lagg(4) MAC address selection proposal

[Andrew Thompson]

On 18 April 2012 12:39, Ed Maste <emaste at freebsd.org> wrote:
> When a new lagg(4) interface is created the link layer address from the
> first port in the group is assigned to the lagg and to all other lagg
> port members.  This means the address assigned to the lagg is different
> if specified as, for example, "laggport em0 laggport em1" vs
> "laggport em1 laggport em0".
>
> The code in lagg_port_create(), in if_lagg.c that chooses the first
> l2 address:
>
>   575  if (SLIST_EMPTY(&sc->sc_ports)) {
>   576          sc->sc_primary = lp;
>   577          lagg_lladdr(sc, IF_LLADDR(ifp));
>   578  } else {
>   579          /* Update link layer address for this port */
>   580          lagg_port_lladdr(lp, IF_LLADDR(sc->sc_ifp));
>   581  }
>
> For the current modes lagg supports this probably doesn't matter much,
> but we have some improvements in the pipeline for which this behaviour
> is undesirable.  (The first of which is an interface for choosing a
> different master; this allows a failover lagg to be set to transmit on a
> new port, without changing link states.  With the current behaviour this
> causes all ports in the lagg to then change their l2 address.)
>
> In looking into potential solutions I found that the bridgestp code in
> bridge(4) searches the list of associated MAC addresses and uses the
> lowest one when it needs to select one from a group.  I'd like to
> propose using the same logic for lagg's MAC address selection.  Can
> anyone foresee an issue with this change?  (I'm not aware of any lagg
> use cases that rely on the current behaviour.)

I do not foresee any issues. What we also need is a event trigger for
various pseudo interfaces when the mac or primary interface changes,
this would allow arp/nd6 to rebroadcast.


Andrew