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

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Message-ID: <20230604092304.gkcdccgfda5hjitf@skbuf>
Date: Sun, 4 Jun 2023 12:23:04 +0300
From: Vladimir Oltean <olteanv@...il.com>
To: Arınç ÜNAL <arinc.unal@...nc9.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 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.