cd11IfVlanEncryptKeyTransmit 1.3.6.1.4.1.9.9.272.1.1.1.9.1.6

Only one of the 4 keys of a VLAN can be the transmit key. Setting any one of the 4 keys to 'true', the agent will automatically change the value of cd11IfVlanEncryptKeyTransmit of the other 3 keys to 'false' if they exist.