[ovs-discuss] why the datapath-id choose the minimum non-local MAC address among all of the ports in bridge

[Numan Siddique]

On Fri, Jan 29, 2021 at 5:31 AM Ben Pfaff <blp at ovn.org> wrote:
>
> On Thu, Jan 28, 2021 at 09:57:49AM +0800, taoyunupt wrote:
> > At 2021-01-28 09:29:45, "Ben Pfaff" <blp at ovn.org> wrote:
> > >On Wed, Jan 27, 2021 at 06:13:12PM +0800, taoyunupt wrote:
> > >> Hi,
> > >>     If no configure of "datapath-id" in other_config of br, then it will  choose the minimum non-local MAC address among all of the ports in bridge. The relevant code is in the find_local_hw_addr function（https://github.com/openvswitch/ovs/blob/master/vswitchd/bridge.c#L2322）.
> > >>
> > >>     In this case, If a new  port with miner mac, it will change the dpid ,and the mac of br-int. It will cause frequently reset with SDN controller by ofproto_set_datapath_id function.
> > >>     Is this reasonable？
> > >
> > >This solution is the best one we know.  It yields a predictable port MAC
> > >address and datapath ID given a particular collection of ports.  It's
> > >been that way for perhaps 10 years or so.
> > >
> >
> > >I don't know what a miner mac is.
> >
> >
> > The "miner" should be "smaller"， sorry for that.
> >
> >
> > If a new  port with smaller MAC, it will change the dpid ,and the MAC of  bridge, so  a connection  reset will happen. I don't understand why the mac address of the port affects the mac address of the bridge . I think hold a MAC address(such as default ea?) steady is neccesary, what do you think?
>
> The reason that this happens is exactly to keep the MAC address of the
> bridge steady.
>
> Here's the use case it was designed to address.  It started with
> XenServer, but other hypervisors work similarly.  Each physical NIC that
> might have VMs on it gets put into a bridge, and then the IP address for
> that NIC (if any) gets migrated from the pnic to the bridge device.  You
> want the bridge device to have the same MAC address as the physical NIC,
> so taking the minimum MAC address does that.  Adding virtual NICs
> doesn't change it because OVS ignores random MACs.
>
> If you want a stable MAC, but you have some different situation, set
> your own MAC.

I think it would be helpful if this reply goes in the FAQ.

Thanks
Numan


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