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

Ethan Jackson ethan at nicira.com
Wed Aug 6 04:44:40 UTC 2014

Based on my (long ago) reading of the LACP spec, only supporting a
single aggregator is a valid configuration.  Furthermore, it's what
makes the most sense given the structure of the OVS bonding
configuration.  I'd really rather not make a non standard change to
the protocol to support a buggy upstream mlag implementation cause I
don't know how it could affect other less buggy switches.  My
preferences is to shelve this for now FWIW.


On Tue, Aug 5, 2014 at 2:16 PM, Flavio Leitner <fbl at redhat.com> wrote:
> 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
>   bandwidth.
>   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.
> """
> Documentation/networking/bonding.txt
> That would avoid problems with transient states like the reported one.
> fbl
>> 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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