So let's start with a simple typology to illustrate how spending tree works.
Why would you require spending tree in a switched network.
Sindhis typology we have to stay connected to switch one switch one in turn is connected to to switch
t and switch to has host be connected to it.
So very simple topology.
Now if the link went down between switch one and switch to host they wouldn't be able to communicate
with host B and vice versa.
So you are 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 these switches have just booted up and they MAC address tables or camp
tables or empty and to help explain this issue.
Let's add McEnroe's 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 cash tables are empty
on the two switches.
Now in a sense of frame to be the destination address and the frame will be B and the source address
will be a so a sending a frame to B 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 1 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 in which it arrived.
So in this example the frame you've got a port too 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 parts of the switch.
However if the switch had many ports let's say ninety six ports an incoming frame on one port could
be replicated out of over 90 ports on the switch that increases the amount of traffic sent in your network
quite dramatically.
So in this typology What does switch to do with the frame received and Pt. 1 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 1 and then
it's going to flood the frame of all ports so it'll flooded out a port to as well as Port three and
this example host B will receive a frame from host day.
However the switch also receives the frame on port three and this is where it gets a bit confusing.
A From the switches point of view.
Is it pulled 1 or is it in port 3.
So in this example it's going to update its MAC address table to state that AY's and Port three because
the frame the NOW example arrived on port three later then on port 1.
So it's going to update the MAC address table entry to state that is now available on 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.
So host B has now received the frame twice once from the original frame that arrived on port 1 S.O.P
and secondly for the frame that arrived in port 3.
So this can get confusing for end devices because they are receiving the same frame multiple times the
MAC address table is also changing the first frame that arrived on.
Pulled one allowed the switch to update its MAC address table to state that A can be found on port 1.
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 they 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 was available on port one but now sees that it's available in Port three but it
gets worse.
When the frame arrived on port 1 the switch updated its MAC address table to state that A can be found
in port 1 but it also flooded the frame out of both port 2 and Port three in the apology.
The frame was received by host B but in addition the frame was sent back to switch one so switch one
has received was a frame that it's st to switch to and switch one.
And now updates its MAC address table to state that A is available on port three.
Now when switch one received the frame it not only updates its MAC address table but it also floods
the frame out of all ports except the port in which it arrived.
So the frame arrived on port Three mine is flooded out of port one and out of port two.
So this gets confusing for homestay because it's receiving the frame that it's originally sent.
But not only is a receiving the frame that it's sent.
So which one is also sending the same frame back to switch to.
And what is switch to going to do with the frame.
It's going to flood it.
So it's going to send a copy to host B.
Host B is now received the same frame three times but it will also send the frame back to switch one
as well as updating its MAC address table to now state that a is on port 1.
So originally when it received the first frame it thought that it was on port 1 then when it received
the frame and Port three it thought that a was on port three and now it thinks that a is on port 1.
So we've got a lot of mac address instability in the MAC address table post be 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.