Workshop

Figure 14.4 How two stations talk via a switch.
Always remember that there’s a segment on each switch port (whether it’s shown or not). In other words,
when you see a diagram showing a point-to-point link between a PC and a switch, or a server and a
switch, it’s unlikely that someone will show the segment in between the node and the switch; instead, just
one line between the switch and the node will be shown. However, the segment is there!
Wait and Switch
There are two kinds of switching methods:
• Cut through Like it sounds, cut through switching starts transmitting the packet as it comes
through, and it’s very fast. Of course, if the packet ends up being an error packet, the switch has
just introduced an error on the other end.
• Store and forward With store and forward switching, the switch receives the entire packet and
then forwards it on to the destination segment. When errors occur here, the switch has the option
of dropping the packet rather than propagating the error. Of course, because the switch waits for
the entire packet to be received before forwarding it to the destination, there’s delay involved.
Store and forward versus cut through involves a tradeoff of reliability versus speed; if you’re
experiencing problems with your switched network, this is something to keep in mind. The price of speed
can sometimes be more than you’re willing to pay.
Broadcast News
If you have large numbers of programs, servers, or print servers that rely on data link–level broadcasts
(that is, data link traffic destined to all stations on a network), remember that each broadcast no longer
goes to one segment. When switching segments together, a broadcast on one propagates to all segments.
This can result in large amounts of traffic and can cause unwanted problems.
You’ll want to think about when to switch and when to route. Two networks with different protocol
addresses do not share data link–level broadcasts.
Again, a switch is really dumb. It doesn’t know anything about the conversation on the phone—it merely
knows who’s doing the talking. Can the switch be configured to divide two different segments based on
protocols? Sure, but then the switch is basically acting as a router.
Trees Save You