Re: Why is STP turned off?

["Daniel P. Berrange" <berrange@redhat.com>]

On Thu, May 01, 2008 at 10:05:16PM +0100, Keir Fraser wrote:
> On 1/5/08 21:29, "Caitlin Bestler" <Caitlin.Bestler@neterion.com> wrote:
> 
> >> All bridge interfaces but the external interface are guest vif's which
> >> are
> >> typically not hiding bridges. This simple topology does not require
> >> STP.
> >> 
> >>  -- Keir
> > 
> > The guest vifs are indeed very unlikely to be acting as bridges.
> > And any switch that only has a single uplink and N internal links
> > (none of which lead to a Bridge) can indeed decide not be an 802.1
> > Bridge and therefore not run spanning tree.
> > 
> > But if Xen is not running spanning tree and one of the Guest VIFs
> > *does* run spanning tree the results can be quite messy. An explicit
> > warning on this might make sense.
> 
> Actually I can't remember why we originally turned off STP. It may have been
> because it took longer for the bridge to 'settle' when new vifs came online.
> On the other hand I may simply have turned off STP along with other
> parameters (hello/learning latencies) as part of a blanket effort to make
> the bridge dumb but efficient. If others have had good experiences with STP
> enabled we could consider re-enabling it in the default bridge
> configuration.

STP does cause problems with DHCP - particularly during installation we
have found problems with STP taking along time to settle causing the
DHCP requests to time out. So I'd recommend keeping it turned off by 
default.

Dan.
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