Re: [Xen-devel] [PATCH 1/3] bridge: preserve random init MAC address

["Luis R. Rodriguez" <mcgrof@xxxxxxxx>]

On Tue, Mar 18, 2014 at 08:10:56PM -0700, Stephen Hemminger wrote:
> On Wed, 12 Mar 2014 20:15:25 -0700
> "Luis R. Rodriguez" <mcgrof@xxxxxxxxxxxxxxxx> wrote:
> 
> > As it is now if you add create a bridge it gets started
> > with a random MAC address and if you then add a net_device
> > as a slave but later kick it out you end up with a zero
> > MAC address. Instead preserve the original random MAC
> > address and use it.
> 
> What is supposed to happen is that the recalculate chooses
> the lowest MAC address of the slaves. If there are no slaves
> it might as well just calculate a new random value. There is
> not great merit in preserving the original defunct address.
> 
> Or something like this
> --- a/net/bridge/br_stp_if.c  2014-02-12 08:21:56.733857356 -0800
> +++ b/net/bridge/br_stp_if.c  2014-03-18 20:09:09.334388826 -0700
> @@ -235,6 +235,9 @@ bool br_stp_recalculate_bridge_id(struct
>                       addr = p->dev->dev_addr;
>  
>       }
> +
> +     if (addr == br_mac_zero)
> +             return false;  /* keep original address */
>  
>       if (ether_addr_equal(br->bridge_id.addr, addr))
>               return false;   /* no change */
> 
> that just keeps the old value.

The old value could be a port which got root blocked, I think
it can be confusing to see that happen. Either way feel free to
make the call, I'll provide more details below on perhaps one reason
to keep the original MAC address.

> The bridge is in a meaningless state when there are no ports,

Some virtualization topologies may want a backend with no link (or
perhaps one which is dynamic, with the option to have none) to the
internet but just a bridge so guests sharing the bridge can talk to 
each other. In this case bridging can be used only to link the
batch of guests.

In this case the bridge simply acts as a switch, but also can be used as the
interface for DHCP, for example. In such a case the guests will be doing
ARP to get to the DHCP server. There's a flurry of ways one can try to get
all this meshed together including enablign an ARP proxy but I'm looking
at ways to optimize this further -- but I'd like to address the current
usage cases first.

> and when the first port is added back it will be used as the
> new bridge id.

Sure. Let me know how you think we should proceed with this patch based
on the above.

  Luis

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