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

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Date:   Fri, 30 Mar 2018 15:54:51 -0400
From:   Chas Williams <3chas3@...il.com>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
Cc:     Roopa Prabhu <roopa@...ulusnetworks.com>,
        David Miller <davem@...emloft.net>,
        netdev <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 29, 2018 at 9:02 PM, Toshiaki Makita
<makita.toshiaki@....ntt.co.jp> wrote:
> 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.

It only changes the default behavior when you are using VLAN aware bridges.
The behavior remains the same otherwise.  I don't know if VLAN aware bridges
are that popular yet so there probably isn't any particular
expectation from those
bridges.

I don't think those drops are unexpected.  If a user has misconfigured
the bridge
we can't be expected to fix that for them.  It is the user's
responsbility to ensure
that the ports on the VLAN have a size consistent with the traffic
they expect to
pass.

>>
>> 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
>