First, you need to get the interface index to create the check through snmp. You can try a snmpwalk with this OID:
1.3.6.1.4.1.2011.5.25.157.1.8.1.1.1 or
1.3.6.1.4.1.2011.5.25.157.1.8.1.1.4
Once you get the SubInterface INDEX, then you can request the traffic.