MTU

[John0616]

We have a site to site VPN running over a DSL line between two SonicWALL appliances. A while back we began having issues at the remote site with domain logons taking close to 20 minutes. After much searching I found that the issue was able to be resolved by reducing the MTU to 1420 across the network. I changed the MTU setting on the SonicWALL and then went through and set the MTU on all the machines in the network to 1420 as well thinking that this could possibly make for smoother communication across the board. What I now find strange is that when I test the network using ping -f -1 1420 It now says this has to be fragmented and will not send a packet larger than 1392. If I lower the MTU to 1392 then I can't send a packed larger than 1364. In other words for some reason I can not send a packet any more than 28 less than whatever I set the MTU. What am I missing here? I am not really an expert on this. Is this normal or is there something else that I should be setting? I am not currently having any serious issues although the communication across the network at times is not that smooth in spite of the fact that it is a very small network and really that congested. I was hoping to get things running smoother though.