In my network I have a 3560 Gbit switch using a 9k jumbo frame, and 2 PC's that are currently now using a 7500 MTU. The two PC's are just connected to the switch, which is using Gbit SFP's. Both NIC's are actually capable of using jumbo frames and a 9k MTU, but when I set them to actually use the 9k MTU - when traffic is sent between them using a ping sweep with a size 7800, the traffic starts fragmenting (I worked it out by setting the df-bit at both 7799 and 7800. At 7799 it doesn't fragment, but at 7800 it does. Makes no sense because its just 2pc's connected to one 3560 switch with everything configured to use 9k jumbo frames). So that's why I've set my MTU to 7500 rather than 9k at the moment. Why won't Windows allow me to get the full 8960 byte MSS (i.e. 9k jumbo frame minus the 20byte tcp and ip header)? That's my first question! According to the much loved internet, Windows should be capable of using a MSS of 64KB, aka 65535 bytes, which is way more than the 8960 I want it to use. So I don't understand why my data is getting fragmented at 7799 bytes?
The command I used to test this was in Windows > ping 192.168.1.100 -f -l 7799. I put this command on both hosts A and host B (just call them that for simplicity). Host A just didn't get the reply when the command was issued. But when I put the command into Host B, it said something like df-bit set but need to fragment. So I assume Host B is potentially the one with the issue? I'm really stuck on how to troubleshoot, or go any further with this. But I'm sure of one thing. This is doing my nut in! Can anyone help?