Specifies a protocol used by Network Layer entities in End Systems (ES) and Intermediate Systems (IS) to maintain routing information. Defines: transmission procedures for configuration and routing information between ES and IS; the encoding of protocol data units; procedures for protocol control information interpretation; functional requirements for implementations conforming with this standard. References: ISO 7498; 7498 Add. 1 and 4; 8208; 8348 Add. 1 and 2; ISO 8473; 8648; 8802; CCITT X.25.