than one layer of the OSI model, and sometimes multiple protocols commu-
nicate within a single layer. In the same way, when we build physical imple-
mentations of hierarchical networks, we may have many devices in a single
layer, or we might have a single device performing functions at two layers.
The definition of the layers is logical, not physical.
archy, the core layer is responsible for transporting large amounts of traf-
fic both reliably and quickly. The only purpose of the network's core layer
is to switch traffic as fast as possible. The traffic transported across the
core is common to a majority of users. However, remember that user data
is processed at the distribution layer, which forwards the requests to the
core if needed.
volumes of traffic, so speed and latency are driving concerns here. Given
the function of the core, we can now consider some design specifics. Let's
start with some things we don't want to do.
lists, routing between virtual local area networks (VLANs), and
packet filtering.
routers). If performance becomes an issue in the core, give prefer-
ence to upgrades over expansion.
that facilitate both speed and redundancy, such as FDDI, Fast Ether-
net (with redundant links), or even ATM.