Learning by Doing: CISCO Certified Network ... - SCN Research

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Workstation Workstation Workstation Workstation
“A” “B” “C” “D”
Figure 4—Small switched network.
so why not just use it now? A switch is more scalable than a hub. And, third, switches
are cool. Many of my cohorts and colleagues believe switching will become more
prevalent in networking than routing. We use switches at the core of our networks, not
routers. Switches only use layer 2 information to make decisions. Routers need layer 2
and 3 information to make decisions so they tend to be slower (in geek-speak: switches
have less latency than routers).
So where were we? Oh yeah, switches eliminate collision domain problems.
Let’s look at our network diagram again. Now we have many collision domains (one per
port) and one big broadcast domain. Workstation A and D could communicate almost
instantaneously with each other or to other ports and their devices.
But we still have that one big broadcast domain hanging out there…don’t get me
wrong big broadcast domains aren’t necessarily bad but we would like to keep them as
small as possible. As we said earlier a broadcast domain is used for network
“maintenance.” One analogy for a broadcast domain may be the public address system
in your classroom. The staff can make announcements to the whole school or can
communicate with just an individual classroom. By keeping the broadcast domain as
small as possible we keep our “overhead” traffic as minimal as possible and, therefore,
lessen any possible network traffic.
You may have heard someone refer to Novell as a “chatty” network. What they
really mean is there is a lot of network broadcasting on the broadcast channel. Each
networking device in a Novell uses “SAP” (Service Advertising Protocol). Periodically
every single device in a Novell network sends out a broadcast “here I am!” message over
the broadcast channel (typically every 60 seconds). As you can deduce if you had 100
devices this could create a lot of traffic. Other protocol suites use the broadcast address
channel, albeit to a lesser extent. TCP/IP uses the broadcast channel for ARP/RARP
(Address Resolution Protocol, Reverse Address Resolution Protocol). These are used
when the workstations are booted that need to find their IP or MAC addresses if they
have not been “statically” configured. You will learn more about ARP/RARP later.
Now let’s say our company is growing so we need to add in another network.
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