Zabbix Aggregate Checks and “Unsupported Item Key”

Problem:

Trying to create an aggregate check in Zabbix to measure total bandwidth in a group of servers using ‘grpsum’, get status ‘Not supported’ and error message ‘Unsupported item key.’

Solution:

It’s not anywhere obvious in the docs  or after googling the error, but the item type needs to be changed from ‘Zabbix agent’ to ‘Zabbix aggregate’