The mode of filling in the remote ID suboption
of DHCP option 82 in VLAN.
'sysname' indicates using the device sysname to fill in
the suboption.
'userDefine' indicates using the string defined
by hh3cDhcpSnoop2IfVlanRIDStr to fill in the suboption.
If the value of this object is set to 'userDefine',
hh3cDhcpSnoop2IfVlanRIDStr must also be set.