Networks and networking
have grown exponentially over the last 15 years. They’ve had to evolve at light
speed just to keep up with huge increases in basic mission critical user needs
such as sharing data and printers, as well as more advanced demands such as
voice and video conferencing. Unless everyone who needs to share network
resources is located in the same office area (an increasingly uncommon
situation), the challenge is to connect the sometimes many relevant networks
together so all users can share the network resources.
It’s also likely that
at some point, you’ll have to break up one large network into a number of
smaller ones because user response has decreased to a trickle as the network
grew and grew and LAN traffic congestion reached overwhelming proportions.
Breaking up a larger network into a number of smaller ones is called network
segmentation, and it’s accomplished using routers, switches, and bridges.
Possible causes of LAN traffic congestion are:
* Too many hosts in a broadcast domain
* Broadcast storms
* Multicasting
* Low bandwidth
* Adding hubs for connectivity to the
network
* A large amount of ARP or IPX traffic (IPX
is a Novell routing protocol)
Routers are used to
connect networks together and route packets of data from one network to
another. Cisco became the de facto standard of routers because of their
high-quality router products, great selection, and fantastic service. Routers,
by default, break up a broadcast domain, which is the set of all devices on a
network segment that hear all broadcasts sent on that segment. Breaking up a
broadcast domain is important because when a host or server sends a network
broadcast, every device on the network must read and process that
broadcast—unless you’ve got a router. When the router’s interface receives this
broadcast, it can respond by basically saying “Thanks, but no thanks,” and
discard the broadcast without forwarding it on to other networks. Even though
routers are known for breaking up broadcast domains by default, it’s important
to remember that they break up collision domains as well. Two advantages of
using routers in your network are:
* They don’t forward broadcasts by default.
* They can filter the network based on
layer 3 (Network layer) information (i.e., IP address).
Four router functions
in your network can be listed as:
* Packet switching
* Packet filtering
* Internetwork communication
* Path selection
Switches aren’t used to
create internetworks they’re employed to add functionality to an internetwork
LAN. The main purpose of a switch is to make a LAN work better—to optimize its
performance—providing more bandwidth for the LAN’s users. And switches don’t
forward packets to other networks as routers do. Instead, they only “switch”
frames from one port to another within the switched network.
Fig -1
Unlike layer 2 switches
that forward or filter frames, routers (layer 3 switches) use logical
addressing and provide what is called packet switching. Routers can also
provide packet filtering by using access-lists (discussed later), and when
routers connect two or more networks together and use logical addressing (IP),
this is called an internetwork. Lastly, routers use a routing table (map of the
internetwork) to make path selections and to forward packets to remote
networks.
Notice that the router
is found at center stage, and that it connects each physical network together!
The best network connected to the router is the LAN switch network on the left,
because each port on that switch breaks up collision domains. But it’s not all
good—all devices are still in the same broadcast domain. If your broadcast
domains are too large, the users have less bandwidth and are required to
process more broadcasts, and network response time will slow to a level that
could cause office riots.
Obviously, the best
network is one that’s correctly configured to meet the business requirements of
the company it serves. LAN switches with routers, correctly placed in the
network, are the best network design. This book will help you understand the
basics of routers and switches so you can make tight, informed decisions on a
case-by-case basis.
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