I'm trying out the NAV virtual appliance (4.1.0), talking to 25 or so Cisco switches. It was working fine but after a few weeks, one of the switches is showing a problem, "SNMP agent down". The SNMP agent is not down. The switch is fine, its configuration hasn't changed. I can talk SNMP to it fine using the same credentials from other machines, and the NAV VM can talk SNMP fine to all of the other switches - all have the same SNMP credentials, and most are the same model of Cisco switch. (I was going to test from the NAV VM or watch the network traffic but the appliance doesn't come with the snmp tools or tcpdump, and is isolated from the Internet so not easily able to install them)
Restarting the switch made no difference.
I've had a rummage in the logs without seeing anything obvious. Any clues where to look to find why NAV is unable to talk SNMP to this one switch suddenly, after working fine for several weeks?