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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 in this topology, let’s determine why certain ports are set to forwarding 2 00:00:07,000 --> 00:00:10,000 align:middle line:84% and why certain points are blocking 3 00:00:10,000 --> 00:00:12,000 align:middle line:84% so we'll work through the Spanning Tree process. 4 00:00:12,000 --> 00:00:17,000 align:middle line:84% The first decision that needs to be made is election of root bridge. 5 00:00:17,000 --> 00:00:20,000 align:middle line:84% So 1 of the switches in the topology 6 00:00:20,000 --> 00:00:23,000 align:middle line:84% needs to become the root of the Spanning Tree. 7 00:00:23,000 --> 00:00:27,000 align:middle line:84% So on switch 1 as we saw previously 8 00:00:27,000 --> 00:00:30,000 align:middle line:84% sh spanning-tree 9 00:00:30,000 --> 00:00:34,000 align:middle line:84% shows us that this switch or bridge is the root of the Spanning Tree. 10 00:00:34,000 --> 00:00:43,000 align:middle line:84% Switch 2 is not the root of this Spanning Tree. 11 00:00:43,000 --> 00:00:46,000 align:middle line:84% So in the output here we can see 12 00:00:46,000 --> 00:00:49,000 align:middle line:84% that it has a path cost to get to the root switch 13 00:00:49,000 --> 00:00:52,000 align:middle line:84% it sees that the root bridge or root switch 14 00:00:52,000 --> 00:00:55,000 align:middle line:84% has the root ID with priority of this 15 00:00:55,000 --> 00:01:00,000 align:middle line:84% and MAC address of this which is different to the local switch MAC address. 16 00:01:00,000 --> 00:01:04,000 align:middle line:84% So first decision, how is the root determined? 17 00:01:04,000 --> 00:01:07,000 align:middle line:84% It's based on lowest bridge ID 18 00:01:07,000 --> 00:01:10,000 align:middle line:84% which consist of the priority and MAC address. 19 00:01:10,000 --> 00:01:16,000 align:middle line:84% Switch 1 has the same priority as switch 2, 32769 20 00:01:16,000 --> 00:01:19,000 align:middle line:84% so that can't be used to determine the Spanning Tree root. 21 00:01:19,000 --> 00:01:23,000 align:middle line:84% So the tie breaker is based on the MAC address. 22 00:01:23,000 --> 00:01:25,000 align:middle line:84% so lowest MAC address will win 23 00:01:25,000 --> 00:01:29,000 align:middle line:84% switch 1 has a lower MAC address when compared to switch 2 24 00:01:29,000 --> 00:01:35,000 align:middle line:84% once again 0011 is the same on both switches 25 00:01:35,000 --> 00:01:41,000 align:middle line:84% but notice c6ea is greater and c6ac in hexadecimal 26 00:01:41,000 --> 00:01:44,000 align:middle line:84% so switch 1 becomes the root of the Spanning Tree. 27 00:01:44,000 --> 00:01:49,000 align:middle line:84% So that’s the first decision determine who the root bridge is. 28 00:01:49,000 --> 00:01:52,000 align:middle line:84% Once again priority 32768 is the default. 29 00:01:52,000 --> 00:01:56,000 align:middle line:84% So we’ve now determined who the root bridge is or root switch is. 30 00:01:56,000 --> 00:02:03,000 align:middle line:84% The next decision is that every non root switch needs to determine its root port 31 00:02:03,000 --> 00:02:06,000 align:middle line:84% the root port is at its best port to get to the root bridge. 32 00:02:06,000 --> 00:02:11,000 align:middle line:84% The root port is chosen based on lowest path cost. 33 00:02:11,000 --> 00:02:13,000 align:middle line:84% If there’s a tie-breaker on that 34 00:02:13,000 --> 00:02:18,000 align:middle line:84% then it's based on lowest neighbor bridge ID if path cost are the same. 35 00:02:18,000 --> 00:02:22,000 align:middle line:84% if that can't be used to determine the root port 36 00:02:22,000 --> 00:02:24,000 align:middle line:84% then the lowest port priority is used 37 00:02:24,000 --> 00:02:26,000 align:middle line:84% the port priority is 128 by default 38 00:02:26,000 --> 00:02:32,000 align:middle line:84% and if that can't be used then the lowest port ID is used as a tie-breaker. 39 00:02:32,000 --> 00:02:36,000 align:middle line:84% So first decision is based on lowest path cost. 40 00:02:36,000 --> 00:02:41,000 align:middle line:84% Here’s a table showing you the path cost of Cisco switches 41 00:02:41,000 --> 00:02:48,000 align:middle line:84% they are either based on a 1998 IEEE cost or 2004 IEEE cost. 42 00:02:48,000 --> 00:02:57,000 align:middle line:84% In the 1998 cost values, a 10 Meg link has a cost of 100, 100 Meg 19, I gig 4 43 00:02:57,000 --> 00:03:06,000 align:middle line:84% and 10 gig 2 and the IEEE cost in 2004 and later the cost changed to the following. 44 00:03:06,000 --> 00:03:11,000 align:middle line:84% So in our topology, we have gig interfaces on the switches 45 00:03:11,000 --> 00:03:15,000 align:middle line:84% and if we look at the path cost of various ports 46 00:03:15,000 --> 00:03:19,000 align:middle line:84% notice the value associated is 4 47 00:03:19,000 --> 00:03:23,000 align:middle line:84% so these gigabit links have a path cost value of 4 48 00:03:23,000 --> 00:03:28,000 align:middle line:84% which means that the switches are using the old path cost method 49 00:03:28,000 --> 00:03:32,000 align:middle line:84% to determine the best path to a destination. 50 00:03:32,000 --> 00:03:38,000 align:middle line:84% now the first decision is to determine the root port based on the path cost 51 00:03:38,000 --> 00:03:45,000 align:middle line:84% in this topology, we have gigabit 0/0 connected directly to switch 1 52 00:03:45,000 --> 00:03:48,000 align:middle line:84% gigabit 0/1 is also directly connected to switch 1 53 00:03:48,000 --> 00:03:52,000 align:middle line:84% gigabit 0/3 is connected to a hub 54 00:03:52,000 --> 00:03:54,000 align:middle line:84% which inturn connected to switch 1. 55 00:03:54,000 --> 00:04:01,000 align:middle line:84% So the path cost of gigabit 0/3 would be 8 56 00:04:01,000 --> 00:04:04,000 align:middle line:84% if there was a switch connected here 57 00:04:04,000 --> 00:04:10,000 align:middle line:84% but at the moment the path cost is 4 because we have a hub instead of a switch. 58 00:04:10,000 --> 00:04:16,000 align:middle line:84% So we have 3 ports with the same path cost to get to switch 1. 59 00:04:16,000 --> 00:04:20,000 align:middle line:84% on switch 2 we can type sh spanning root as an example 60 00:04:20,000 --> 00:04:28,000 align:middle line:84% and we can see that gigabit 0/0 was chosen as the root port to get to switch 1 61 00:04:28,000 --> 00:04:31,000 align:middle line:84% but that couldn’t have been determined based on the path cost 62 00:04:31,000 --> 00:04:35,000 align:middle line:84% it would need to be determined base on something else. 63 00:04:35,000 --> 00:04:39,000 align:middle line:84% So once again sh spanning-tree 64 00:04:39,000 --> 00:04:47,000 align:middle line:84% so on switch 2 its chosen gigabit 0/0 as its root port. 65 00:04:47,000 --> 00:04:51,000 align:middle line:84% Can path cost be used to determined the best path to the root bridge 66 00:04:51,000 --> 00:04:55,000 align:middle line:84% based on its port numbers and the answer is no 67 00:04:55,000 --> 00:04:59,000 align:middle line:84% the path cost of this link is 4, the path cost of this link is 4 68 00:04:59,000 --> 00:05:03,000 align:middle line:84% the path cost of this link is 4 69 00:05:03,000 --> 00:05:06,000 align:middle line:84% but that can’t be used as the determining factor. 70 00:05:06,000 --> 00:05:09,000 align:middle line:84% So the next choice is neighbor bridge ID. 71 00:05:09,000 --> 00:05:12,000 align:middle line:84% Now in this example switch, 2 is connected to switch 1 72 00:05:12,000 --> 00:05:16,000 align:middle line:84% on 2 ports that are directly connected to switch 1. 73 00:05:16,000 --> 00:05:20,000 align:middle line:84% so the neighboring bridge ID on both this port is the same 74 00:05:20,000 --> 00:05:22,000 align:middle line:84% so that cannot be used at as the tie-breaker 75 00:05:22,000 --> 00:05:26,000 align:middle line:84% the next decision criteria is based on priority 76 00:05:26,000 --> 00:05:29,000 align:middle line:84% but the priority of the ports are the same 77 00:05:29,000 --> 00:05:32,000 align:middle line:84% so that can't be used as a tie-breaker. 78 00:05:32,000 --> 00:05:35,000 align:middle line:84% So the port number is used as the tie-breaker. 79 00:05:35,000 --> 00:05:39,000 align:middle line:84% 1 is a lower number than 2 80 00:05:39,000 --> 00:05:44,000 align:middle line:84% so hence gigabit 0/0 is chosen as the root port in this topology. 81 00:05:44,000 --> 00:05:49,000 align:middle line:84% now once the root ports are chosen on a per segment basis 82 00:05:49,000 --> 00:05:52,000 align:middle line:84% a designated port needs to be chosen. 83 00:05:52,000 --> 00:05:53,000 align:middle line:84% The easiest way to work this out 84 00:05:53,000 --> 00:06:00,000 align:middle line:84% is imagine that you have a PC in the middle of this cable 85 00:06:00,000 --> 00:06:03,000 align:middle line:84% and it needs to get to the root bridge 86 00:06:03,000 --> 00:06:07,000 align:middle line:84% using either the port on the left or the port on the right. 87 00:06:07,000 --> 00:06:08,000 align:middle line:84% So if I had a PC in this topology 88 00:06:08,000 --> 00:06:12,000 align:middle line:84% which port would it used to get to the root bridge 89 00:06:12,000 --> 00:06:13,000 align:middle line:84% and hopefully, it's fairly obvious 90 00:06:13,000 --> 00:06:18,000 align:middle line:84% that this port is closer to the root bridge than this port. 91 00:06:18,000 --> 00:06:23,000 align:middle line:84% and hence on this segment gigabit 0/0 to gigabit 0/0 92 00:06:23,000 --> 00:06:29,000 align:middle line:84% this port, port 0/0 on switch 1 is the designated port 93 00:06:29,000 --> 00:06:33,000 align:middle line:84% a designated port is the best port to use 94 00:06:33,000 --> 00:06:37,000 align:middle line:84% on a per segment basis to get to the root bridge. 95 00:06:37,000 --> 00:06:41,000 align:middle line:84% So this port is the best port to use on this top segment 96 00:06:41,000 --> 00:06:44,000 align:middle line:84% to get to the root bridge. What about this segment? 97 00:06:44,000 --> 00:06:48,000 align:middle line:84% So on this segment imagine once again that you had a PC here 98 00:06:48,000 --> 00:06:52,000 align:middle line:84% what's it's best port to use to get to the root bridge? 99 00:06:52,000 --> 00:06:56,000 align:middle line:84% Well it would be this port here on switch 1 100 00:06:56,000 --> 00:07:01,000 align:middle line:84% and once again on switch 1, we can see that by typing sh spanning-tree 101 00:07:01,000 --> 00:07:08,000 align:middle line:84% notice gigabit 0/1 on switch 1 is the designated port for this segment. 102 00:07:08,000 --> 00:07:10,000 align:middle line:84% The same it's true for this segment 103 00:07:10,000 --> 00:07:14,000 align:middle line:84% which is the best port to use to get to the root bridge? 104 00:07:14,000 --> 00:07:20,000 align:middle line:84% It's gonna be gigabit 0/3 on switch 1. 105 00:07:20,000 --> 00:07:25,000 align:middle line:84% And on this segment looking at layer 2 switches running spanning 3 106 00:07:25,000 --> 00:07:29,000 align:middle line:84% this port is the best port to use to get to the root bridge. 107 00:07:29,000 --> 00:07:33,000 align:middle line:84% So we've now chosen a designated port for this top link 108 00:07:33,000 --> 00:07:38,000 align:middle line:84% the 2nd link, these links through the hub as well as this link. 109 00:07:38,000 --> 00:07:41,000 align:middle line:84% The last remaining link is this link 110 00:07:41,000 --> 00:07:47,000 align:middle line:84% and the best port to use to get to the root bridge is this port on switch 2. 111 00:07:47,000 --> 00:07:51,000 align:middle line:84% Any other ports on the network will go blocking. 112 00:07:51,000 --> 00:07:56,000 align:middle line:84% So this port gigabit 0/1 is put in the blocking state 113 00:07:56,000 --> 00:08:00,000 align:middle line:84% and so is gigabit 0/3 also put in the blocking state. 114 00:08:00,000 --> 00:08:07,000 align:middle line:84% Now in Rapid Spanning Tree or Rapid PVST these are known as alternate ports. 115 00:08:07,000 --> 00:08:11,000 align:middle line:84% In other words, on these hubs as an example, if we have a PC connected to it. 116 00:08:11,000 --> 00:08:13,000 align:middle line:84% If this link went down 117 00:08:13,000 --> 00:08:18,000 align:middle line:84% PCs could send traffic into the network using this alternate port 118 00:08:18,000 --> 00:08:23,000 align:middle line:84% because it would now transition to the forwarding state when this link goes down. 13790