The value of tmnxDhcpServerCfgUserIdent specifies
which method is used by the local DHCP server to uniquely identify
a user.
When the value of tmnxDhcpServerCfgAddrType is set to 'ipv4(1)',
this object can only be set to 'macCircuitId(1)'.
When the value of tmnxDhcpServerCfgAddrType is set to 'ipv6(2)',
this object cannot be set to 'macCircuitId(1)'. The default
value is 'duid(2)'.