network protocols handbook.pdf

266Protocols GuideISO Protocols - Network LayerProtocol NameIS-IS: Intermediate System toIntermediate System RoutingProtocol8 16 bitIntradomain routing protocoldiscriminatorLength indicatorVersion/protocol ID extensionID lengthR R R PDU type VersionReservedMaximum area addressesProtocol DescriptionIntermediate System-to-Intermediate System (IS-IS) is a routingprotocol developed by the ISO. It is a link-state protocol whereISs (routers) exchange routing information based on a singlemetric to determine network topology. It behaves similar to OpenShortest Path First (OSPF) in the TCP/IP network.In an IS-IS network, there are End Systems, Intermediate Systems,Areas and Domains. End systems are user devices. Intermediatesystems are routers. Routers are organized into localgroups called ‘areas’, and several areas are grouped togetherinto a ‘domain’. IS-IS is designed primarily for providing intradomainrouting or routing within an area. IS-IS, working in conjunctionwith CLNP, ES-IS and IDRP, provides complete routingover the entire network.IS-IS routing makes use of two-level hierarchical routing. Level1 routers know the topology in their area, including all routersand hosts, but they do not know the identity of routers or destinationsoutside of their area. Level 1 routers forward all traffic fordestinations outside of their area to a level 2 router within theirarea which knows the level 2 topology. Level 2 routers do notneed to know the topology within any level 1 area, except to theextent that a level 2 router may also be a level 1 router within asingle area.IS-IS adapted to carry IP network information is called IntegratedIS-IS. Integrated IS-IS has the most important characteristicnecessary in a modern routing protocol: It supports VLSM andconverges rapidly. It is also scalable to support very large networks.There are two types of IS-IS addresses:Network Service Access Point (NSAP) - NSAP addresses identifynetwork layer services, one for each service running.Network Entity Title (NET) - NET addresses identify networklayer entities or processes instead of services.Devices may have more than one of each of the two types ofaddresses. However NET’s should be unique, and the SystemID portion of the NSAP must be unique for each system.Protocol StructureIS-IS PDU Header:• Intradomain routing protocol discriminator - Networklayer protocol identifier assigned to this protocol• Length indicator - Length of the fixed header in octets.• Version/protocol ID extension - Equal to 1.• ID length - Length of the ID field of NSAP addressesand NETs used in this routing domain.• R - Reserved bits.• PDU type - Type of PDU. Bits 6, 7 and 8 are reserved.• Version - Equal to 1.• Maximum area addresses - Number of area addressespermitted for this intermediate system’s area.Format of NSAP for IS-IS:AFI< IDP > < DSP >IDI< HO-DSP >Contents assigned by authority identified inIDI field< Area Address > < ID > < SEL >• IDP - Initial Domain Part• AFI - Authority and Format Identifier (1-byte); Providesinformation about the structure and content ofthe IDI and DSP fields.• IDI - Initial Domain Identifier (variable length)• DSP - Domain Specific Part• HO-DSP - High Order Domain Specific Part• Area Address (variable)• ID - System ID (1- 8 bytes)• SEL - n-selector (1-byte value that serves a functionsimilar to the port number in Internet Protocol).Related protocolsOSPF, ES-IS, CLNP, IDRP, CONPSponsor SourceIS-IS is defined in ISO (www.iso.org) 10589 and reviewed byIETF (www.ietf.org) RFC 1629.Referencehttp://www.javvin.com/protocol/rfc1629.pdfGuidelines for OSI NSAP Allocation in the Internet