MTU

[Mola9850]

Hi
My switch is a WS-12-250A with firmware 1.2.2 and i have just made a test about what my MTU had to lie on , and it says 1472 +20 bytes that are reserved for the IP header and 8 bytes that are be allocated for the ICMP Echo Request header so all in all 1500 MTU but with Netonix switch you can only go down to 1518 MTU
why is that ??

[sirhc]

As it is we are running into problems where people should be increasing the MTU on the switch to 1522 because of the use of simple VLANs.

We are going to make the default MTU soon to be 1522 or 1528 on the next firmware push.

1518 is fine if you are handing the switch a VLAN Tag and then stripping it off when you push it out a port but if you push a VLAN tag out a port you really need 1522.

Then there are other protocols that add additional bytes to the headed like TZSP and others.

When the MTU is too small it causes weird behavior and unless you really know what your looking for people often think the switch is misbehaving when in reality it is not.
Most switches add 4 bytes or more behind the scene to whatever MTU you configure in their UI we do not.

Having the MTU larger than you need on a device in the middle (such as the switch) does not hurt anything as the MTU size is determined by the end points of the stream.

Really the only way a 1500 MTU on our switch would be correct is if it is on a FLAT segment with NO VLANS or using any other protocols at all that add bytes to the header which is VERY RARELY the case in a WISP environment.

In the beginning I set our minimum/default to 1518 and in hind sight I wish I had made it 1522 or 1528 which would prevent a LOT of problems.

Here are some other posts discussing MTU
viewtopic.php?f=17&t=823&p=6213&hilit=MTU#p6210
viewtopic.php?f=6&t=698&p=5204&hilit=qinq#p5234

And there have been multiple threads with people having an issue that just increasing the MTU solved so we are debating making the default 1522 or 1528

We could allow people to set the MTU to 1500 or even less which probably will result in tons of tech support threads of networks stopping.

[Mola9850]

Ty Chris
always great to get a quick and helpful answer from you .
I will wait and see what happens in the new firmware and eave it a 1518

[sirhc]

I strongly recommend changing it to 1522 or 1528

[adairw]

It's for mikrotik, but good visual example of MTU.
http://wiki.mikrotik.com/wiki/Manual:Ma ... uterBoards
We run everything at above 1530 (1560 typically) because we use MPLS/VPLS and VLAN's. We wanted to make sure that we could cover ANYTHING we might ever want to transport. PPPoE over VPLS over VLAN, etc.. But honestly 1530 is all we've ever needed

Some of the older rockets only supported a max mtu of 1524 and had to be replaced. The same goes for all of the Mikrotik RB4xx line of routers. They don't support large enough MTU's

Chris, I'd almost just say set the MTU to 1530 and be done with it. 99.9% of people would be covered.

[Mola9850]

ok i will try to set it to 1530 and see :).

[cbl]

Why don't you just max each port out to 9600? That way when you have a need for a larger MTU than 1522/1528/1530 you know your Netonix will be capable of switching it through. We already set our core 10G links to max MTU to avoid any future MTU related weirdness in the future.

[sirhc]

Well people can chose to do that, set the MTU to MAX on the switch, it does not hurt as the switch does not generate any packets except for the UI / CLI packets and the Linux OS use MTU path discovery plus those packets are relatively small anyway I think.