The Functions of Each Layer

Each hierarchical layer in the network design is responsible for preventing unnecessary traffic from being forwarded to the higher layers (and then being discarded at a higher point in the network or by the receiving stations). The goal is to allow only relevant traffic to traverse the network and thereby reduce the load on the network. If this goal is met, the network can scale more effectively. The three layers of a hierarchy are as follows:

• The access layer

• The distribution layer

The Access Layer

In accordance with its name, the access layer is where the end devices connect to the network— where they gain access to the company network. The Layer 3 devices (such as routers) that guard the entry and exit to this layer are responsible for ensuring that all local server traffic does not leak out to the wider network. QoS classification is performed here along with other technologies that define the traffic that is to traverse the network. Service Advertisement Protocol (SAP) filters for NetWare and AppleTalk's GetZoneLists are implemented here, in reference to the design consideration of client/server connectivity.

The Distribution Layer

The distribution layer is responsible for determining access across the campus backbone by filtering out unnecessary resource updates and by selectively granting specific access to users and departments. Access lists are used not just as traffic filters, but as the first level of rudimentary security.

Access to the Internet is implemented here, requiring a more sophisticated security or firewall system.

The Core Layer

The responsibility of the core layer is to connect the entire enterprise. At the pinnacle of the network, reliability is of the utmost importance. A break in the network at this level would result in the incapability of large sections of the organization to communicate. To ensure continuous connectivity, the core layer should be designed to be highly redundant, and, as much as possible, all latency should be removed. Because latency is created when decisions are required, decisions relating to complex routing decisions, such as filters, should not be implemented at this layer. They should be implemented at the access or distribution layers, leaving the core layer with the simple duty of relaying the data as fast as possible to the remote site. In some implementations, QoS is implemented at this layer to ensure a higher priority to certain packets to prevent them from being lost during high congestion periods.

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