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Re: [Xen-users] XenNetworking - Reason for NOARP on eth1/br1?

On Mon, 2006-02-20 at 10:53 +1300, Nicholas Lee wrote:
> http://wiki.xensource.com/xenwiki/XenNetworking
> I'm wondering about this requirement:
> before you connect a physical interface to a bridge, remember to reset
> it's mac and turn arp off. For example:
> # ip link set eth1 down
> # ip link set eth1 mac fe:ff:ff:ff:ff:ff arp off
> # brctl addif br1 eth1
> # ip link set eth1 up
> NOARP does not seem to be set by default (netrwork-bridge) on eth0. Is
> there a reason for this?

If you look carefully at network-bridge, you will find the line in the
function "op_start" that runs "ip link set ${pdev} down arp off".  This
command shuts down the physical interface and disables ARP at the same

ARP is the way a host learns an ethernet MAC address associated with an
IP address.  The reason ARP is disabled on the physical interface is
that once the xenbr0 bridge is brought up, the physical interface is
simply being used as a transparent virtual connection between the
external network and the xenbr0 bridge.  It does not have any IP address
associated with it, so there is no reason for it to support ARP.

Dom0's IP address and MAC address get moved from that physical interface
to a virtual interface (and the confusing part is that the virtual
interface also assumes the OS device name "eth0").  ARP does get enabled
on the virtual interface "eth0", since that is the interface with the
dom0's IP address on it.

So, the same thing needs to occur whenever you create an internal bridge
and attach a physical interface to it.  You disable ARP on that physical
interface and set it's MAC address to a generic bridge MAC that isn't
really used by IP at all.  You then put IP addresses and enable ARP on
the domU's virtual ethernet interfaces that connect to the internal

I hope that clears things up a little.


Patrick Wolfe

email:   pwolfe@xxxxxxxxxxxxxx

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