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

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Message-ID: <9034b7d8-dfb7-f1fa-44b1-d5dc95653724@lab.ntt.co.jp>
Date:   Fri, 30 Mar 2018 10:02:38 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Roopa Prabhu <roopa@...ulusnetworks.com>,
        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 2018/03/30 1:49, Roopa Prabhu wrote:
> 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.

+1

This makes sense to me.

-- 
Toshiaki Makita