Sunday, November 25, 2012

Checking Jumbo Packets

Jumbo packets are an important way to increase performance on an iscsi network. Jumbo packets carry more data in each packet. The only requirements for jumbo packets is that the network interface of the server, the network switch, and iscsi storage of the network must be able to handle the larger packets.

Typically jumbo packets have a MTU size of 9000. With setting up our iscsi network I wanted to test to make sure MTU size was working across all components. From the server I ran the following command pinging the interface of the filer.

ping -f -l 8958

The -l is to set the size of the packet, and the -f does not allow the packet to be fragmented. 

If the packets return, then jumbo packets are working correctly. If not, then we need to check the MTU size on each part of your network.

Why use 8958 instead of 9000? Protocol overhead - each packet has a certain amount of data that is required for network packet to work.

12 gap + 8 preamble + 18 header + 4 trailer = 42 bytes.