Jumbo MTU and switches

Dima

11/27/23, 8:01 PM

I have a network with several file servers, workstations and application servers. For simplicity we can consider it all attached to a single switch, say Dell 4032F with maximum Jumbo MTU size of 9216. I would appreciate your help with some questions related to MTU. So my first question is:

What is this (9216) - HW MTU or IP MTU according to classification (in other words, does it include 14-bytes frame header or not)? It is set for a whole device, not a port, according to documentation, but it does not state what is that.
Is it a problem, when a switch has a larger MTU value then devices attached?
When I setup MTU in Linux, I am putting 9216 in NetworkManager's settings - should I deduct 14 bytes and put 9202 or even 9000? What is MTU meant in OS setup?
And the last question: VMWare does support only 9000 byte MTU as Jumbo. Does it mean that everything in my network should have that value of MTU or not?

Thank you.

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Mircea Vutcovici

11/27/23, 8:08 PM

You need to make distinction between `IP MTU` and `Ethernet Frame Size`.

Dima

11/27/23, 8:20 PM

Frame size = IP MTU or HW MTU + L2 Header (14 bytes). But not for IOS and JunOS: they do include L2 header into MTU.

TomTom

11/27/23, 9:21 PM

If you have a LIST - make them bullet points, preferably numbered, not a text wasteland.

Score:1

Server

TomTom

11/27/23, 9:26 PM

what is this (9216)

Byte per packet. Either on IP or Ethernet level. As we talk of switches, this is Ethernet.

It is set for a whole device, not a port, according to documentation, but it does not state what is that.

It is assumed people learn their trade.

Is it a problem, when a switch has a larger MTU value then devices attached?

No. it is the maximum allowed number. It is a problem if the device sends a larger packet than the switch will handle (in which case the packet should be thrown away by the switch but possibly is partially translated), but it is NOT a problem if the switch can handle more than is sent.

Ethernet has NO negotiation, so the machines must show the SMALLEST MTU the network can handle. IP DOES negotiate (and routers can split packets), but Ethernet does not.

And the last question: VMWare does support only 9000 byte MTU as Jumbo

That would be awful for high speed networks.

Does it mean that everything in my network should have that value of MTU or not?

Depends. Generally: Do not touch MTU until you know what you do - and have a plan how this works and impacts your network. Higher MTU reduces processing on heavy duty networks. That said, 9200 is awfully low for a heavy duty network (i.e. 100g or higher).

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Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Jumbo MTU and switches

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