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These are the user uploaded subtitles that are being translated: 1 00:00:00,000 --> 00:00:04,000 align:middle line:84% At the moment in this topology switch 2 is the root 2 00:00:04,000 --> 00:00:07,000 align:middle line:84% and we can prove that by using sh spanning-tree 3 00:00:07,000 --> 00:00:13,000 align:middle line:84% and notice the local switch is the root for VLAN 1 in the topology. 4 00:00:13,000 --> 00:00:18,000 align:middle line:84% No other VLANs have been enabled 5 00:00:18,000 --> 00:00:22,000 align:middle line:84% so the only Ethernet VLAN existing at the moment is VLAN 1 6 00:00:22,000 --> 00:00:26,000 align:middle line:84% and all ports are VLAN 1 on all the switches. 7 00:00:26,000 --> 00:00:28,000 align:middle line:84% When looking at the BPDUs 8 00:00:28,000 --> 00:00:33,000 align:middle line:84% we can see that the switch is advertising a root path cost of 0 9 00:00:33,000 --> 00:00:37,000 align:middle line:84% and the bridge ID and root identifier are the same. 10 00:00:37,000 --> 00:00:42,000 align:middle line:84% On switch 1 when we capture traffic here 11 00:00:42,000 --> 00:00:49,000 align:middle line:84% it's advertising a root path cost of 4 12 00:00:49,000 --> 00:00:54,000 align:middle line:84% and you can see that the bridge identifier is different to the root identifier 13 00:00:54,000 --> 00:01:04,000 align:middle line:84% So on switch 1, let’s change the spanning-tree vlan 1 root 14 00:01:04,000 --> 00:01:10,000 align:middle line:84% so I'll change the switch to become the root and in this case, it’ll be the primary root 15 00:01:10,000 --> 00:01:13,000 align:middle line:84% that will drop the priority of the switch. 16 00:01:13,000 --> 00:01:16,000 align:middle line:84% previously the switch was using a default priority 17 00:01:16,000 --> 00:01:20,000 align:middle line:84% which we can see in the Wireshark capture 18 00:01:20,000 --> 00:01:26,000 align:middle line:84% it was 32768 + the VLAN number but now sh spanning-tree 19 00:01:26,000 --> 00:01:30,000 align:middle line:84% shows us that the switch priority has been reduced 20 00:01:30,000 --> 00:01:34,000 align:middle line:84% to a value lower than any current root bridge. 21 00:01:34,000 --> 00:01:38,000 align:middle line:84% So the switches now become the root of the Spanning Tree topology. 22 00:01:38,000 --> 00:01:42,000 align:middle line:84% Bridge MAC address and root MAC address is the same. 23 00:01:42,000 --> 00:01:45,000 align:middle line:84% So these are our first Wireshark capture 24 00:01:45,000 --> 00:01:49,000 align:middle line:84% and if I scroll down to the latest advertisements. 25 00:01:49,000 --> 00:01:52,000 align:middle line:84% Notice the root path cost is 0. 26 00:01:52,000 --> 00:01:58,000 align:middle line:84% Bridge ID and root identifier are the same, the priorities have been reduced. 27 00:01:58,000 --> 00:02:02,000 align:middle line:84% So this switch is root with the lower priority. 28 00:02:02,000 --> 00:02:07,000 align:middle line:84% You can see that the bridge priority is 24576 29 00:02:07,000 --> 00:02:12,000 align:middle line:84% extended system ID is 1 bacause this is VLAN 1 30 00:02:12,000 --> 00:02:16,000 align:middle line:84% and the MAC address of the switch is set to the following. 31 00:02:16,000 --> 00:02:19,000 align:middle line:84% So once again, if we look at the Wireshark capture 32 00:02:19,000 --> 00:02:23,000 align:middle line:84% this is capture 114 at the moment. 33 00:02:23,000 --> 00:02:28,000 align:middle line:84% Previously the root identifier and bridge identifier were different. 34 00:02:28,000 --> 00:02:33,000 align:middle line:84% Now, what about switch 2 previously this switch was the root. 35 00:02:33,000 --> 00:02:40,000 align:middle line:84% So notice on capture 1, this switch previously had the root path cost of 0 36 00:02:40,000 --> 00:02:42,000 align:middle line:84% but scrolling down through the captures 37 00:02:42,000 --> 00:02:46,000 align:middle line:84% you can now see that it has a root path cost of 4. 38 00:02:46,000 --> 00:02:51,000 align:middle line:84% Its local bridge identifier has a higher priority. 39 00:02:51,000 --> 00:02:56,000 align:middle line:84% So this switch is no longer the Spanning Tree root. 40 00:02:56,000 --> 00:03:00,000 align:middle line:84% Previously on the switch 3, this was the root port. 41 00:03:00,000 --> 00:03:03,000 align:middle line:84% So here's the output you can see that gigabit 0/0 42 00:03:03,000 --> 00:03:06,000 align:middle line:84% was the root port on switch 3. 43 00:03:06,000 --> 00:03:09,000 align:middle line:84% What is it now? So sh spanning-tree 44 00:03:09,000 --> 00:03:14,000 align:middle line:84% notice gigabit 0/1 is the root port at the moment. 45 00:03:14,000 --> 00:03:18,000 align:middle line:84% so previously on switch 3 this was the root port 46 00:03:18,000 --> 00:03:21,000 align:middle line:84% but the root port is now changed to this port 47 00:03:21,000 --> 00:03:24,000 align:middle line:84% because this switch is now the Spanning Tree root. 48 00:03:24,000 --> 00:03:27,000 align:middle line:84% the path cost using this link is 4 49 00:03:27,000 --> 00:03:32,000 align:middle line:84% where's now the path cost using this link would be 4 + 4 50 00:03:32,000 --> 00:03:35,000 align:middle line:84% in other words 8 to get to the root bridge. 51 00:03:35,000 --> 00:03:40,000 align:middle line:84% Spanning tree topologies can dynamically change 52 00:03:40,000 --> 00:03:47,000 align:middle line:84% so as an example, if on switch 3 has shut down gigabit 0/1 53 00:03:47,000 --> 00:03:50,000 align:middle line:84% and type sh spanning-tree 54 00:03:50,000 --> 00:03:56,000 align:middle line:84% What we can see now is that gigabit 0/0 is now the root port 55 00:03:56,000 --> 00:03:58,000 align:middle line:84% as this port is no longer available. 56 00:03:58,000 --> 00:04:08,000 align:middle line:84% If I no shut that interface and type sh spanning-tree 57 00:04:08,000 --> 00:04:10,000 align:middle line:84% and see the interface is coming up. 58 00:04:10,000 --> 00:04:17,000 align:middle line:84% What you can see now is that gigabit 0/1 has become the root port once again 59 00:04:17,000 --> 00:04:22,000 align:middle line:84% where's this port is now blocking to stop a Spanning Tree loop. 6786