As the name implies, the
Cisco model was developed by
Cisco Systems to simplify the construction of large
networks by dividing them into three
layers. The model dictates specific network configurations that, if followed, will ensure network
scalability and
efficiency.
The three layers of the model are as follows:
Core Layer
Placed at the top of the hierarchy, the
core layer switches large amounts of network traffic. It forms the basic links responsible for the overall integrity of the network, and thus a failure at this level can be catastrophic. Traffic speed at this layer is extremely important, as slowdowns will effect large amounts of
users. Because of this,
routing and other
network-layer functions should not be used. High-speed
data link technologies such as
FDDI and
Fast Ethernet should be implemented here.
Distribution Layer
This layer functions as an
intermediary between the upper and lower layers. It is responsible for deciding which
packets will access the core layer, as well as the implementation of
Access Contol Lists (
ACLs) and other packet
filtering technologies. Packet
routing should occur at this layer, as well as security features such as
firewalls and
VLANs.
Broadcast domains are also created at this layer.
Access Layer
At the bottom of the hierarchy, this layer controls user and
workgroup access to the network. Local network resources should be placed on this layer, as well as
security features that require a more localized
scope.
Collision domains are created here by the use of
routers and
switches. Because of the simplicity at this level,
static routing can often be used in place of
dynamic routing protocols such as
RIP and
IGRP.