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Date: Mon, 17 Aug 2009 15:37:25 -0700
From: Stephen Hemminger <shemminger@linux-foundation.org>
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Subject: Re: [Bridge] [RFC] bridge: prevent hairpin and STP problems?
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To: "Fischer, Anna" <anna.fischer@hp.com>
Cc: "bridge@osdl.org" <bridge@linux-foundation.org>, "Dickson,
	Mike (ISS Software)" <mike.dickson@hp.com>, "arnd@arndb.de" <arnd@arndb.de>, "netdev@vger.kernel.org" <netdev@vger.kernel.org>, "bridge@lists.linux-foundation.org" <bridge@lists.linux-foundation.org>, David Miller <davem@davemloft.net>, "ptcongdon@ucdavis.edu" <ptcongdon@ucdavis.edu>, "adobriyan@gmail.com" <adobriyan@gmail.com>, "evb@yahoogroups.com" <evb@yahoogroups.com>

On Mon, 17 Aug 2009 21:16:04 +0000
"Fischer, Anna" <anna.fischer@hp.com> wrote:

> > Subject: [RFC] bridge: prevent hairpin and STP problems?
> > 
> > Do we need to add this to block Spanning Tree from being enabled
> > with hairpin mode? I am not sure what the exact usage of hairpin
> > mode and if it is possible to create loops and get STP confusion.
> > 
> > For comment only, do not apply as is.
> 
> Your patch disables STP on the whole bridge if one or more ports
> are set to hairpin mode.
> 
> However, I don't really see that this is necessary. 
> 
> A hairpin mode port should not reflect BPDUs, because otherwise the
> connected port would think it has detected a loop. The hairpin mode
> port should still be able to generate BPDUs though, and in any case
> the bridge should still be able to run STP.
> 
> The hairpin patch we submitted reflects packets on the forwarding /
> data path whereas BPDUs are processed with a separate hook, so we
> should not be reflecting BPDUs back out of a hairpin mode port.

So if user is using hairpin properly, the STP would work. In fact
it would be a good thing since it would detect looping configurations.