[ovs-dev] [PATCH RFC v2] lacp: Prefer slaves with running partner when selecting lead

Flavio Leitner fbl at redhat.com
Tue Aug 5 21:16:03 UTC 2014

On Mon, Aug 04, 2014 at 12:08:48PM -0700, Andy Zhou wrote:
> Zoltan,
> Sorry it took a while to get back to you.  I am just coming up to
> speed on OVS LACP implementation, so my understanding may not be
> correct.  Please feel free to point them out If I am wrong.
> According to wikipeida MC-LAG entry, there is no standard for it, they
> are mostly designed and implemented by vendors.
> After reading through the commit message, and comparing with the
> 802.1AX spec, I feel this seems like there is a bug in the MC-LAG
> implementation/configuration issue. When the partner on port A comes
> back again, should it wait for MC-LAG sync before using the default
> profile to exchange states with OVS?

I agree that it sounds like a problem in the MC-LAG.  However, I also
agree that OVS could do better.

The aggregation selection policy is somewhat a gray area not defined
in any spec. The bonding driver offers ad_select= parameter which
allows to switch to the new aggregator only if, for instance, all the
ports are down in an active aggregator.

The Team driver implementing 802.3ad also provides the policy selection
parameter.  The default is to consider the prio in the LACPDU, but you
can also tell to not select any other aggregator if the current one is
still usable, or per bandwidth or per number of ports available.

My suggestion if we want to change something is to stick with bonding
driver default behavior regarding to select a new aggregator:
table or 0

  The active aggregator is chosen by largest aggregate

  Reselection of the active aggregator occurs only when all
  slaves of the active aggregator are down or the active
  aggregator has no slaves.

  This is the default value.

That would avoid problems with transient states like the reported one.


> On Mon, Jul 14, 2014 at 3:11 PM, Ben Pfaff <blp at nicira.com> wrote:
> > On Tue, Jul 08, 2014 at 05:35:57PM +0100, Zoltan Kiss wrote:
> >> This patch modifies the LACP selection logic by prefering a slaves with up and
> >> running partners when looking for a lead.
> >> That fixes the following scenario:
> >> - bond has 2 ports, A and B, their other ends are in separate chassis with
> >>   MC-LAG sync
> >> - the partner of port A is restarted
> >> - port B is still working
> >> - the partner on port A comes back, but temporarily it is using a default
> >>   config, as MC-LAG haven't synced yet
> >> - apparently that default config has a sys_priority which is smaller than the
> >>   other, still running port, plus completely different sys_id
> >> - therefore OVS choose port A despite it won't ever comes up into
> >>   collecting-distributing state
> >> - and port B is disabled, causing the whole bond goes down
> >>
> >> Checking through the 802.1ax standard, when port A comes up again, the two
> >> links fall apart due to the different LAG IDs. They should be attached to
> >> different Aggregators, and the Aggregators should live separately. In OVS there
> >> is no such concept as Aggregator, but I think it should be said that it has only
> >> one Aggregator, and it has an unique policy to choose which ports can join.
> >> Although changing the chassis' default config can also fix this, detecting
> >> such problems quite hard, therefore I think it is still valid to improve things
> >> in OVS side.
> >> Btw. the Linux kernel bonding drivers' LACP implementation allows more
> >> aggregators, and therefore it could handle this situation properly.
> >>
> >> Signed-off-by: Zoltan Kiss <zoltan.kiss at citrix.com>
> >
> > I verified that the unit tests still pass with this applied.
> >
> > Andy Zhou said he'd review the patch.
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