The bits of tBgpInstanceTransportTunnel specify whether VPNs can
exchange routes across autonomous system boundaries, providing model
B connectivity. The 'no' form of the command disallows ASBRs to
advertise VPRN routes to their peers in other autonomous systems. If a
bit is set, the corresponding mechanism will be used to exchange
routes.
The mechanisms represented by each bit are:
ldp(0) - This allows LDP based LSPS to be used as transport from
the ASBR to local PE routers
rsvp-te(1) - This allows RSVP-TE based LSPS to be used as transport
from the ASBR tolocal PE routers
mpls(2) - This allows both LDP and RSVP-TE to be used to resolve
the BGP nexthop for VPRN routes in an associated VPRN
instance.