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So let's start with a simple typology to illustrate how spending tree works.

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Why would you require spending tree in a switched network.

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Sindhis typology we have to stay connected to switch one switch one in turn is connected to to switch

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t and switch to has host be connected to it.

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So very simple topology.

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Now if the link went down between switch one and switch to host they wouldn't be able to communicate

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with host B and vice versa.

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So you are probably going to want to implement some kind of redundancy between those two switches by

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adding an additional link.

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So that's great because you now have a network redundancy in case one of the links goes down.

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However that introduces problems in a switched environment.

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It's generally recommended in networks today that you implement some type of redundancy.

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So in this example you have two links between your two switches but that will introduce additional problems

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which will now discuss.

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Let's assume for the moment that these switches have just booted up and they MAC address tables or camp

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tables or empty and to help explain this issue.

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Let's add McEnroe's tables to the topology so that you can see how the MAC address tables are updated

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when traffic is sent from one host to another.

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So let's assume that in this topology the switches have just come up.

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In other words they've been rebooted or powered up and the MAC address tables or cash tables are empty

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on the two switches.

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Now in a sense of frame to be the destination address and the frame will be B and the source address

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will be a so a sending a frame to B and when it arrives at Switch one switch one will read the source

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MAC address of the frame and the switch will see that the source addresses a the switch will update

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its MAC address table to state that A can be found on port 1 MAC address.

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B however is not in the MAC address table.

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So the switch will flood the frame out of all ports except on the port in which it arrived.

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So in this example the frame you've got a port too as well as Port three.

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It does that because it doesn't know where mac address B is.

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Now this is obviously a very simple typology.

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In this example the frame is only being sent out of two parts of the switch.

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However if the switch had many ports let's say ninety six ports an incoming frame on one port could

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be replicated out of over 90 ports on the switch that increases the amount of traffic sent in your network

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quite dramatically.

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So in this typology What does switch to do with the frame received and Pt. 1 the source address once

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again IS-A and the destination address is be what will the switch do with the frame.

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Well firstly it's going to update its MAC address table to state that A can be found on port 1 and then

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it's going to flood the frame of all ports so it'll flooded out a port to as well as Port three and

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this example host B will receive a frame from host day.

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However the switch also receives the frame on port three and this is where it gets a bit confusing.

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A From the switches point of view.

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Is it pulled 1 or is it in port 3.

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So in this example it's going to update its MAC address table to state that AY's and Port three because

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the frame the NOW example arrived on port three later then on port 1.

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So it's going to update the MAC address table entry to state that is now available on port three.

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The switch will also flood the frame out of all ports.

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So it's going to flood it out of port one and out of port.

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So host B has now received the frame twice once from the original frame that arrived on port 1 S.O.P

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and secondly for the frame that arrived in port 3.

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So this can get confusing for end devices because they are receiving the same frame multiple times the

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MAC address table is also changing the first frame that arrived on.

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Pulled one allowed the switch to update its MAC address table to state that A can be found on port 1.

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However the frame that arrived on port three now indicates to the switch that a can be found on port

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three.

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So the switch needs to update its MAC address table to state that they can be now found on port three.

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So this introduces instability in the MAC address table.

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So we have end devices receiving frames multiple times and we have MAC address instability because the

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switch thought that was available on port one but now sees that it's available in Port three but it

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gets worse.

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When the frame arrived on port 1 the switch updated its MAC address table to state that A can be found

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in port 1 but it also flooded the frame out of both port 2 and Port three in the apology.

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The frame was received by host B but in addition the frame was sent back to switch one so switch one

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has received was a frame that it's st to switch to and switch one.

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And now updates its MAC address table to state that A is available on port three.

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Now when switch one received the frame it not only updates its MAC address table but it also floods

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the frame out of all ports except the port in which it arrived.

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So the frame arrived on port Three mine is flooded out of port one and out of port two.

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So this gets confusing for homestay because it's receiving the frame that it's originally sent.

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But not only is a receiving the frame that it's sent.

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So which one is also sending the same frame back to switch to.

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And what is switch to going to do with the frame.

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It's going to flood it.

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So it's going to send a copy to host B.

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Host B is now received the same frame three times but it will also send the frame back to switch one

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as well as updating its MAC address table to now state that a is on port 1.

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So originally when it received the first frame it thought that it was on port 1 then when it received

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the frame and Port three it thought that a was on port three and now it thinks that a is on port 1.

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So we've got a lot of mac address instability in the MAC address table post be receiving the same frame

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multiple times.

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But the biggest issue here is that the frame gets sent back to switch one gets flooded again gets sent

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back to switch to.

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And this process continues over and over again.

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We have a loop in this topology with the frame being duplicated and sent round and round and round between

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these two switches.