So let's start with a simple topology to illustrate how spending tree works.
Why would you require spending tree in a switched network?
Since topology we have to stay connected to switch one.
Switch one in turn is connected to switch two and switch two has host be connected to it.
So very simple topology.
Now, if the link went down between switch one and switch to host, I wouldn't be able to communicate
with host B and vice versa.
So you probably going to want to implement some kind of redundancy between those two switches by adding
an additional link.
So that's great because you now have a network redundancy in case one of the links goes down.
However, that introduces problems in a switched environment.
It's generally recommended in networks today that you implement some type of redundancy.
So in this example, you have two links between your two switches, but that will introduce additional
problems, which will now discuss.
Let's assume for the moment that the switches have just booted up and they mac address tables or camp
tables or empty.
And to help explain this issue, let's add Mac address tables to the topology so that you can see how
the MAC address tables are updated when traffic is sent from one host to another.
So let's assume that in this topology the switches have just come up.
In other words, they've been rebooted or powered up and the Mac address tables or camp tables are empty
on the two switches.
Now, in a sense a frame to be the destination address in the frame will be B and the source address
will be A.
So are you sending a frame to be?
And when it arrives at switch one switch one will read the source Mac address of the frame and the switch
will see that the source addresses a.
The switch will update its Mac address table to state that a can be found on Port one.
Mac address.
B, however, is not in the Mac address table, so the switch will flood the frame out of all ports
except on the port on which it arrived.
So in this example, the frame will go to port two as well as port three.
It does that because it doesn't know where Mac Address B is.
Now, this is obviously a very simple typology.
In this example, the frame is only being sent out of two ports of the switch.
However, if the switch had many ports, let's say 96 ports, an incoming frame on one port could be
replicated out of over 90 ports on that switch.
That increases the amount of traffic sent in your network quite dramatically.
So in this topology, what does switch to do with the frame received on Port one?
The source address once again is a and the destination address is be.
What will the switch do with the frame?
Well, firstly, it's going to update its Mac address table to state that a can be found on Port one
and then it's going to flood the frame out of all ports.
So it'll flooded out of port two as well as Port three.
So in this example, host B will receive the frame from host A.
However, the switch also receives the frame on Port three, and this is where it gets a bit confusing.
Where is a from the switchers point of view?
Is that on Port one or is it on port three?
So in this example, it's going to update its Mac address table to state that A is on Port three because
the frame in our example arrived on Port three a later, then on Port one.
So it's going to update the Mac address table entry to state that A is now available and Port three.
The switch will also flood the frame out of all ports.
So it's going to flood it out of Port one and out of port two.
So Host B has now received the frame twice, once from the original frame that arrived on Port One sent
to OSB and secondly for the frame that arrived on Port three.
So this can get confusing for end devices because they receiving the same frame multiple times.
The Mac address table is also changing.
The first frame that arrived on Port one allowed the switch to update its Mac address table to state
that a can be found on Port one.
However, the frame that arrived on Port three now indicates to the switch that A can be found on Port
three.
So the switch needs to update its Mac address table to state that A can be now found on Port three.
So this introduces instability in the Mac address table.
So we have end devices receiving frames multiple times and we have Mac address instability because the
switch thought that A was available on Port one, but now sees that it's available on Port three.
But it gets worse.
When the frame arrived on Port one, the switch updated its Mac address table to state that A can be
found on Port One.
But it also flooded the frame out of both Port two and Port three in this topology.
The frame was received by Host B, but in addition the frame was sent back to switch one.
So switch one has received as a frame that it sent to switch two and switch one and now updates its
Mac address table to state that A is available on Port three.
Now when to switch one received the frame.
It not only updates its Mac address table, but it also floods the frame out of all ports except to
the port in which it arrived.
So the frame arrived on Port three and is flooded out of Port one and out of port two.
So this gets confusing for Host A because it's receiving the frame that it's originally sent.
But not only is a receiving the frame that it sent switch one is also sending the same frame back to
switch two.
And what is switch two going to do with the frame?
It's going to flood it.
So it's going to send a copy to host B.
Host P is now received the same frame three times, but it'll also send the frame back to switch one.
As well as updating its Mac address table to now state that A is on Port one.
So originally when it received the first frame, it thought that A was on port one.
Then when it received the frame and port three, it thought that A was in port three and now it thinks
that A is on port one.
So we've got a lot of Mac address instability in the MAC address table.
Post B's receiving the same frame multiple times.
But the biggest issue here is that the frame gets sent back to switch one, gets flooded again, gets
sent back to switch to, and this process continues over and over again.
We have a loop in this topology with the frame being duplicated and sent round and round and round between
these two switches.