lagg(4) and failover

Andrew Thompson thompsa at
Tue Aug 12 15:50:22 UTC 2008

On Tue, Aug 12, 2008 at 09:24:30PM +1000, Peter Jeremy wrote:
> On 2008-Aug-12 18:55:52 +0800, Eugene Grosbein <eugen at> wrote:
> >On Tue, Aug 12, 2008 at 12:37:15PM +0200, Marian Hettwer wrote:
> >
> >> I'm using lagg(4) on some of our servers and I'm just wondering how the
> >> failover is implemented.
> As far as I can tell, not especially well :-(.  It doesn't seem to detect
> much short of layer 1 failure.  In particular, shutting down the switch
> port will not trigger a failover.
> >> The manpage isn't quite clear:
> >> 
> >>      failover     Sends and receives traffic only through the master port. 
> >> If
> >>                   the master port becomes unavailable, the next active port
> >> is
> >>                   used.  The first interface added is the master port; any
> >>                   interfaces added after that are used as failover devices.
> >> 
> >> What is meant by "becomes unavailable"? Is it just the physical link which
> >> needs to become unavailable to trigger a failover?
> It seems to be,
> >Yes. It seems you need lacp protocol described later in the manual.
> Actually, lacp and failover are used differently: lacp is primarily
> used to increase the bandwidth between the host and the switch whilst
> failover is used for redundancy.
> With lacp, all the physical interfaces must be connected to a single
> switch.  With failover, the physical interfaces will normally be
> connected to different switches (so a failure in one switch will not
> cause the loss of all connectivity.

Actually you can use lacp in failover mode by connecting interfaces to
different switches. It will only bundle an aggregation to one switch at
a time but if that becomes unavailable then it will automatically choose
the next switch.


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