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SubjectRe: [Xen-devel] [RFC v2 1/4] bridge: enable interfaces to opt out from becoming the root bridge
On Tue, 2014-02-18 at 13:02 -0800, Luis R. Rodriguez wrote:
> On Sun, Feb 16, 2014 at 10:57 AM, Stephen Hemminger
> <> wrote:
> > On Fri, 14 Feb 2014 18:59:37 -0800
> > "Luis R. Rodriguez" <> wrote:
> >
> >> From: "Luis R. Rodriguez" <>
> >>
> >> It doesn't make sense for some interfaces to become a root bridge
> >> at any point in time. One example is virtual backend interfaces
> >> which rely on other entities on the bridge for actual physical
> >> connectivity. They only provide virtual access.
> >>
> >> Device drivers that know they should never become part of the
> >> root bridge have been using a trick of setting their MAC address
> >> to a high broadcast MAC address such as FE:FF:FF:FF:FF:FF. Instead
> >> of using these hacks lets the interfaces annotate its intent and
> >> generalizes a solution for multiple drivers, while letting the
> >> drivers use a random MAC address or one prefixed with a proper OUI.
> >> This sort of hack is used by both qemu and xen for their backend
> >> interfaces.
> >>
> >> Cc: Stephen Hemminger <>
> >> Cc:
> >> Cc:
> >> Cc:
> >> Signed-off-by: Luis R. Rodriguez <>
> >
> > This is already supported in a more standard way via the root
> > block flag.
> Great! For documentation purposes the root_block flag is a sysfs
> attribute, added via 3.8 through commit 1007dd1a. The respective
> interface flag is IFLA_BRPORT_PROTECT and can be set via the iproute2
> bridge utility or through sysfs:
> mcgrof@garbanzo ~/linux (git::master)$ find /sys/ -name root_block
> /sys/devices/pci0000:00/0000:00:04.0/0000:02:00.0/net/eth0/brport/root_block
> /sys/devices/vif-3-0/net/vif3.0/brport/root_block
> /sys/devices/virtual/net/vif3.0-emu/brport/root_block
> mcgrof@garbanzo ~/devel/iproute2 (git::master)$ cat
> /sys/devices/vif-3-0/net/vif3.0/brport/root_block
> 0
> mcgrof@garbanzo ~/devel/iproute2 (git::master)$ sudo bridge link set
> dev vif3.0 root_block on
> mcgrof@garbanzo ~/devel/iproute2 (git::master)$ cat
> /sys/devices/vif-3-0/net/vif3.0/brport/root_block
> 1
> So if we'd want to avoid using the MAC address hack alternative to
> skip a root port userspace would need to be updated to simply set this
> attribute after adding the device to the bridge. Based on Zoltan's
> feedback there seems to be use cases to not enable this always for all
> xen-netback interfaces though as such we can just punt this to
> userspace for the topologies that require this.
> The original motivation for this series was to avoid the IPv6
> duplicate address incurred by the MAC address hack for avoiding the
> root bridge. Given that Zoltan also noted a use case whereby IPv4 and
> IPv6 addresses can be assigned to the backend interfaces we should be
> able to avoid the duplicate address situation for IPv6 by using a
> proper random MAC address *once* userspace has been updated also to
> use IFLA_BRPORT_PROTECT. New userspace can't and won't need to set
> this flag for older kernels (older than 3.8) as root_block is not
> implemented on those kernels and the MAC address hack would still be
> used there. This strategy however does put a requirement on new
> kernels to use new userspace as otherwise the MAC address workaround
> would not be in place and root_block would not take effect.

Can't we arrange things in the Xen hotplug scripts such that if the
root_block stuff isn't available/doesn't work we fallback to the
existing fe:ff:ff:ff:ff usage?

That would avoid concerns about forward/backwards compat I think. It
wouldn't solve the issue you are targeting on old systems, but it also
doesn't regress them any further.


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