Specifies the encapsulation used by the circuit to carry IP datagrams.
Encapsulations are as specified in RFC1483 Multiprotocol Interconnect
over ATM. The 'autoconfig' value will recognize both VC Mux and LLC
formats. The 'aal5' and 'aal0' values are used for encapsulation
methods for transport of ATM frames and cells over IP and MPLS networks
when no encapsulation method is specified.