Re: [pve-devel] [PATCH docs/manager/qemu-server v2 0/3] Make VirtIO network devices always inherit MTU from bridge

[Thomas Lamprecht <t.lamprecht@proxmox.com>]

Am 17.04.25 um 12:48 schrieb Stefan Hanreich:
> The current default behavior for VirtIO network devices is to default to 1500
> MTU, unless otherwise specified. This is inconvenient in cases where the MTU is
> not the default value (e.g. for VXLAN VNets or bridges with jumbo frames).
> Containers already inherit the MTU of the bridge, if not set, so change the
> behavior of VMs to be more in line with containers. This also makes using
> non-standard MTUs more convenient and less error-prone since users do not have
> to remember setting the MTU everytime they configure a network device on such a
> brige.

Hmm, does this have regression potential for bridges with a too high MTU?
I.e., one where the MTU works for LAN but not for anything going beyond that,
which is odd but can be working fine I think? At least as long as no host and
no CT uses this bridge for communicating with endpoints outside the LAN.

FWIW, we could also tie this behavior to a machine version to avoid changing
the behavior for any existing VM. But I would be fine with applying this only
for PVE 9 then and add a notice to the pve8to9 checker script that lists all
VMs that will change their MTU including the respective value.


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