Re: macvtap with disconnected physical interface

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On 5/3/22 4:31 AM, Daniel P. Berrangé wrote:
On Mon, May 02, 2022 at 03:42:05AM +0200, Gionatan Danti wrote:
Dear list,
I just discovered the hard way that if the lower lever physical interface of
a macvlan bridge is disconnected (ie: by unplugging the eth cable, resulting
in no carrier), inter-guest network traffic from all virtual machines bound
to the disconnected interface is dropped.

This behavior surprises me, as with the classic bridges I can disconnect the
underlying physical interface without causing any harm to inter-guest

Am I doing something wrong, or this really is the expected behavior? If so,
can I force the macvtap interfaces to bridge traffic even when the
underlying physical interface is disconnected?

Can you share the <interface> configuration for your guest NIC so we
can see how it is setup.

I can't say that I've ever tried this, since my only reason for using macvtap is to provide the guests with direct connectivity to the physical network, and unplugging the physdev negates that. The behavior you describe doesn't surprise me all that much though, since the physical device in the case of a host bridge isn't an integral part of the bridge (it's just one more device attached to a port), while the physical device and macvlan bridge a much more closely associated.

I'm Cc'ing Michael Tsirkin to see if he has more authoritative information on whether or not the macvtaps connected to a macvlan bridge can communicate amongst themselves when the physdev is disconnected.

In the meantime, is there a reason you don't want to just use a standard host bridge that's not connected to any physdev? The one thing I can think of is that you might not want to allow communication between the host and guests, but as long as the bridge itself isn't given an IP address, that won't be possible (at least at the level of IP).

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