netdev - Re: [PATCH net-next 27/30] net: dsa: mt7530: introduce BPDU trapping for MT7530 switch

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Date: Sun, 4 Jun 2023 12:39:33 +0300
From: Arınç ÜNAL <arinc.unal@...nc9.com>
To: Vladimir Oltean <olteanv@...il.com>
Cc: Sean Wang <sean.wang@...iatek.com>, Landen Chao
 <Landen.Chao@...iatek.com>, DENG Qingfang <dqfext@...il.com>,
 Daniel Golle <daniel@...rotopia.org>, Andrew Lunn <andrew@...n.ch>,
 Florian Fainelli <f.fainelli@...il.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Matthias Brugger <matthias.bgg@...il.com>,
 AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
 Russell King <linux@...linux.org.uk>,
 Richard van Schagen <richard@...terhints.com>,
 Richard van Schagen <vschagen@...com>,
 Frank Wunderlich <frank-w@...lic-files.de>,
 Bartel Eerdekens <bartel.eerdekens@...stell8.be>, erkin.bozoglu@...ont.com,
 mithat.guner@...ont.com, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH net-next 27/30] net: dsa: mt7530: introduce BPDU trapping
 for MT7530 switch

On 4.06.2023 12:23, Vladimir Oltean wrote:
> On Sun, Jun 04, 2023 at 11:51:33AM +0300, Arınç ÜNAL wrote:
>>> If the switch doesn't currently trap BPDUs, isn't STP broken?
>>
>> No, the BPDU_PORT_FW bits are 0 after reset. The MT7620 programming guide
>> states that frames with 01:80:C2:00:00:00 MAC DA (which is how the BPDU
>> distinction is being made) will follow the system default which means the
>> BPDUs will be treated as normal multicast frames.
>>
>> Only if all 3 bits are set will the BPDUs be dropped.
> 
> Right, if you don't trap BPDUs just to the CPU but flood them, I believe
> the STP protocol won't behave properly with switching loops. Worth testing.

I've got no interest spending time playing around with STP at the moment 
so I'm going to pass.

Arınç