It is EVERY device on the subnut that is showing down. I have cleared all events and they immediately come back as "down". Zenoss also never shows any graph data for any of my snmp devices. I only have one broadcast domain...all devices are in that domain and there are no router hops anywhere in the LAN.
I just fired up a test/demo of SevOne (another NMS system), it sees all my devices and is pulling graph metrics just like the others do (Zabbix, OpenNMS, Cacti and Spiceworks)...Zenoss is the ONLY NMS system that can't get the data and keeps showing all devices down.