netdev - Re: [PATCH net-next] bridge: Allow max MTU when multiple VLANs present

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Date:   Thu, 29 Mar 2018 09:49:01 -0700
From:   Roopa Prabhu <roopa@...ulusnetworks.com>
To:     Chas Williams <3chas3@...il.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Stephen Hemminger <stephen@...workplumber.org>,
        Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
Subject: Re: [PATCH net-next] bridge: Allow max MTU when multiple VLANs present

On Thu, Mar 22, 2018 at 9:53 PM, Roopa Prabhu <roopa@...ulusnetworks.com> wrote:
> On Thu, Mar 22, 2018 at 8:34 AM, Chas Williams <3chas3@...il.com> wrote:
>> If the bridge is allowing multiple VLANs, some VLANs may have
>> different MTUs.  Instead of choosing the minimum MTU for the
>> bridge interface, choose the maximum MTU of the bridge members.
>> With this the user only needs to set a larger MTU on the member
>> ports that are participating in the large MTU VLANS.
>>
>> Signed-off-by: Chas Williams <3chas3@...il.com>
>> ---
>
> Acked-by: Roopa Prabhu <roopa@...ulusnetworks.com>
>
> This or an equivalent fix is necessary: as stated above, today the
> bridge mtu capped at min port mtu limits all
> vlan devices on top of the vlan filtering bridge to min port mtu.

On further thought, since this patch changes default behavior, it may
upset people. ie with this patch, a vlan device
on the bridge by default will now use the  bridge max mtu and that
could cause unexpected drops in the bridge driver
if the xmit port had a lower mtu. This may surprise users.

The other equivalent fix i was thinking about is to keep the default
behavior as is, and allow a max mtu to be
configured on the bridge. This will allow a sys admin to fix the
current mtu limitations if
deployments require it.

we will submit an incremental patch to re-work this patch to restore
default behavior.