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PowerConnect48
2 Bronze

MTU Sizes Jumbo Frames

I have a few questions on how the PowerConnect 6248 handles MTU sizes:

1.  It appears if I have a host using jumbo frames at 9000 MTU then I have to use a MTU  of 9216 to match?

2.  I can apply a MTU size under a VLAN using the "ip mtu" command but I can't set it to 9216 since I get an error back saying "Value is out of range. The valid range is 68 to 9198."  I understand why since 9216 isn't even a valid standard for jumbo frames.  So is the MTU size on the port vs VLAN suppose to be different to support a MTU of 9000?

3.Do I have to apply a MTU size to a VLAN?  My intent is not all VLANs on the physical port need jumbo frames.

13 Replies
Moderator
Moderator

RE: MTU Sizes Jumbo Frames

Sounds like the switch is on an older firmware revision. Getting the firmware up to date should enable the larger MTU setting.

http://dell.to/1vHRJit

Daniel Covey
Dell EMC| Enterprise Support Services
Get support on Twitter:@DellCaresPRO
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PowerConnect48
2 Bronze

RE: MTU Sizes Jumbo Frames

I don't understand your response of:

1. "Getting the firmware up to date should enable the larger MTU setting."

9216 MTU is not even in the Ethernet Jumbo Frame standard, so why would it support larger?

2. I'm on the latest firmware A36, so no changes have been made to this release.

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Moderator
Moderator

RE: MTU Sizes Jumbo Frames

Older firmware revisions had a lower maximum mtu size. In the newer revisions they increased the mtu limit.

It should let you set it to 9216. Can you post up the output of the command ,console>show version. This will let us check what revision the switch is on, which image the revision is installed on, and if that image is active.

You could also run the command [tag:iscsi] enable

When iSCSI is enabled, the following actions occur:

• The MTU on all ports and port-channels is set to 9216 (jumbo frames are

enabled).

• Flow control is globally enabled, if it is not already enabled.

• iSCSI LLDP monitoring starts to automatically detect Dell EqualLogic

arrays.

Daniel Covey
Dell EMC| Enterprise Support Services
Get support on Twitter:@DellCaresPRO
Download our QRL app:iOS, Android, Windows
Dell Networking Resources

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PowerConnect48
2 Bronze

RE: MTU Sizes Jumbo Frames

Daniel -

Would you mind reviewing my initial questions as I don't believe you fully understand them.  See below on my firmware.

console#show ver

Image Descriptions

image1 : default image

image2 :

Images currently available on Flash

--------------------------------------------------------------------

unit      image1      image2     current-active        next-active

--------------------------------------------------------------------

   1    3.3.11.2     3.3.4.1             image1             image1

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Moderator
Moderator

RE: MTU Sizes Jumbo Frames

Thanks for the output from the show command, it looks good.

With MTU size you need to leave enough room for the headers to be added. IPv4 =20, TCP =20, icsi can vary, etc. So MTU 9216 is not required, but is used to ensure there is enough space.

However with the firmware the switch is on should allow the mtu to be set to 9216. I just connected to one we have in the lab and it allowed me to set a port to 9216.

If you enter an interface and enter this command # mtu ?, what does it show as possible options?

Daniel Covey
Dell EMC| Enterprise Support Services
Get support on Twitter:@DellCaresPRO
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PowerConnect48
2 Bronze

RE: MTU Sizes Jumbo Frames

Daniel

You are referring to a physical port.  Please see below, I am referring to a VLAN interface.

2.  I can apply a MTU size under a VLAN using the "ip mtu" command but I can't set it to 9216 since I get an error back saying "Value is out of range. The valid range is 68 to 9198."  I understand why since 9216 isn't even a valid standard for jumbo frames.  So is the MTU size on the port vs VLAN suppose to be different to support a MTU of 9000?

3.Do I have to apply a MTU size to a VLAN?  My intent is not all VLANs on the physical port need jumbo frames.

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Moderator
Moderator

RE: MTU Sizes Jumbo Frames

I apologize, I was reading that incorrectly. With ip mtu on the VLAN interface, this value includes the 802.1q tag, but does not include the Ethernet header and CRC trailer. So, the maximum Ethernet frame size, including the Ethernet header/trailer, is 9198 + 18 = 9216 bytes.

The physical interfaces includes everything, meaning you can set it to 9216.

Daniel Covey
Dell EMC| Enterprise Support Services
Get support on Twitter:@DellCaresPRO
Download our QRL app:iOS, Android, Windows
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PowerConnect48
2 Bronze

RE: MTU Sizes Jumbo Frames

OK so back to question

3.Do I have to apply a MTU size to a VLAN?  My intent is not all VLANs on the physical port need jumbo frames.

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Moderator
Moderator

RE: MTU Sizes Jumbo Frames

ip mtu setting is used when a packet is sent on an interface. When applied to a VLAN interface, the ip mtu command will be followed when the VLAN sends the packet to another VLAN. So if your jumbo frames are all staying in the same VLAN, then you shouldn't need to worry about it. However if the jumbo frames will frequently be moving from VLAN to VLAN, i would set the ip mtu to 9198.

Daniel Covey
Dell EMC| Enterprise Support Services
Get support on Twitter:@DellCaresPRO
Download our QRL app:iOS, Android, Windows
Dell Networking Resources

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