Original subtitles

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So let’s start with a simple topology to illustrate how Spanning Tree works.

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Why would you require Spanning Tree in a switch network?

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So in this topology we have host A connected to switch 1

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switch 1 in turn is connected to switch 2 and switch 2 has host B connected to it.

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So very simple topology.

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Now if your link went down between switch 1 and switch 2

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host A wouldn’t be able to communicate with host B and vice versa.

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So you probably gonna want to implement some kind of redundancy

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between those switches by adding an additional link.

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So that’s great because you now have network redundancy

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in case one of the links goes down

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however, that introduces problems in a switch environment.

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It’s generally recommended in networks today

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that you implement some type of redundancy.

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So in this example, you have 2 links between your 2 switches

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but that will introduce additional problems which we'll now discuss.

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Let’s assume for the moment

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that the switches have just booted up

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and their MAC address tables or cam tables are empty

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and to help explain this issue

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let's add MAC address tables to the topology

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so that you can see how the MAC address tables are updated

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when traffic is sent from 1 host to another.

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So let’s assume that in this topology

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the switches have just come up

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in other words, they've been rebooted or powered up

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and the MAC address tables or cam tables are empty on the 2 switches.

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now when A sends a frame to B the destination address on the frame will be B

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and the source address will be A

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so A is sending a frame to B and when it arrives at switch 1

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switch 1 will read the source MAC address of the frame

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and the switch will see that the source address is A.

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

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MAC address B, however, is not in the MAC address table.

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So the switch will flood the frame out of all ports

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except on the port in which it will arrive.

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So in this example, the frame will go out port 2 as well as port 3.

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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 topology.

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In this example, the frame is only being sent out of 2 ports of the switch.

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however, if the switch had many ports, let’s say 96 ports

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an incoming frame on 1 port could be replicated out of over 90 ports on that switch.

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That increases the amount of traffic sent in your network quite dramatically.

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So in this topology what does switch 2 do with the frame received in port 1.

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The source address once again is A and the destination address is B

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what will the switch do with the frame?

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Well firstly its gonna update its MAC address table

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to state that A can be found on port 1

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and then it's gonna flood the frame out of all ports.

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So they’ll flood out of port 2 as well as port 3.

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so in this example, host B will receive the frame from host A

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however, the switch also receive the frame on port 3

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and this is where it gets a bit confusing

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where is A from the switches point of view, is it on port 1 or is it on port 3?

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So in this example, its gonna update its MAC address table

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to state that A is on port 3

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because their frame in our example arrived on port 3 but later then on port 1

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so its gonna update the MAC address table entry

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to state that A is now available on port 3.

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The switch will also flood the frame out of all ports

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so it’s gonna flood it out of port 1 and out of port 2.

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so host B has now received the frame twice

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once from the original frames that arrived on port 1 sent to host B

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and secondly for the frame that arrived on port 3.

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So this can get confusing for any devices

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because they're receiving the same frame multiple times.

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The MAC address table is also changing

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the first frame that arrived on port 1

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allowed this switch to update it's MAC address table

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to state that A can be found on port 1, however, the frame that arrived on port 3

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now indicates to the switch that A can be found on port 3

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so the switch needs to update its MAC address table

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to state that A can be now found on port 3.

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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

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and we have MAC address instability

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because the switch thought that A was available on port 1

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but now sees that it's available on port 3

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but it get worst, when the frame arrived on port 1

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

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but it also flooded the frame out of both port 2 and port 3 in this topology.

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The frame was received by host B

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but in an addition, the frame was sent back to switch 1.

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So switch 1 has received the frame that it sent to switch 2

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and switch 1 now updates its MAC address table to state that A is available on port 3.

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Now when switch 1 receives the frame it not only updates its MAC address table

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but it also floods the frame out of all ports except to the port which it arrived.

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so the frame arrived on port 3 it's flooded out of port 1 and out of port 2

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so this gets confusing for host A

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because it's receiving the frame that it originally sent.

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but not only is A receiving the frame that it sent

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switch 1 is also sending the same frame back to switch 2

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and what a switch 2 gonna do with the frame?

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it's gonna flood it, so it's gonna send a copy to host B

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host B has now received the same frame 3 times

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but it will also send the frame back to switch 1

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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 A was on port 1

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then when it received the frame on port 3, it thought that A was on port 3

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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.

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Host B is receiving the same frame multiple times

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but the biggest issue here is that

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the frame get sent back to switch 1, gets flooded again

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get sent back to switch 2 and this process continues over and over again.

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We have a loop in this topology with the frame being duplicated

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and sent round and round and round between these 2 switches.