Since iSCSI networks have been growing in popularity the past couple of years, more people have been trying to use jumbo frames to eek out a little more speed. This article is going to tell you how to test your jumbo frames after getting it configured.

The theory behind why raising the MTU will give better performance is fairly simple. (Please excuse my crude description as I tried to keep it very simple). Data is diced up into sections to make transmission easier. Normal networks split data in 1,500 byte segments. A frame is then put around that data to tell it where to go. If you start splitting that data into 9,000 byte segments, there will be fewer frames created. Theoretically this will give you less overhead, and slightly faster speeds.

There are three places that you could run into errors if you start changing MTU sizes. The switch, host computer, or client computer. Most of the time the host will be a SAN or iSCSI tape drive. The client will usually be a server accessing the SAN or tape drive. So to test connectivity we will try to ping everything, just like troubleshooting standard network connectivity issues. The difference is that we will be setting the -f and -L switches.

The -f switch tells the computer that you do not want you ping packet split up at all. The -L switch sets the size of the ping. Let’s try these out right now. Try to ping your router using those switches and a 1600 byte packet.

It will look similar to this: ping -f -L 1600 192.168.1.1

It should tell you that the Packet needs to be fragmented. If you lower the size to 1000 then it will ping successfully. Now let’s get onto you iSCSI network and try some larger file sizes.

First step will be to ping your switch with a large packet size and see if it works.

Here is a good example test: ping -f -L 5000 172.16.0.10

If it does not work then try a small packet. If that goes through then you have a discrepancy in MTU size.

If both Pings make it through the switch just fine you should try to ping the host device using the same commands.  This will tell you where your packets are getting dropped, and hopefully save you a lot of time trouble shooting.