Wednesday, June 01, 2016

Force10 Operating System 9.10 changes maximum MTU size

Force10 operating system (aka FTOS, DNOS) always had the maximal configurable MTU size per port 12000 bytes. I have just been informed by former colleague of mine that it is not the case since FTOS 9.10 and above. Since FTOS 9.10 the maximum MTU size per switch port is 9261. If you used MTU 12000 then after upgrade to firmware 9.10 the MTU should be adjusted automatically. But I have been told that it is automatically adjusted to standard MTU 1500 therefore if you use Jumbo Frames (9000 bytes payload) it is necessary to change configuration before upgrade from 12000 to 9261.

Disclaimer: I had no chance to test it so I don't guarantee all information on this post are correct. 

UPDATE: Please read comments below this article for further information and great Martin's explanation of real MTU behavior. Thanks Martin and Kaloyan for your comments.

Martin's comment:
MTU 12000 in configuration was not reflecting real hardware MTU of underlaying chipset, after upgrade to 9.10 it's just adjusted to reflect real hardware MTU. Tested on S4048 9.10(0.1). When you boot into 9.10 you can see log messages saying that configuration is adjusted to reflect real maximum hardware MTU.
Also in configuration
 ethswitch1(conf-if-te-1/47)#mtu ?  
 <594-12000> Interface MTU (default = 1554, hardware supported maximum = 9216)  
 ethswitch1(conf-if-te-1/47)#mtu   

                                 

3 comments:

Kaloyan said...

Hello David,

I have upgraded couple of our switches to Dell Networking OS Version : 9.10(0.1)
and checked the ports with mtu 12000. It seems that there is no change to the MTU reported on the ports.

show interfaces tengigabitethernet 0/10
MTU 12000 bytes, IP MTU 11982 bytes

Martin Zidek said...

MTU 12000 in configuration was not reflecting real hardware MTU of underlaying chipset, after upgrade to 9.10 it's just adjusted to reflect real hardware MTU. Tested on S4048 9.10(0.1). When you boot into 9.10 you can see log messages saying that configuration is adjusted to reflect real maximum hardware MTU.

Also in configuration

ethswitch1(conf-if-te-1/47)#mtu ?
<594-12000> Interface MTU (default = 1554, hardware supported maximum = 9216)
ethswitch1(conf-if-te-1/47)#mtu

David Pasek said...

Thanks both of you for very valuable comments. It makes perfect sense now. I'm sure it will help other guys dealing with Force10 networking.