Re: can't bridge an I/F with jumbo to taps, deleted bridge 'flags' are sticky if you remake them

In reply to: George Michaelson : "Re: can't bridge an I/F with jumbo to taps, deleted bridge 'flags' are sticky if you remake them"
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From: Kristof Provost <kp_at_FreeBSD.org>
Date: Mon, 17 Jan 2022 08:53:54 UTC

On 17 Jan 2022, at 6:09, George Michaelson wrote:
> I tested on FreeBSD 13 and the behaviour is the same.
>
> But, I want to withdraw an imputation this is a bug. The underlying
> behaviour is that you can't bond an MTU9000 and an MTU1500 interface.
> If the base device is set jumbo, then the tapX instances must be set
> jumbo. once you set MTU to match, it works fine on both FreeBSD12 and
> on FreeBSD13.
>
> It's probably a documentation nit, if anything: bridged devices should
> have identical MTU. The ifconfig bridge command will refuse to addm
> <device> if the MTU is not correct for the any other member of the
> bridge group.
>
Note that the if_bridge behaviour around MTU changes will change in 14.
In 14 it becomes possible to change the MTU on the bridge itself, which will then adjust the MTU for all member interfaces:

https://cgit.freebsd.org/src/commit/sys/net/if_bridge.c?id=33306493825b291a308c0d37396e82de458f6cfe

(And also https://cgit.freebsd.org/src/commit/sys/net/if.c?id=01ad0c0079646a31afa20e1f07e5b62b82613493 which means that interfaces belonging to a bridge no longer allow MTU changes.)

Br,
Kristof