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| 7.1 |  | IS-IS Fundamentals |
| 7.1.1 |  | OSI protocols |

The OSI protocols are the product of an international program formed to develop data networking protocols and other standards that facilitate multivendor equipment interoperability. The OSI program grew out of a need for international networking standards and is designed to facilitate communication between hardware and software systems despite differences in underlying architectures.

The OSI specifications were conceived and implemented by two international standards organizations, the International Organization for Standardization (ISO) and the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). The world of OSI internetworking includes various network services with the following characteristics:

- Independent of underlying communications infrastructure
- End-to-end transfer
- Transparency
- Quality of service (QoS) selection
- Addressing

The OSI protocol suite supports numerous standard protocols at the physical, data link, network, transport, session, presentation, and application layers.

OSI CLNS is a network layer service similar to bare IP service. A CLNS entity communicates over Connectionless Network Protocol (CLNP) with its peer CLNS entity. CLNP is the OSI equivalent of IP.

CLNP is an OSI network layer protocol that carries upper layer data and error indications over connectionless links. CLNP provides the interface between CLNS and upper layers. CLNS does not perform connection setup or termination because paths are determined independently for each packet that is transmitted through a network. In addition, CLNS provides best-effort delivery, which means that no guarantee exists that data will not be lost, corrupted, misordered, or duplicated. CLNS relies on transport layer protocols to perform error detection and correction.

OSI network layer addressing is implemented by using two types of hierarchical addresses, Network Service Access Point (NSAP) addresses and Network Entity Titles (NET). An NSAP is a conceptual point on the boundary between the network and the transport layers. The NSAP is the location at which OSI network services are provided to the transport layer. Each transport layer entity is assigned a single NSAP.

The OSI protocol suite specifies two routing protocols at the network layer, End System-to-Intermediate System (ES-IS) and Intermediate System-to-Intermediate System (IS-IS). In addition, the OSI suite implements two types of network services, connectionless service and connection-oriented service.

| Web Links Introduction to Intermediate System-to-Intermediate System Protocol http://www.cisco.com/warp/public/cc/pd/ iosw/prodlit/insys_wp.htm