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These are the user uploaded subtitles that are being translated: 1 00:00:00,000 --> 00:00:09,000 align:middle line:84% In this topology, I have 2 switches which are configured to run PVST not Rapid PVST+ 2 00:00:09,000 --> 00:00:13,000 align:middle line:84% but simply PVST I'll show you that config in a moment. 3 00:00:13,000 --> 00:00:15,000 align:middle line:84% Router 1 is connected to switch 1 4 00:00:15,000 --> 00:00:17,000 align:middle line:84% and router 2 is connected to switch 2 5 00:00:17,000 --> 00:00:24,000 align:middle line:84% and the routers are simply acting as edge devices or PCs in this topology. 6 00:00:24,000 --> 00:00:28,000 align:middle line:84% I’ve also got a hub connected to switch 1 and switch 2. 7 00:00:28,000 --> 00:00:36,000 align:middle line:84% Here’s switch 1, sh run | include span 8 00:00:36,000 --> 00:00:42,000 align:middle line:84% as you can see at the moment the switches configured for PVST 9 00:00:42,000 --> 00:00:46,000 align:middle line:84% I'll explain extended system IDs in more detail later 10 00:00:46,000 --> 00:00:49,000 align:middle line:84% but essentially it means that the priority of the switches 11 00:00:49,000 --> 00:00:53,000 align:middle line:84% based on the priority and VLAN number. 12 00:00:53,000 --> 00:00:55,000 align:middle line:84% So as an example, sh Spanning Tree 13 00:00:55,000 --> 00:01:02,000 align:middle line:84% this switch has a bridge ID consisting of the priority 32769 14 00:01:02,000 --> 00:01:07,000 align:middle line:84% which is the default of 32768+ the extended system ID of 1 15 00:01:07,000 --> 00:01:13,000 align:middle line:84% because we're looking at VLAN 1 and MAC address of the following. 16 00:01:13,000 --> 00:01:15,000 align:middle line:84% this switch is currently the root 17 00:01:15,000 --> 00:01:21,000 align:middle line:84% What I want you to see is that the Spanning Tree enabled here is IEEE 18 00:01:21,000 --> 00:01:25,000 align:middle line:84% so in this output, it looks like you're running 802.1D 19 00:01:25,000 --> 00:01:29,000 align:middle line:84% but actually, the switch is configured for Per-VLAN Spanning Tree. 20 00:01:29,000 --> 00:01:34,000 align:middle line:84% Per-VLAN Spanning Tree is compatible with 802.1D switches 21 00:01:34,000 --> 00:01:38,000 align:middle line:84% and therefore we can see IEEE in the output here. 22 00:01:38,000 --> 00:01:46,000 align:middle line:84% Here switch 2 sh run | include span 23 00:01:46,000 --> 00:01:55,000 align:middle line:84% switches configured to use PVST, extended system IDs are being used 24 00:01:55,000 --> 00:02:03,000 align:middle line:84% on this switch, the brIDge ID consist of the priority 32769 25 00:02:03,000 --> 00:02:11,000 align:middle line:84% which is 32768 the default + the VLAN number which is VLAN 1 in this example 26 00:02:11,000 --> 00:02:14,000 align:middle line:84% this is the MAC address of the switch. 27 00:02:14,000 --> 00:02:19,000 align:middle line:84% So we have 2 switches, one has this MAC address 28 00:02:19,000 --> 00:02:26,000 align:middle line:84% one has this MAC address, switch 1 has become the root of the Spanning Tree 29 00:02:26,000 --> 00:02:31,000 align:middle line:84% because it has a lower MAC address when compared to this switch. 30 00:02:31,000 --> 00:02:34,000 align:middle line:84% So because of the lower MAC address 31 00:02:34,000 --> 00:02:39,000 align:middle line:84% notice AC is lower than EA in hexadecimal 32 00:02:39,000 --> 00:02:42,000 align:middle line:84% switch 1 became the root of the Spanning Tree. 33 00:02:42,000 --> 00:02:45,000 align:middle line:84% What you’ll also notice is that on switch 1 34 00:02:45,000 --> 00:02:48,000 align:middle line:84% all ports are forwarding in the topology 35 00:02:48,000 --> 00:02:52,000 align:middle line:84% the ports that are currently connected are those ports 36 00:02:52,000 --> 00:02:59,000 align:middle line:84% and they're all forwarding on switch 2, however, port 1 37 00:02:59,000 --> 00:03:02,000 align:middle line:84% which is gigabit 0/0 is the root port 38 00:03:02,000 --> 00:03:08,000 align:middle line:84% and it's forwarding has path cost of 4, gigabit 0/1 is blocking 39 00:03:08,000 --> 00:03:12,000 align:middle line:84% or discarding to use the industry standard term. 40 00:03:12,000 --> 00:03:20,000 align:middle line:84% Port 2 is forwarding, port 3 is blocking. 41 00:03:20,000 --> 00:03:22,000 align:middle line:84% So this port is also blocking. 42 00:03:22,000 --> 00:03:26,000 align:middle line:84% Root switches forward on all ports. 43 00:03:26,000 --> 00:03:30,000 align:middle line:84% Before I show you how port status were determined 44 00:03:30,000 --> 00:03:32,000 align:middle line:84% let’s have a look at the BPDUs 45 00:03:32,000 --> 00:03:36,000 align:middle line:84% So I'll start capture on that link 46 00:03:36,000 --> 00:03:43,000 align:middle line:84% and what we can see here in Wireshark is a Spanning Tree BPDU. 47 00:03:43,000 --> 00:03:47,000 align:middle line:84% So it’s using 802.3 Ethernet frames 48 00:03:47,000 --> 00:03:52,000 align:middle line:84% notice the destination address is the well-known MAC address for Spanning Tree 49 00:03:52,000 --> 00:03:58,000 align:middle line:84% it is a multicast/broadcast address from this MAC address. 50 00:03:58,000 --> 00:04:00,000 align:middle line:84% Here’s switch 1 51 00:04:00,000 --> 00:04:10,000 align:middle line:84% notice the MAC address of the switch 00:11:c6:ac:dd: 52 00:04:10,000 --> 00:04:14,000 align:middle line:84% and we're currently looking at port 3 on the switch 53 00:04:14,000 --> 00:04:19,000 align:middle line:84% so notice dd00 but this is dd03 54 00:04:19,000 --> 00:04:24,000 align:middle line:84% because in Spanning Tree that’s the port that we're currently looking at. 55 00:04:24,000 --> 00:04:28,000 align:middle line:84% if we went and look at port 2 as an example 56 00:04:28,000 --> 00:04:35,000 align:middle line:84% notice the MAC address ends in 02 57 00:04:35,000 --> 00:04:40,000 align:middle line:84% we’ve got port 00 01 02 and 03 58 00:04:40,000 --> 00:04:46,000 align:middle line:84% so back in Wireshark here’s our capture sent out of port 3 59 00:04:46,000 --> 00:04:50,000 align:middle line:84% in Spanning Tree we can see the spanning 3 version 60 00:04:50,000 --> 00:04:53,000 align:middle line:84% so this is Spanning Tree 0 61 00:04:53,000 --> 00:04:56,000 align:middle line:84% because in this port its actually using 802.1D 62 00:04:56,000 --> 00:04:58,000 align:middle line:84% or the original version of Spanning Tree. 63 00:04:58,000 --> 00:05:04,000 align:middle line:84% The root identifier is 32768, there’s the VLAN number 64 00:05:04,000 --> 00:05:10,000 align:middle line:84% and there’s the MAC address of the switch, which we can see clearly here 65 00:05:10,000 --> 00:05:17,000 align:middle line:84% so notice root identifier, there’s the information of 32768 VLAN 1 66 00:05:17,000 --> 00:05:20,000 align:middle line:84% there is the MAC address of the switch. 67 00:05:20,000 --> 00:05:24,000 align:middle line:84% The root path cost is 0 because this switch is the root 68 00:05:24,000 --> 00:05:27,000 align:middle line:84% so there’s no cost to get to the root. 69 00:05:27,000 --> 00:05:33,000 align:middle line:84% There’s the port identifier and here are some timers used in Spanning Tree. 70 00:05:33,000 --> 00:05:38,000 align:middle line:84% Now when a switch boots up all ports are put into a blocking state 71 00:05:38,000 --> 00:05:44,000 align:middle line:84% they're then moved to other states based on timers in 802.1D 72 00:05:44,000 --> 00:05:49,000 align:middle line:84% When the Spanning Tree switch boots up all ports are put into the blocking state 73 00:05:49,000 --> 00:05:54,000 align:middle line:84% after 20 seconds called the max age timer ports moved 74 00:05:54,000 --> 00:05:56,000 align:middle line:84% to what’s called the listening state. 75 00:05:56,000 --> 00:06:00,000 align:middle line:84% if a switch is already up and you connect your cable to the port 76 00:06:00,000 --> 00:06:02,000 align:middle line:84% in other words, the links goes up 77 00:06:02,000 --> 00:06:04,000 align:middle line:84% it starts at the listening state 78 00:06:04,000 --> 00:06:08,000 align:middle line:84% ports will then move to the learning state 79 00:06:08,000 --> 00:06:13,000 align:middle line:84% based on the forward delay which is 15 seconds in duration 80 00:06:13,000 --> 00:06:19,000 align:middle line:84% and after 15 seconds ports transition from the learning state to the forwarding state. 81 00:06:19,000 --> 00:06:24,000 align:middle line:84% So an 802.1D or PVST it can take 50 seconds 82 00:06:24,000 --> 00:06:27,000 align:middle line:84% for ports to start forwarding on switches 83 00:06:27,000 --> 00:06:31,000 align:middle line:84% because they move from blocking to listening to learning to forwarding. 84 00:06:31,000 --> 00:06:34,000 align:middle line:84% Now in the listening state, they are sending BPDUs 85 00:06:34,000 --> 00:06:37,000 align:middle line:84% but not updating their MAC address tables. 86 00:06:37,000 --> 00:06:40,000 align:middle line:84% in the learning state, BPDUs are sent 87 00:06:40,000 --> 00:06:43,000 align:middle line:84% and the MAC address tables of switches are updated. 88 00:06:43,000 --> 00:06:47,000 align:middle line:84% So only if based on the Spanning Tree calculation 89 00:06:47,000 --> 00:06:49,000 align:middle line:84% it’s determined that a port can be opened 90 00:06:49,000 --> 00:06:54,000 align:middle line:84% a port is set to the forwarding state after 50 secs. 91 00:06:54,000 --> 00:06:58,000 align:middle line:84% when the switch comes up or typically if the switch is already up 92 00:06:58,000 --> 00:07:02,000 align:middle line:84% and you plug in a cable into that switch after 30 secs. 93 00:07:02,000 --> 00:07:04,000 align:middle line:84% The port will start forwarding. 94 00:07:04,000 --> 00:07:08,000 align:middle line:84% So in the BPDU we can see the max age timer 95 00:07:08,000 --> 00:07:11,000 align:middle line:84% and the forwarding delay timer. 96 00:07:11,000 --> 00:07:15,000 align:middle line:84% this is determined by the root bridge 97 00:07:15,000 --> 00:07:17,000 align:middle line:84% so on switch 1 which is the root bridge 98 00:07:17,000 --> 00:07:20,000 align:middle line:84% we can see that the hello timer 99 00:07:20,000 --> 00:07:24,000 align:middle line:84% in other words, BPDU hellos are sent out every 2 seconds 100 00:07:24,000 --> 00:07:27,000 align:middle line:84% the max age time is 20 seconds 101 00:07:27,000 --> 00:07:30,000 align:middle line:84% and the forwarding delay timer is 15 seconds 102 00:07:30,000 --> 00:07:35,000 align:middle line:84% and that’s what we see in the BPDUs as captured in our topology. 11722