This attribute indicates how the LNN is transported over the link
group. It can be one of three types:
trunk - the LNN is supported by Topology over a Passport to
Passport link
internal gateway - the LNN is supported by an address plan (but not
Topology) over a Passport to Passport link
external gateway - the LNN is supported by an address plan (but not
Topology) over a Passport to non-Passport link