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These are the user uploaded subtitles that are being translated: 1 00:00:00,000 --> 00:00:07,000 align:middle line:84% So let’s start with a simple topology to illustrate how Spanning Tree works. 2 00:00:07,000 --> 00:00:10,000 align:middle line:84% Why would you require Spanning Tree in a switch network? 3 00:00:10,000 --> 00:00:15,000 align:middle line:84% So in this topology we have host A connected to switch 1 4 00:00:15,000 --> 00:00:21,000 align:middle line:84% switch 1 in turn is connected to switch 2 and switch 2 has host B connected to it. 5 00:00:21,000 --> 00:00:24,000 align:middle line:84% So very simple topology. 6 00:00:24,000 --> 00:00:28,000 align:middle line:84% Now if your link went down between switch 1 and switch 2 7 00:00:28,000 --> 00:00:33,000 align:middle line:84% host A wouldn’t be able to communicate with host B and vice versa. 8 00:00:33,000 --> 00:00:36,000 align:middle line:84% So you probably gonna want to implement some kind of redundancy 9 00:00:36,000 --> 00:00:41,000 align:middle line:84% between those switches by adding an additional link. 10 00:00:41,000 --> 00:00:45,000 align:middle line:84% So that’s great because you now have network redundancy 11 00:00:45,000 --> 00:00:47,000 align:middle line:84% in case one of the links goes down 12 00:00:47,000 --> 00:00:52,000 align:middle line:84% however, that introduces problems in a switch environment. 13 00:00:52,000 --> 00:00:56,000 align:middle line:84% It’s generally recommended in networks today 14 00:00:56,000 --> 00:00:58,000 align:middle line:84% that you implement some type of redundancy. 15 00:00:58,000 --> 00:01:03,000 align:middle line:84% So in this example, you have 2 links between your 2 switches 16 00:01:03,000 --> 00:01:07,000 align:middle line:84% but that will introduce additional problems which we'll now discuss. 17 00:01:07,000 --> 00:01:09,000 align:middle line:84% Let’s assume for the moment 18 00:01:09,000 --> 00:01:12,000 align:middle line:84% that the switches have just booted up 19 00:01:12,000 --> 00:01:16,000 align:middle line:84% and their MAC address tables or cam tables are empty 20 00:01:16,000 --> 00:01:19,000 align:middle line:84% and to help explain this issue 21 00:01:19,000 --> 00:01:23,000 align:middle line:84% let's add MAC address tables to the topology 22 00:01:23,000 --> 00:01:27,000 align:middle line:84% so that you can see how the MAC address tables are updated 23 00:01:27,000 --> 00:01:30,000 align:middle line:84% when traffic is sent from 1 host to another. 24 00:01:30,000 --> 00:01:32,000 align:middle line:84% So let’s assume that in this topology 25 00:01:32,000 --> 00:01:35,000 align:middle line:84% the switches have just come up 26 00:01:35,000 --> 00:01:38,000 align:middle line:84% in other words, they've been rebooted or powered up 27 00:01:38,000 --> 00:01:43,000 align:middle line:84% and the MAC address tables or cam tables are empty on the 2 switches. 28 00:01:43,000 --> 00:01:51,000 align:middle line:84% now when A sends a frame to B the destination address on the frame will be B 29 00:01:51,000 --> 00:01:53,000 align:middle line:84% and the source address will be A 30 00:01:53,000 --> 00:01:58,000 align:middle line:84% so A is sending a frame to B and when it arrives at switch 1 31 00:01:58,000 --> 00:02:02,000 align:middle line:84% switch 1 will read the source MAC address of the frame 32 00:02:02,000 --> 00:02:05,000 align:middle line:84% and the switch will see that the source address is A. 33 00:02:05,000 --> 00:02:12,000 align:middle line:84% the switch will update its MAC address table to state that A can be found on port 1. 34 00:02:12,000 --> 00:02:16,000 align:middle line:84% MAC address B, however, is not in the MAC address table. 35 00:02:16,000 --> 00:02:20,000 align:middle line:84% So the switch will flood the frame out of all ports 36 00:02:20,000 --> 00:02:22,000 align:middle line:84% except on the port in which it will arrive. 37 00:02:22,000 --> 00:02:26,000 align:middle line:84% So in this example, the frame will go out port 2 as well as port 3. 38 00:02:26,000 --> 00:02:31,000 align:middle line:84% It does that because it doesn’t know where MAC address B is. 39 00:02:31,000 --> 00:02:34,000 align:middle line:84% Now, this is obviously a very simple topology. 40 00:02:34,000 --> 00:02:39,000 align:middle line:84% In this example, the frame is only being sent out of 2 ports of the switch. 41 00:02:39,000 --> 00:02:45,000 align:middle line:84% however, if the switch had many ports, let’s say 96 ports 42 00:02:45,000 --> 00:02:52,000 align:middle line:84% an incoming frame on 1 port could be replicated out of over 90 ports on that switch. 43 00:02:52,000 --> 00:02:57,000 align:middle line:84% That increases the amount of traffic sent in your network quite dramatically. 44 00:02:57,000 --> 00:03:03,000 align:middle line:84% So in this topology what does switch 2 do with the frame received in port 1. 45 00:03:03,000 --> 00:03:08,000 align:middle line:84% The source address once again is A and the destination address is B 46 00:03:08,000 --> 00:03:11,000 align:middle line:84% what will the switch do with the frame? 47 00:03:11,000 --> 00:03:14,000 align:middle line:84% Well firstly its gonna update its MAC address table 48 00:03:14,000 --> 00:03:17,000 align:middle line:84% to state that A can be found on port 1 49 00:03:17,000 --> 00:03:20,000 align:middle line:84% and then it's gonna flood the frame out of all ports. 50 00:03:20,000 --> 00:03:24,000 align:middle line:84% So they’ll flood out of port 2 as well as port 3. 51 00:03:24,000 --> 00:03:29,000 align:middle line:84% so in this example, host B will receive the frame from host A 52 00:03:29,000 --> 00:03:34,000 align:middle line:84% however, the switch also receive the frame on port 3 53 00:03:34,000 --> 00:03:36,000 align:middle line:84% and this is where it gets a bit confusing 54 00:03:36,000 --> 00:03:42,000 align:middle line:84% where is A from the switches point of view, is it on port 1 or is it on port 3? 55 00:03:42,000 --> 00:03:45,000 align:middle line:84% So in this example, its gonna update its MAC address table 56 00:03:45,000 --> 00:03:48,000 align:middle line:84% to state that A is on port 3 57 00:03:48,000 --> 00:03:56,000 align:middle line:84% because their frame in our example arrived on port 3 but later then on port 1 58 00:03:56,000 --> 00:04:00,000 align:middle line:84% so its gonna update the MAC address table entry 59 00:04:00,000 --> 00:04:02,000 align:middle line:84% to state that A is now available on port 3. 60 00:04:02,000 --> 00:04:07,000 align:middle line:84% The switch will also flood the frame out of all ports 61 00:04:07,000 --> 00:04:10,000 align:middle line:84% so it’s gonna flood it out of port 1 and out of port 2. 62 00:04:10,000 --> 00:04:15,000 align:middle line:84% so host B has now received the frame twice 63 00:04:15,000 --> 00:04:21,000 align:middle line:84% once from the original frames that arrived on port 1 sent to host B 64 00:04:21,000 --> 00:04:24,000 align:middle line:84% and secondly for the frame that arrived on port 3. 65 00:04:24,000 --> 00:04:27,000 align:middle line:84% So this can get confusing for any devices 66 00:04:27,000 --> 00:04:30,000 align:middle line:84% because they're receiving the same frame multiple times. 67 00:04:30,000 --> 00:04:34,000 align:middle line:84% The MAC address table is also changing 68 00:04:34,000 --> 00:04:37,000 align:middle line:84% the first frame that arrived on port 1 69 00:04:37,000 --> 00:04:39,000 align:middle line:84% allowed this switch to update it's MAC address table 70 00:04:39,000 --> 00:04:45,000 align:middle line:84% to state that A can be found on port 1, however, the frame that arrived on port 3 71 00:04:45,000 --> 00:04:50,000 align:middle line:84% now indicates to the switch that A can be found on port 3 72 00:04:50,000 --> 00:04:53,000 align:middle line:84% so the switch needs to update its MAC address table 73 00:04:53,000 --> 00:04:56,000 align:middle line:84% to state that A can be now found on port 3. 74 00:04:56,000 --> 00:04:59,000 align:middle line:84% So this introduces instability in the MAC address table. 75 00:04:59,000 --> 00:05:04,000 align:middle line:84% so we have end devices receiving frames multiple times 76 00:05:04,000 --> 00:05:07,000 align:middle line:84% and we have MAC address instability 77 00:05:07,000 --> 00:05:10,000 align:middle line:84% because the switch thought that A was available on port 1 78 00:05:10,000 --> 00:05:13,000 align:middle line:84% but now sees that it's available on port 3 79 00:05:13,000 --> 00:05:17,000 align:middle line:84% but it get worst, when the frame arrived on port 1 80 00:05:17,000 --> 00:05:22,000 align:middle line:84% the switch updated its MAC address table to state that A can be found on port 1 81 00:05:22,000 --> 00:05:28,000 align:middle line:84% but it also flooded the frame out of both port 2 and port 3 in this topology. 82 00:05:28,000 --> 00:05:31,000 align:middle line:84% The frame was received by host B 83 00:05:31,000 --> 00:05:35,000 align:middle line:84% but in an addition, the frame was sent back to switch 1. 84 00:05:35,000 --> 00:05:40,000 align:middle line:84% So switch 1 has received the frame that it sent to switch 2 85 00:05:40,000 --> 00:05:48,000 align:middle line:84% and switch 1 now updates its MAC address table to state that A is available on port 3. 86 00:05:48,000 --> 00:05:54,000 align:middle line:84% Now when switch 1 receives the frame it not only updates its MAC address table 87 00:05:54,000 --> 00:05:59,000 align:middle line:84% but it also floods the frame out of all ports except to the port which it arrived. 88 00:05:59,000 --> 00:06:06,000 align:middle line:84% so the frame arrived on port 3 it's flooded out of port 1 and out of port 2 89 00:06:06,000 --> 00:06:09,000 align:middle line:84% so this gets confusing for host A 90 00:06:09,000 --> 00:06:13,000 align:middle line:84% because it's receiving the frame that it originally sent. 91 00:06:13,000 --> 00:06:17,000 align:middle line:84% but not only is A receiving the frame that it sent 92 00:06:17,000 --> 00:06:22,000 align:middle line:84% switch 1 is also sending the same frame back to switch 2 93 00:06:22,000 --> 00:06:25,000 align:middle line:84% and what a switch 2 gonna do with the frame? 94 00:06:25,000 --> 00:06:29,000 align:middle line:84% it's gonna flood it, so it's gonna send a copy to host B 95 00:06:29,000 --> 00:06:32,000 align:middle line:84% host B has now received the same frame 3 times 96 00:06:32,000 --> 00:06:36,000 align:middle line:84% but it will also send the frame back to switch 1 97 00:06:36,000 --> 00:06:42,000 align:middle line:84% as well as updating its MAC address table to now state that A is on port 1. 98 00:06:42,000 --> 00:06:48,000 align:middle line:84% So originally when it received the first frame, it thought that A was on port 1 99 00:06:48,000 --> 00:06:52,000 align:middle line:84% then when it received the frame on port 3, it thought that A was on port 3 100 00:06:52,000 --> 00:06:56,000 align:middle line:84% and now it thinks that A is on port 1 101 00:06:56,000 --> 00:07:01,000 align:middle line:84% so we’ve got a lot of MAC address instability in the MAC address table. 102 00:07:01,000 --> 00:07:05,000 align:middle line:84% Host B is receiving the same frame multiple times 103 00:07:05,000 --> 00:07:07,000 align:middle line:84% but the biggest issue here is that 104 00:07:07,000 --> 00:07:12,000 align:middle line:84% the frame get sent back to switch 1, gets flooded again 105 00:07:12,000 --> 00:07:17,000 align:middle line:84% get sent back to switch 2 and this process continues over and over again. 106 00:07:17,000 --> 00:07:23,000 align:middle line:84% We have a loop in this topology with the frame being duplicated 107 00:07:23,000 --> 00:07:28,000 align:middle line:84% and sent round and round and round between these 2 switches. 12882