Designing-And Redesigning-Today's Local Area Network
by Art Wittmann
You've built a network and it runs reliably! Congratulations;
you're doing better than many. But you know - deep in the pit
of your stomach - that sooner or later it's going to be time to
rebuild your network.
There are three big reasons to build a new network and a couple
of lesser ones. The three biggies are:
- A substantial increase in the number of users on your network.
- A substantial change in the power of the average user's workstation
- New applications emerge that demand more or different network services.
Lesser reasons include:
- Making the network more manageable for changes, moves and adds.
- Adding redundancy and improving reliability of the network
- Updating out-of-date equipment
Note that the big reasons are very much user driven, while the
lesser reasons are mostly driven by the needs of the network manager
- that's you. It isn't that your needs are inconsequential, however,
satisfied users are what turns the budget wheels, so they get
were the only game in town for adding bandwidth to networks.
Some larger networks have been built with transparent
however, these networks usually proved to be difficult to scale
have become much more
popular over the last few years and now offer the features necessary
to build a large, reliable high-performance network. Switching
hubs initially where nothing more than multiport bridges, offering
little more than bandwidth. Now, with
and some layer-three protocol processing, switching hubs can be
used to safely build economical high-performance networks.
In this article we will take a fairly progressive view of routers
and switches. Our philosophy throughout will be to switch where
you can and route where you must. Some vendor has probably already
coined the phrase, but it is a good catch-all for the advice that
we will provide throughout this document.
Our reasoning here is simple:
1. Routers are software-driven devices that excel in flexibility
and feature sets. Generally, much, if not all of the routing decisions
are determined by algorithms run on general purpose RISC CPUs.
Because of this, routers are:
a. Expensive on a per-port basis. CPUs and memory cost a lot of
money and router vendors extract heavy margins to support their
software development efforts.
b. Routers are not particularly fast. CPU algorithms take time
to run and, given the chance, you and I usually load up routers
with all kinds of rules and control lists that must be checked
on a per-packet basis, slowing the router even more.
c. Routers are a great way to get from a trusted
to an untrusted segment. Generally traffic between such segments
(say, between an engineering department and a marketing department)
is orders of magnitude less than within a department. Also, for
all of the reasons we gave for routers being slow above, they
also make great firewalls.
Switches, on the other hand, are firmware-driven devices. Virtually
all of what they do has been committed to silicon in the form
of Application Specific Integrated Circuits (ASICs). Custom ASICs
can provide lightening fast algorithmic processing, but they allow
for fairly little flexibility in the algorithm run. As a result
a. Simple. They take in packets, find a path for them and spit
them back out another port. Network managers can't set up a large
number of parameters on a per-switch basis,
b. Cheap. Particularly Ethernet switches have been reduced down
to a few chips and usually only one major chip per port. They
are beginning to rival the price of non-switched intelligent hubs.
c. Effective. Because they are simple devices, they can deliver
exactly what you want - bandwidth to users. New technology like
virtual LANs (VLANs) and the ability to deliver VLAN traffic outside
of the box make the manageable on most all networks.
Another assumption we'll make is that you need more performance
and flexibility out of your network. We'll also look closely at
some management issues. These are three key areas that matter
to you as a network admin. and that vendors use to distinguish
Table of Contents
November 15, 1996
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