Networks should be designed top-down but
implemented bottom-up. A good architect starts with
the client's goals and waits until the structure is
defined before pouring the foundation. A good
network architect remains focused on your
particular goals, from design through
configuration, to ensure that each layer will
support those above.
Ideally, each layer is independent of the rest
and can be upgraded in place. Although existing
standards do provide for such modular designs,
changes and upgrades are more difficult and
expensive toward the bottom of the stack. It is
essential, therefore, that your design considers
existing and emerging standards at every level.