This International Standard specifies a protocol to be used by Boundary Intermediate systems (defined in 3.6) to acquire and maintain information for the purpose of routeing NPDUs between different routeing domains. Figure 1 illustrates the field of application of this International Standard. This International Standard specifies: — the procedures for the exchange of inter-domain reachability and path information between BISs — the procedures for maintaining inter-domain routeing information bases within a BIS — the encoding of protocol data units used to distribute inter-domain routeing information between BISs — the functional requirements for implementations that claim conformance to this International Standard The procedures are defined in terms of: — interactions between Boundary Intermediate systems through the exchange of protocol data units — interactions between this protocol and the underlying Network Service through the exchange of service primitives — constraints on policy feasibility and enforcement which must be observed by each Boundary Intermediate system in a routeing domain The boundaries of Administrative Domains are realized as artifacts of the placement of policy constraints and the aggregation of network layer reachability information; they are not manifested explicitly in the protocol. The protocol described in this International Standard operates at the level of individual routeing domains. The establishment of administrative domains is outside the scope of this International Standard.