This is straight from the proposed annex:
B/IP router tables shall contain the following information for each logical port:
(a) the B/IP address for this port,
(b) if the port is to be used to communicate with nodes on a network directly connected by virtue of the router having an IP network number in common with one of the IP subnets comprising the B/IP network, the BACnet network number of the network, else
if the port is to be used to communicate with nodes on a network directly connected by virtue of the router registering as a foreign device with a BBMD, the BACnet network number of the network served by the BBMD and the B/IP address of the BBMD offering foreign device registration,
(c) a list of network numbers reachable through this port along with the B/IP address of the next router on the path to each network number and the reachability status, as defined in 6.6.1, of each such network.
Note that one implication of these rules is that two B/IP networks may each have routers that talk to each other directly. While this would normally be considered inefficient in an IP environment (remember the furor in Philadelphia when the concept of B/IP Half-Routers was suggested?!), it DOES provide a mechanism whereby cooperating routers could be set up as a "secure tunnel" through an otherwise impenetrable firewall.