Firmware 8.1 and custom MTU

I’m trying to diagnose a problem with my Balance One where it only can pass about 250mbps of traffic. See Balance One Core Underperforming Throughput

As a result, I’ve been poking around at all my settings, and realized that for both of my networks,
WAN / [Network] / Physical Interface Settings / MTU Is set to Custom: 1440.

I’m 99% sure I never made this change - while I do vaguely understand what MTU is, I’m sure I never changed it.


  • I’m assuming that Auto is the default, right?
  • I have two WANs, cable and fiber, both from Spectrum. What’s the best MTU. Should I set it back to Auto?
  • How did this get set to a Custom value. Is this a bug in one of the firwmare updates I’ve done?
  • Is anyone else seeing this?

This page: How to determine the optimal MTU and MSS size suggests that 1440 is in fact the default. And I did the ping tests suggested, and ended up with 1440 as the best size for me. So maybe this is just a GUI issue, where it should say Auto but for some reason it says “Custom: 1440” ? I think this is a bug, or a least, somewhat confusing UI design.

However, after switching both to Auto, both WANs are now set at MTU 1500. So I’m a bit confused. I don’t see any performance difference with Auto, so I guess I’ll leave it there, but it seems like the FAQ and the help popups could be updated.

@soylentgreen I think the default is 1440 and has been for some time. When you upgrade versions sometimes the device starts to think you entered the value when you in fact just took the default. Now with that said, this particular topic is evolving. I use T-Mobile and AT&T and they used to be 1440 and 1430 respectively. Early this year T-Mobile was working at 1500 but I was still stuck with 1430 for AT&T. Last week through some troubleshooting and testing I figured out AT&T is now giving me 1500 as well. I did a happy dance! No more fragmented packets all over the place, but since cell carriers are changing by the day, it might take a little time for default values to follow suit.

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