 1400 in each server that must be accessed through the VPN, if I don't do that I have packet loss, anybody have experienced this before, or know how to avoid this problem without modifying the MTU size.

Best Regards,

[Richard Burts]

I implemented a fairly large VPN based network for a customer. We were having problems that were MTU related. I found that the command ip tcp adjust-mss was very effective in solving our problem. This command will adjust the MSS which will control the MTU for all TCP sessions that go through the router.

You do have to configure it on the routers at the edge but you do not need to do anything to the end stations.

HTH

Rick

[burbidge]

In my experience, 'tcp adjust-mss' will work, but obviously not for non-TCP communications (i.e. UDP / ICMP / other).

If you are running applications across your VPNs that do not use TCP (such as TFTP and possibly VoIP) you may need to adjust the MTU size on the relevant hosts. For everything uses TCP, the 'tcp adjust-mss' should solve your problems.

Kevin