Think I’d next test with two physical machines, hot/cold, with identical MAC etc but both connected via an intermediate switch onto the same port on the SH3

Agreed. Think we’re coming to the same conclusion. It’s the either the ports on the SH3 in modem mode being ignored or the the active link isn’t being fully released by the active NIC or there is some MAC caching or other going on by the switch chip in the SH3.

Devices connected via Intermediate switch in to the same SH3 port needed to prove or disprove this theory.

Okays thanks Dan. I guess I'll just have to use one machine then with no backup or ditch Virgin media. I do wonder if a static IP would work as I could just enter that into both machines, then the SH3 doesn't have to assign one.

Yip. With an intermediate switch neither machine gets an IP

Simulated crash by powering off, removing the physical cable so there's definitely not link active.

Unfortunately using a switch results in no IP given to any machine.

Just checking that it's a dumb switch, not one that would be trying to get an IP for a management interface

It was a dumb switch yeah. I also tried it on a managed one with and without a VLAN.

In reply to a post by mrjay84:

Unfortunately using a switch results in no IP given to any machine.

I think in this scenario the Hub 3 gets two devices sending ARPs and it sees both at the same time and doesn't use either.

The Hub 3 is not designed for this, the assumption is you are connecting a computer or a physical hardware router to the Hub.

It will have a lot of bugs in the firmware, its from Virgin Media !!

In reply to a post by mrjay84:

It was a dumb switch yeah. I also tried it on a managed one with and without a VLAN.

Did you define a new VLAN that wasn't shared with the default / management interface of the switch (typically VLAN 1) and then set the ports for the SH3 and the 'routers' to untag for that new VLAN and not be members of any other or default VLAN?

The Hub 3 in modem mode will "lock" to the MAC address of the connected machine. If you power off this machince and power on another one, the first MAC address will still be locked to the Hub 3.

The Hub 3 needs to be powered off when the machines are swapped. It's the same advise we give to users on the VM Forums when either changing from Router Mode to Modem mode, or replacing a third party Router.

If you can power off the Hub in-between swapping the machines it should work.