Jumbo frames configuration

I would like to enable jumbo frames on my network due to the file sizes that we are constantly moving around (large images and video files). I am not quite sure though what to set my devices mtu size to. I have the following devices and their allowable mtu:

Mac computers max mtu: 9000
Netgear GSM7324 max mtu: 9216
Intel NIC on Windows server max mtu: 4088, 9014, 16128

No VLANs involved anywhere.

I want the most efficient transfer between the Mac computers. This is where most of the heavy lifting is done. Any large transfers between the Macs and the Windows server are almost always from a Mac to the Windows server.

So my question I guess is what should I configure as the mtu for each device? I can manually enter a number on the Macs and switch, but the Intel NIC only has those presets I can choose from.

bhilgenkampAsked:
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Don JohnstonInstructorCommented:
You would use the lowest common denominator. In your case, it looks like 9,000 bytes.

Unless the values on the Windows server are absolutes. Then you've got some issues.

But I suspect that this is a terminology issue. It sounds like the Mac's are showing the MTU. The Netgear and the Windows server is the frame size.


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bhilgenkampAuthor Commented:
Ahh. You are correct about the terminology. I didn't realize there was a difference, so I didn't take that into consideration. What is the difference between the two terms, and is there a way to do conversion between them?
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bhilgenkampAuthor Commented:
Ok I did a little googling and found out the difference. Thank you for your response and helping me learn something today.
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Don JohnstonInstructorCommented:
MTU is the maximum packet size. When a packet is encapsulated in a frame, 18 bytes are added (6 byte destination address, 6 byte source address, 2 byte type/length and 4 byte FCS).

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bhilgenkampAuthor Commented:
Actually I guess I have a little add on question. In the example I was reading about MTU vs frame size there were 18 bytes used for MAC/LLC. If I am understanding this correctly then selecting the 9014 frame size on the Intel NIC would be a 8996 mtu. If I set my Macs to 9000 mtu then would this cause fragmentation when sending to the 8996 mtu Intel NIC?
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Don JohnstonInstructorCommented:
There's an adding wrinkle. Some vendors don't count the FCS when they refer to the frame size. So while Intel may say a 9,014 byte frame, if they don't count the FCS, then the MTU would be 9,000 bytes.


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bhilgenkampAuthor Commented:
Ok great. Would it be okay to set the switch ports to 9216 or should everything match exactly?
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Don JohnstonInstructorCommented:
As long as the switch is set to a larger frame than is being used, you're okay.
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bhilgenkampAuthor Commented:
Sounds good. Thanks for all the help!
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bhilgenkampAuthor Commented:
I've got everything switched over to an mtu of 9000 on my Macs. However, doing a ping set to not fragment says "Message too long" for anything over a size of 8164. Is this correct, or should I be able to set the size all the way up to 9000 without it getting fragmented?

e2:~ brent$ ping -Ds 8165 192.168.1.200
PING 192.168.1.200 (192.168.1.200): 8165 data bytes
ping: sendto: Message too long
--- 192.168.1.200 ping statistics ---
1 packets transmitted, 0 packets received, 100% packet loss

e2:~ brent$ ping -Ds 8164 192.168.1.200
PING 192.168.1.200 (192.168.1.200): 8164 data bytes
8172 bytes from 192.168.1.200: icmp_seq=0 ttl=64 time=0.632 ms
--- 192.168.1.200 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max/stddev = 0.632/0.632/0.632/0.000 ms

Is this okay or should I change all of my machines' mtu to a smaller size?
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