Networking Basics The OSI Reference Model

This section covers the Open System Interconnection (OSI) seven-layer reference model and common examples of each Individual OSI layer. CCIE candidates must fully understand and appreciate the OSI model, because almost every routed protocol in use today is based on its architecture. The OSI model was developed by a standards body called the International Organization for Standardization (ISO) to provide software developers with a standard architecture to develop protocols (such as IP). For example, the OSI model allows a PC to communicate with a UNIX device.

NOTE ISO developed the OSI reference model in 1984. Layers 1 and 2 are implemented in hardware and Layers 3 through 7 are typically implemented in software. Layer 2 is broken up into two smaller sublayers: the software-based LLC sublayer and the hardware-based MAC sublayer.

Table 1-1 displays the seven layers of the OSI model.

Table 1-1 OSI Seven-Layer Model

Layer Name

Layer Number

Application

Layer 7

Presentation

Layer 6

Session

Layer 5

Transport

Layer 4

Network

Layer 3

Data link

Layer 2

Physical

Layer 1

The following sections cover each layer and provide protocol examples for each.

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