ISDN RRs cause no additional section processing. Note that ISDN, in Q.931, permits any IA5 character in the general case. IN ISDN 150862028003217 sh. A host connected to the ISDN may be able to use both the X25 and ISDN addresses, with the local prefix added. The RT resource record provides a route-through binding for hosts that do not have their own direct wide area network addresses.
Smaller numbers indicate more preferred routes.
The format of the RT RR is class insensitive. IN RT 2 Relay. IN RT 90 Relay. If it is itself in the set of RT records, it discards any RTs with preferences higher or equal to its own. 61-69, March 1989.