This notification signifies that a TACACS+ authorization server has transitioned to 'up' or 'down'. Note: if rbnTacacsState's value is 'down' and rbnTacacsReason's value is 'packetTimeout', then rbnTacacsUserName's value will be a username. For all other cases, rbnTacacsUserName's value will be a zero-length string. If rbnTacacsServerReason is 'serverError', then rbnTacacsServerMsg may contain a string from the server, otherwise the value is a zero-length string.