@kitten couldn't have been, the WiFi is a different range of IPs than the ethernet... yet Unifi network analysis and the Airview thing showed that the Shield was extremely noisy (even when supposedly turned off) and STP blocking occurred just on the Shield ethernet port once all STP priorities were set on all switches.
Am slightly confused too, but in part this is a thing I only learned this morning having been frustrated my network kept blacking out at one end of the house.