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 provides connectivity between several parts of the access 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 by interconnecting distribution layer devices. 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 all areas of the network. 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.