MTU Test

MagikMark · Post by **MagikMark** » Wed Oct 24, 2018 7:06 pm

Hi Philip!

I was pondering on the result of the ff MTU test with and without OpenVPN:

1. Using Asus 5300 router with and without VPN I get 1500

2, Using Airport Extreme without VPN I get 1500 and with VPN 1400

I use the ping -f -l command as well as your TCP optimizer

How come MTU test is the same using Asus?

Post by **Philip** » Thu Oct 25, 2018 4:26 pm

I assume pings are not send through the VPN tunnel, if you are getting 1500 MTU.

MagikMark · Post by **MagikMark** » Thu Oct 25, 2018 6:22 pm

Do you happen to know what's causing this?

Post by **Philip** » Fri Oct 26, 2018 9:15 am

Well, you said it yourself, it depends on the router. Not sure why exactly, but I assume one is sending pings through the VPN tunnel and the other isn't. It is hard to tell without additional info.

You can investigate further by using a traceroute, for example - see what are the first couple of hops, is it your ISP's IP address, or your VPN IP ?

MagikMark · Post by **MagikMark** » Sun Oct 28, 2018 6:51 pm

First hop is the VPN IP

Post by **Philip** » Sun Oct 28, 2018 7:17 pm

So, where does the VPN terminate, is the VPN client in the router, or some Windows/client side software?
Are you measuring the MTU with the router's ping test, command prompt, or using the TCP Optimizer?

What I am trying to determine is, is it possible that the router sends out pings without using the VPN.

MagikMark · Post by **MagikMark** » Sun Oct 28, 2018 10:27 pm

1. When running the test, it starts with the VPN server and ends in the target IP for both routers
2. VPN is from Windows client app
3. Tests is being run under windows command prompt and TCP Optimizer