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Three Stages
All switches regardless of the brand and various enhancements they carry,
have something in common, it's the three stages (sometimes 2 stages) they
go through when powered up and during operation. These are as follows:
- Loop Avoidance (Optional)
Let's have a look at them to get a better understanding!
Address Learning
When a switch is powered on, the MAC filtering table is empty. When a
device transmits and an interface receives a frame, the switch places
the source address in the MAC filtering table remembering the interface
the device on which it is located. The switch has no choice but to flood
the network with this frame because it has no idea where the destination
device is located.
If a device answers and sends a frame back, then the switch will take
the source address from that frame and place the MAC address in the database,
associating this address with the interface that received the frame.
Since the switch has two MAC addresses in the filtering table, the devices
can make a point-to-point connection and the frames will only be forwarded
between the two devices. This makes layer-2 switches better than hubs.
As we explained early on this page, in a hub network all frames are forwarded
out to all ports every time. Most desktop switches these days can hold
upto 8000 MAC addresses in their table, and once the table is filled,
then starting with the very first MAC entry, the switch will start overwritting
the entries. Even tho the number of entries might sound big .. it only
takes a minute or two to fill it up, and if a workstation dosen't talk
on the network for that amount of time, then chances are that its MAC
address has been removed from the table and the switch will forward to
all ports the packet which has as a destination this particular workstation.
And after the first frame has been successfully received by Node
2, Node 2 sends a reply to Node
1, check out what happens:
Notice how the frame is not transmitted to every node on the switch.
The switch by now has already learned that Node
1 is on the first port, so it send it straight there without delay.
From now on, any communication between the two will be a point-to-point
connection :
Forward/Filter Decision
When a frame arrives at the switch, the first step is to check the destination
hardware address, which is compaired to the forward/filter MAC database.
If the destination hardware address is known, then it will transmit it
out the correct port, but if the destination hardware address is not known,
then it will broadcast the frame out of all ports, except the one which
it received it from. If a device (computer) answers to the broadcast,
then the MAC address of that device is added to the MAC database of the
switch.
Loop Avoidance (Optional)
It's always a good idea to have a redundant link between your switches,
in case one decides to go for a holiday. When you setup redundant switches
in your network to stop failures, you can create problems. Have a look
at the picture below and I'll explain:
The above picture shows an example of two switches which have been placed
in the network to provide redundancy in case one fails. Both switches
have their first port connected to the upper section of the network, while
their port 2 is connected to the lower section of the same network. This
way, if Switch A
fails, then Switch B
takes over, or vice versa.
Things will work fine until a broadcast come along and causes alot of
trouble. For the simplicity of this example, I am not going to show any
workstations, but only the server which is going to send a broadcast over
the network, and keep in mind that this is what happens in real life if
your switch does not support Spanning-Tree
Protocol (STP), this is why I stuck the "Optional"
near the "Loop Avoidance"
at the start of this section:
It might look a bit messy and crazy at a first glance but let me explain
what is going on here.
The Server for one reason or another decides to do a broadcast. This
First Round (check arrow)
broadcast is sent down to the network cable and firstly reaches
Port 1 on Switch
A. As a result, since Switch
A has Port 2
connected to the other side of the lan, it sends the broadcast out to
the lower section of the network, this then is sent down the wire and
reaches Port 2
on Switch B
which will send it out Port
1 and back onto the upper part of the network.
At this point, as the arrows indicate (orange
colour) the Second
Round of this broadcast starts.
So again... the broadcast reaches Port
1 of Switch
A and goes out
Port
2 back down to
the lower section of the network and back up via Port
2 of Switch
B. After it comes
out of Port
1 of Switch
B, we get the
Third
Round, and then
the Fourth
Round, Fifth
Round and keeps
on going without stopping.....! This is what we call a Broadcast
Storm.
A Broadcast Storm will repeat constantly, chewing up the valueble bandwidth
on the network. This is a major problem, so they had to solve it one way
or another, and they did... with the Spanning-Tree
Protocol or STP
in short. What STP does, is to find the redundant links, which this case
would be Port 2
of Switch B
and shut it down, thus eliminating the posibility of looping to occur.
Lan Switch Types
At the begining of this page we said that the switches are fast, therefor
have low latency. This latency does vary and depends on what type of switching
mode the switch is operating at. You might recall seeing these three switching
modes at the beginning: Store
& Forward, Cut-Through
and Fragment
Free.
The picture below shows how far the different switching modes check the
frame:
So what does this all mean ? Switching modes ? I Don't understand !
Let's Explain!
The fact is that switches can operate in one of the three modes. Some
advance switches will allow you to actually pick the mode you would like
it to operate in, while others don't give you any choice. Let's have a
quick look at each mode:
Store & Forward mode
This is one of the most popular swtiching methods. In this mode, when
the switch receives a frame from one of it's ports, it will store it in
memory, check it for errors and corruption, and if it passes the test,
it will forward the frame out the designated port, otherwise, if it discovers
that the frame has errors or is corrupt, it will discard it. This method
is the safest, but also has the highest latency.
Cut-Through (Real Time)
Cut-Through switching is the second most popular method. In this mode,the
switch reads the frame until it learns the destination MAC address of
the frame it's receiving. Once it learns it, it will forward the frame
straight out the designated port without delay. This is why we say it's
-Real Time-, there is no delay or error checking done to the frame.
Fragment Free
The Fragment free switching method is mainly used to check for frames
which have been subject to a collision. The frame's first 64 bytes are
only checked before forwarding the frame out the designated port. Reason
for this is because almost all collisions will happen within the first
64 bytes of a frame. If there is a corruption in the first 64 bytes, it's
most likely that that frame was a victim of a collision.
Just keep one important detail in mind:
When you go out to buy a switch, make sure you check the amount of memory
it has. Alot of the cheap switches which support the Store & Forward
mode have very small amounts of memory buffer (256KB- 512KB) per port.
The result of this is that you get a major decrease in performance when
you have more than 2 computers communicating via that switch cause there
isn't enough memory to store all incoming packets (this also depends on
th swtiching type your switch supports), and you eventually get packets
being discarded.
The table below is a guide on what amounts of memory you should be looking
at for switches of different configuration :
Bridges
Bridges are really just like switches, but there are a
few differences which we will mention, but not expand upon. These are
the following:
That's pretty much as far as we will go with the bridges since they are
pretty much old technology and you probably won't see many around.
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