What happens if MTU is too high?

The maximum number of messages to transfer is 1,000 per day.

To allow for the delivery of the next 100k messages (the size of a daily transaction), it is necessary to make our transaction expire in less than one minute.

Each transaction that has been running for over one minute before they expire, will be deleted by Postfix. If we can make sure that MTU stays at least 1,500 bytes, we can still let each 100k transaction to run.

That's what makes a good message classifier: you need to learn how to recognize your own messages! Here, we are using two rules: A Message-ID greater than 100K. The Message-ID of a message is the size of the message's data (headers, and content). But if we keep MTU for 100K, even a message with Message-Id >100K is going to be dropped.

It's the size of the data, not the size of the whole message, which can cause the problem we described here. Fortunately, Postfix uses a mechanism to check messages by scanning them up until the last rule match, then check again if there are more matching rules.

It's enough to have the two above rules and we'll be safe.

What should my switch MTU be?

I currently have an 8K cable from my desktop to a wireless router, and a 6K cable from the wireless router to a wireless bridge.

I am trying to switch to WDS, but the WDS settings on the AP seem to only let me set the switch MTU for either 2K or 4K. Is there a way to set the switch MTU to 4K?

The AP is a Netgear R6250v2 running DD-WRT v24-sp2. 4K isn't actually a supported MTU for WDS. You'd have to use a custom SSID.

Should I set MTU to 1500?

We have a customer trying to migrate (using a third party vendor) from a Cisco 1841 switch to a Juniper 1513. The link speed will be in the 300-1200 kbs range with plenty of room for growth.

The third party vendor says that I should set MTU to 1500 as they are unable to negotiate the lower MTU's and may cause more packet loss. I know there are a number of different values (2048, 1514, 64536, etc.) which I can specify in the switch configuration (either via CLI or config-register) but would like some expert opinion on the most appropriate setting? MTU of 1500 will work fine. This is because in your question you mention that "we are able to grow up to 1200" -- in this case MTU is only going to impact packet size if you're seeing packet loss before that point. If the switch drops frames below the MTU then it can take them into the next switch up at the higher MTU and thus no packet loss occurs. If you had packet loss between 1200 and 1500 then by that logic 1200 would not be large enough, so if you need maximum stability in the 1500-2000 range, then it is better to go with the lower value.

If I had that much of bandwidth and capacity to grow and not want to use the full 1500 MTU then by that logic I might as well use 1466 which means one less interface and some extra wires (but none of my own).