How to discover the MTU size to be set for having your uplink work correctly with your ISP.

In some circumstances or with some ISP's the standard MTU size of 1500 is to high. If that happens you will notice strange network behaviour like downloads which always stop after a while or connections which will not work at all.

You can discover the MTU size by sending special icmp packets of the packetsize you think is your needed MTU size. If the packet passes the MTU size is ok, if not it is to high and you may lower the packetsize and retry until you find the size which is comfortable for your ISP.

In order to send the icmp packets do the following:

1. Log in to your EFW and choose a server which you actually can ping.
2. Ping that host with the following command:
   
   ping -M do -s 1460 <host>
   
   If the actual MTU size is ok for icmp packets of the size 1460, you will get ping replies like this:
   
   PING 10.10.10.10 (10.10.10.10) 1460(1488) bytes of data.
   1468 bytes from 10.10.10.10: icmp_seq=1 ttl=49 time=75.2 ms
   
   If the actual MTU size is to big for packets of the size 1460, you will get the following error message:
   
   PING 10.10.10.10 (62.116.64.82) 1461(1489) bytes of data.
   ping: sendmsg: Message too long
   
3. Retry with different packet sizes (the value after the -s option), until you find the size which is at the limit to work ok.
4. The value which ping shows you in brackets is your MTU size. In this example: 1460(1488), 1488 is the MTU size you need to configure.