Select an Extreme Networks 5420 Series Universal Switch as the target host to create a monitor — the SNMP service should be up and running on it. Click New Monitor, then check SNMP Custom on the Favorites tab, click Next, and confirm the host. On the next page, click Select... to open the built-in SNMP MIB Browser and type extremeEsrpActivePorts into the Find box to locate it in the OID tree, selecting the specific row/instance you want to monitor since this is a table column, then select it and click OK. It reports the number of physical ports currently members of this VLAN under ESRP (Extreme Standby Router Protocol). On the monitor's Main parameters page you can set the target's SNMP port (default 161), credentials, polling interval, and other settings — see the SNMP Monitor help for details. On the State conditions and Alerting tabs, configure when the monitor should change state and trigger an alert; since this is an INTEGER-type OID, Value bounds is the most useful condition here — trigger an alert if the port count changes unexpectedly, which would indicate a VLAN membership or link-state change. Click Finish to create the monitor; you can adjust any parameter later.