1 00:00:10,450 --> 00:00:14,410 Is spanning tree important in switched networks? 2 00:00:14,430 --> 00:00:17,390 What happens when you disable spanning tree? 3 00:00:17,520 --> 00:00:23,880 Do you actually need spanning tree in a layer 2 Ethernet network? 4 00:00:23,880 --> 00:00:29,300 Okay, so let's see what happens at the moment on both of these switches 5 00:00:29,700 --> 00:00:39,810 A default configuration is being used so show spending tree shows us that spending tree is enabled on 6 00:00:39,810 --> 00:00:40,670 the VLAN 1. 7 00:00:44,260 --> 00:00:53,330 On switch 1 all ports of forwarding switch 1 is the root of the spanning tree, spanning tree is also 8 00:00:53,330 --> 00:01:04,700 running on switch to on VLAN one the switch is not the root switch interface gigabit. 9 00:01:04,730 --> 00:01:18,660 102 is blocking on this switch. So lets disable spanning tree conf t, no spanning tree VLAN 1. On 10 00:01:18,660 --> 00:01:28,630 this side no spanning tree VLAN 1, so on switch 1, show spanning tree shows us that spanning tree is 11 00:01:28,630 --> 00:01:37,650 disabled. On switch 2 show spanning tree shows us that spanning tree is disabled. Notice all ports 12 00:01:37,650 --> 00:01:46,250 are now showing green no ports are being blocked. Please note that I'm running in simulation mode 13 00:01:46,250 --> 00:01:58,620 in packet tracer and what I'm gonna do now is send a ping from PC 1 to PC2, PC 2s IP address 14 00:01:59,700 --> 00:02:09,490 is 10.1.1.2, the MAC address of PC 2 is this. 15 00:02:13,010 --> 00:02:14,150 On PC 1 16 00:02:17,210 --> 00:02:25,190 IP address is 10 1.1.1, MAC address is this. 17 00:02:25,420 --> 00:02:37,420 So what happens if we ping PC 2, we're sending an ICMP message but the PC doesn't know the MAC address 18 00:02:38,260 --> 00:02:49,080 of PC 2, so it's gonna send an ARP into the network which is a broadcast and it's gonna try and 19 00:02:49,080 --> 00:02:58,910 find out the MAC address of PC 2. I'm gonna click capture forward the ARP message is sent to the 20 00:02:58,910 --> 00:02:59,390 switch 21 00:03:02,420 --> 00:03:04,030 notice what happens. 22 00:03:04,020 --> 00:03:14,630 It's sent to switch 2, switch 2 however duplicates the packet and floods it out of all ports so 23 00:03:14,630 --> 00:03:26,950 it goes back to switch 1 on gigabit 101 and it's received by PC 2, so PC 2 is receiving 24 00:03:27,160 --> 00:03:28,480 this broadcast 25 00:03:29,750 --> 00:03:38,140 and now PC 1 is receiving the broadcast that it sent. Notice the source MAC address is 26 00:03:38,140 --> 00:03:48,520 PC 1 destination is broadcast it's looking for the MAC address of PC 2, so PC 1 will drop that 27 00:03:48,520 --> 00:03:49,660 packet. 28 00:03:49,660 --> 00:03:56,440 But notice we now have multiple packets being flooded through the network. 29 00:03:56,500 --> 00:04:06,760 PC 1 has received the ARP message once again so as PC 2. So PC 2 is receiving multiple ARP requests 30 00:04:06,880 --> 00:04:14,760 from the network, the switches are also duplicating packets when we look at the MAC address table 31 00:04:14,760 --> 00:04:15,660 of switch 2, 32 00:04:18,490 --> 00:04:20,240 we can see that 33 00:04:20,230 --> 00:04:28,590 PC 2 is found on gigabit 103 and PC 1 is found on gigabit 102. 34 00:04:31,360 --> 00:04:32,260 Capture forward, 35 00:04:37,070 --> 00:04:46,700 notice now the switch thinks that PC 2 is connected to gigabit 102 whereas in actual fact PC 2 36 00:04:46,700 --> 00:04:50,060 is connected to gigabit 103. 37 00:04:50,090 --> 00:05:00,610 Once again this is the MAC address of PC 2, so the switch is receiving conflicting information 38 00:05:00,640 --> 00:05:05,490 previously it thought that PC 2 is connected to this port. 39 00:05:05,590 --> 00:05:15,350 Now it thinks that PC 2 is connected to gigabit 101, so the switch previously thought that PC 40 00:05:15,440 --> 00:05:26,960 Now it thinks that PC 2 is connected to gigabit 101, so the switch previously thought that PC 41 00:05:26,960 --> 00:05:35,260 2 is connected to 103 which is correct then it's thought that PC 2 is connected to 102 and now it 42 00:05:35,260 --> 00:05:47,120 thinks that PC 2 is connected to 101, previously it's thought that PC 1 is connected to 102 then 43 00:05:47,180 --> 00:05:52,570 to 101 and now to 102. 44 00:05:53,660 --> 00:05:56,670 So the MAC address table is constantly being updated. 45 00:05:58,620 --> 00:06:03,390 This is how broadcast storms happen in live networks 46 00:06:06,160 --> 00:06:09,250 which can bring down an entire network. 47 00:06:09,310 --> 00:06:11,450 We have duplication of packets, 48 00:06:11,650 --> 00:06:21,500 we have mac address table instability, we have hosts receiving the packets that they sent out into the 49 00:06:21,500 --> 00:06:28,300 network, such as here PC 1 receiving its own ARP request message. 50 00:06:29,980 --> 00:06:38,530 Generally, we don't talk to ourselves and in the same way, a PC doesn't send a broadcast to itself like 51 00:06:38,530 --> 00:06:40,710 we see in this network. 52 00:06:40,900 --> 00:06:50,200 And if I continue doing that notice we constantly, we constantly have these ARP messages being duplicated 53 00:06:52,190 --> 00:06:58,470 and flooded through the network. Packet tracer isn't showing all the duplication here 54 00:06:59,580 --> 00:07:08,250 but notice this just continues on and on and on and can cause a broadcast storm and network meltdown 55 00:07:08,730 --> 00:07:11,620 in a real network. 56 00:07:12,060 --> 00:07:17,930 The same original packet is being duplicated multiple times. 57 00:07:17,940 --> 00:07:20,670 Notice this is stolen ARP message 58 00:07:20,670 --> 00:07:26,250 looking for the MAC address of PC 10.1.1.2 59 00:07:26,600 --> 00:07:35,430 and if I continue to capture forward we just see those messages being sent continuously by the switches 60 00:07:35,860 --> 00:07:38,280 throughout the network. 61 00:07:38,280 --> 00:07:40,740 So is spanning tree important? 62 00:07:40,740 --> 00:07:50,370 Definitely, should you disable spanning tree in a layer 2 network? In most cases no, layer 2 switch networks 63 00:07:50,910 --> 00:07:53,160 are a single broadcast domain. 64 00:07:53,310 --> 00:07:57,270 A broadcast gets flooded throughout a layer 2 domain. 65 00:07:57,420 --> 00:08:02,700 If you had a more complex network like this your broadcast storm would be even worse. 66 00:08:02,700 --> 00:08:10,300 Here's a very simple example of what happens when spanning tree is disabled, so I'll go back on to switch 67 00:08:10,300 --> 00:08:16,870 1 and I'll re enable spanning tree. 68 00:08:16,870 --> 00:08:23,140 I'll do something similar on switch 2 enable spanning tree. 69 00:08:23,610 --> 00:08:23,950 Notice 70 00:08:23,950 --> 00:08:29,740 now we see spanning tree messages being sent between the switches. 71 00:08:29,940 --> 00:08:34,010 That's how the switches learn about one another 72 00:08:34,010 --> 00:08:36,289 they are sending out BPDUs. 73 00:08:41,059 --> 00:08:46,250 So in packet tracer, we can see the actual BPDU messages.c 74 00:08:46,250 --> 00:08:54,470 I'll turn this back to real-time and what should happen now is the ports should transition to green 75 00:08:54,470 --> 00:08:56,690 once the switches have learnt about one another 76 00:08:59,900 --> 00:09:08,630 and decided to the root bridge is notice the ports are currently in the learning state, show spanning 77 00:09:08,630 --> 00:09:11,780 tree on switch 2 we see something similar. 78 00:09:11,780 --> 00:09:19,040 These ports are now forwarding, this port is blocking on switch 1 all ports have transitioned to the 79 00:09:19,040 --> 00:09:25,800 forwarding state so we can now see that this port is blocking in Packet Tracer. 80 00:09:26,060 --> 00:09:28,530 So let's do that ping again. 81 00:09:28,690 --> 00:09:38,180 We've got an ARP message being sent into the network that now gets sent to switch 2 across port gigabit 82 00:09:38,190 --> 00:09:39,270 101 83 00:09:42,900 --> 00:09:49,870 it's sent to PC 2 but notice this packet is gonna be dropped. 84 00:09:49,870 --> 00:09:58,420 The packet will not be forwarded out of port 102, spanning trees blocking that broadcast the packet 85 00:09:58,420 --> 00:10:00,850 is only forwarded to PC 2. 86 00:10:04,260 --> 00:10:16,970 We now have the ARP reply from PC 2 back to PC 1 so in the inbound PDU we can see target MAC address 87 00:10:16,980 --> 00:10:18,580 target IP address. 88 00:10:18,780 --> 00:10:21,490 Source MAC address is PC 2, 89 00:10:21,570 --> 00:10:31,760 this is an ARP reply message gets sent across the top link to switch 1 and gets sent to PC 1 and 90 00:10:31,760 --> 00:10:34,730 now PC 1 can send the ICMP message 91 00:10:37,590 --> 00:10:45,180 using the top link to PC 2 and the reply can be sent back 92 00:10:47,330 --> 00:10:48,790 to PC 1. 93 00:10:48,800 --> 00:10:52,750 That's what we wanna see in an ethernet network. 94 00:10:52,790 --> 00:10:59,530 We wanna have the devices communicating with each other. So hopefully this packet tracer demonstration 95 00:10:59,530 --> 00:11:08,980 has shown you how important spanning tree is in a layer 2 switched network even in this small topology 96 00:11:08,980 --> 00:11:15,730 the network breaks when spanning tree is disabled on both switches, make sure that you have a spanning 97 00:11:15,730 --> 00:11:23,440 tree enabled on layer 2 switch networks unless you have a very good reason to disable spanning tree 98 00:11:23,710 --> 00:11:24,610 on your switches.