A layer is identified as a point in the
network where an OSI reference model Layer 3 (network layer) boundary
occurs: The three layers are bound by Layer 3 devices or other devices
that separate the network into broadcast domains. As shown in the
figure, the
three-layer model consists of core, distribution, and access layers,
each of which has specific functions:
- Core layer -- The core layer provides
fast wide-area connections between geographically remote sites,
tying a number of campus networks together in a corporate or
enterprise WAN. Core links are usually point-to-point, and there
are rarely any hosts in the core layer. Core services (for
example, T1/T3, Frame Relay, SMDS) typically are leased from a
telecom service provider.
- Distribution layer -- The distribution
layer gives network services to multiple LANs within a WAN
environment. This layer is where the WAN backbone network is
found, and it is typically based on Fast Ethernet. This layer is
implemented on large sites and is used to interconnect buildings.
- Access layer -- The access layer is
usually a LAN or a group of LANs, typically Ethernet or Token
Ring, that provide users with frontline access to network
services. The access layer is where almost all hosts are attached
to the network, including servers of all kinds and user
workstations.
A three-layer model can meet the needs
of most enterprise networks. However, not all environments require a full three-layer hierarchy. In some cases, a two-layer design may be adequate or even a single layer flat network. Even in these cases, however, a hierarchical structure should be planned or maintained to allow these network designs to expand to three layers as the need arises.
The following sections
discuss in more detail the functions of the three layers. Then, we'll
move on to discuss one- and two-layer hierarchies.
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