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Date:   Fri, 11 Sep 2020 16:20:58 +0300
From:   Ido Schimmel <>
To:     Florian Fainelli <>
CC:     Vladimir Oltean <>,
        netdev <>, Andrew Lunn <>,
        Vivien Didelot <>
Subject: Re: VLAN filtering with DSA

On Thu, Sep 10, 2020 at 11:41:04AM -0700, Florian Fainelli wrote:
> +Ido,
> On 9/10/2020 8:07 AM, Vladimir Oltean wrote:
> > Florian, can you please reiterate what is the problem with calling
> > vlan_vid_add() with a VLAN that is installed by the bridge?
> > 
> > The effect of vlan_vid_add(), to my knowledge, is that the network
> > interface should add this VLAN to its filtering table, and not drop it.
> > So why return -EBUSY?

Can you clarify when you return -EBUSY? At least in mlxsw we return an
error in case we have a VLAN-aware bridge taking care of some VLAN and
then user space tries to install a VLAN upper with the same VLAN on the
same port. See more below.

> I suppose that if you wanted to have an 802.1Q just for the sake of
> receiving VLAN tagged frames but not have them ingress the to the CPU, you
> could install an 802.1Q upper, but why would you do that unless the CPU
> should also receive that traffic?
> The case that I wanted to cover was to avoid the two programming interfaces
> or the same VLAN, and prefer the bridge VLAN management over the 802.1Q
> upper, because once the switch port is in a bridge, that is what an user
> would expect to use.
> If you have a bridge that is VLAN aware, it will manage the data and control
> path for us and that is all good since it is capable of dealing with VLAN
> tagged frames.
> A non-VLAN aware bridge's data path is only allowed to see untagged traffic,
> so if you wanted somehow to inject untagged traffic into the bridge data
> path, then you would add a 802.1Q upper to that switch port, and somehow add
> that device into the bridge. There is a problem with that though, if you
> have mutliple bridge devices spanning the same switch, and you do the same
> thing on another switch port, with another 802.1Q upper, I believe you could
> break isolation between bridges for that particular VID.

At least in mlxsw this is handled by mapping the two {Port, VID} pairs
into different FIDs, each corresponding to a different bridge instance,
thereby maintaining the isolation.

> Most of this was based on discussions we had with Ido and him explaining to
> me how they were doing it in mlxsw.
> AFAIR the other case which is that you already have a 802.1Q upper, and then
> you add the switch port to the bridge is permitted and the bridge would
> inherit the VLAN into its local database.

If you have swp1 and swp1.10, you can put swp1 in a VLAN-aware bridge
and swp1.10 in a VLAN-unaware bridge. If you add VLAN 10 as part of the
VLAN-aware bridge on swp1, traffic tagged with this VLAN will still be
injected into the stack via swp1.10.

I'm not sure what is the use case for such a configuration and we reject
it in mlxsw.

> I did not put much thoughts back then into a cascading set-up, so some
> assumptions can certainly be broken, and in fact, are broken today as you
> experimented.
> -- 
> Florian

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