Hello.

RB4011iGS+ 6.46.6, Winbox

When I clear "Limit broadcasts" in Switch-Port settings and click OK I get
"Couldn't change Swithc Port <ether 3> - vlan mode not supported"
Clicking OK again closes the dialog and apparently saves the setting (see below for 3 screenshots).
This has happened on ports 1, 2, 3 so far.

What does that error message mean and why it started to show up?
Thanks a lot in advance.

switching hardware is a "Thing" in ROS ... did you try to change the mode to fallback ? ( ... fallback normally doesn't hurt)
... maybe it's just an unsopported handle in the interface ?! ... best guess.
.
btw. is this a good old XP-box ?
.

Yes, changing VLAN mode to fallback allows to save cleared "Limit broadcasts" without an error message, but after reopening Winbox still shows "VLAN mode - disabled" and "Limit broadcasts" set, so settings are not saved.
Should I refer to Mikrotik tech support for help?
p.s. do I even need to enable broadcasts? This bridge will host wan-connected ports - two WAN links, WAN sides of 3 routers and my personal playground.

and yes, that is still XP

You cannot do VLAN filtering on the RTL8367 switch chip built into RB4011. It's a hardware limitation, see https://wiki.mikrotik.com/wiki/Manual:S ... p_Features .
Either do it in software on the CPU (https://wiki.mikrotik.com/wiki/Manual:I ... _Filtering) or use a separate switch.

I want to uncheck "Limit broadcasts" and save it.
Winbox produces
"Couldn't change Switch Port <ether 3> - vlan mode not supported"
when I do that.

It's a bug that this window is at all available for the 4011. Neither broadcast throttling setting nor any VLAN mode setting are relevant for that device because its switch chips don't support VLAN header manipulation (or maybe they do but in a way which would collide with RouterOS way of handling switch chips) nor broadcast throttling.

thank you very much for clarification