The type of VLAN encapsulation desired to be used on this
private vlan trunk port.
It is either a particular type, or 'negotiate' meaning
whatever type results from the negotiation.
dot1Q(1) indicates that the port should accept and transmit
packets with IEEE 802.1q VLAN encapsulation
isl(2) indicates that the port should accept and transmit
packets with Inter Switch Link (ISL) VLAN encapsulation
negotiate(3) indicates that the VLAN encapsulation is
negotiated with the far end. The negotiated VLAN
encapsulation can be dot1Q or isl. negotiate(2) is not
allowed if the port does not support negotiation of
VLAN encapsulation type or if its
cpvlanTrunkPortDynamicState is set to onNoNegotiate(2)