Ethernet MTU - size

catalinu

Hello,

I've noticed on the config 10Gig link MTU is 1998 or greater then 1500... Why do we need to change this value to a greater one? and why packets with MTU 1998 don't get dropped ? What happens if the packets have the DF flag set ? What packets have a DF flag set?

Thanks,

Forsaken_GA

do some research into jumbo frames

DevilWAH

There are many reasons you might want to have a large MTU. In things like GIg ether net it can improve the transmission speeds and delay.

its simply how much data can be sent in one frame. so if you are running large data transfers between two devices on a dedicated segment of the network, having large MTU incresses the efficency as there are fewer frames and less over head to deal with.

Also in things like dot1q tagging you double tag a packet with two VLAN tags. so tunnel one vlan inside another. This adds an extra 4bytes (someone correct me if that figure is wrong) which if your frame is already at the 1500 limit would mean it was to big to pass over a link, so you may incress you MTU to say 1520.

this is known as a baby jumbo frame. which is a frame which is only slightly above the standard to allow for protocol over head.

so you have baby jumbo frames that dont carry any more data than standard, jsut extra protocol info,

and Giant/jumbo frames which as you are seeing in the 10gig that allow you to send more data and so more efficient data transfer.

In the case of DF, if you havea packet that is 1550 with the DF falg set it will not pass down a link with an MTU of 1500. if it down not have the DF flag it will be split in to a frame of 1500 bytes and a second of 50bytes. this will now require sending two frames and the devices at each end having to first split the frame and then reasemble. fragmented frames can also create issues with firewalls that have to buffer the packets, and then reassemble them before they can be filtered. so generaly fragmentation is a bad idea.

there can be issues using mutiply MTU's on the same network, so do be carefull. And if the NIC of the server connectd tt a 10gig link is not set to use jumbo frames then even though the link can accept it, the server will still only send at 1500.

Like forsaken said it's worth looking in to this before playing around with it. For non standard MTU's to work correctly you need to insure every step of the way is configured correctly. You can create some really strange happening when you get it wrong.

For example one that happen to me when i was playing with this, was i set the MTU to a low value across a link. when i was connection to switchs for management through SSH they ran fine, but as soon as I tried a ftp transfer to them, the first few packets went OK, but then it failed. it was simply that while the packet size was small MTU was not playing a part. but as soon as the FTP got under way and tried to send full size packet the frames got dropped. So be warned.....