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SubjectRe: [PATCH net] net: vlan: allow vlan device MTU change follow real device from smaller to bigger
On Tue, 22 Feb 2022 11:37:33 +0100 Guillaume Nault wrote:
> What about an explicit option:
>
> ip link add link eth1 dev eth1.100 type vlan id 100 follow-parent-mtu
>
>
> Or for something more future proof, an option that can accept several
> policies:
>
> mtu-update <reduce-only,follow,...>
>
> reduce-only (default):
> update vlan's MTU only if the new MTU is smaller than the
> current one (current behaviour).
>
> follow:
> always follow the MTU of the parent device.
>
> Then if anyone wants more complex policies:
>
> follow-if-not-modified:
> follow the MTU of the parent device as long as the VLAN's MTU
> was not manually changed. Otherwise only adjust the VLAN's MTU
> when the parent's one is set to a smaller value.
>
> follow-if-not-modified-but-not-quite:
> like follow-if-not-modified but revert back to the VLAN's
> last manually modified MTU, if any, whenever possible (that is,
> when the parent device's MTU is set back to a higher value).
> That probably requires the possibility to dump the last
> modified MTU, so the administrator can anticipate the
> consequences of modifying the parent device.
>
> yet-another-policy (because people have a lot of imagination):
> for example, keep the MTU 4 bytes lower than the parent device,
> to account for VLAN overhead.
>
> Of course feel free to suggest better names and policies :).
>
> This way, we can keep the current behaviour and avoid unexpected
> heuristics that are difficult to explain (and even more difficult for
> network admins to figure out on their own).

My $0.02 would be that if we want to make changes that require new uAPI
we should do it across uppers.

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