1 00:00:08,880 --> 00:00:15,000 This is one of multiple LAN or local area network troubleshooting videos. 2 00:00:15,450 --> 00:00:22,230 In this video, we have four switches running in GNS3 switch 1 and switch 2 are configured 3 00:00:22,260 --> 00:00:28,990 as layer 3 switches, switch 3 and switch 4 are configured as layer 2 switches. 4 00:00:29,130 --> 00:00:32,170 So I could change the symbol as an example 5 00:00:35,510 --> 00:00:39,610 to indicate that that's a layer 2 switch. 6 00:00:39,630 --> 00:00:50,410 However, these four switches are running IOS VLAN 2 7 00:00:50,560 --> 00:00:56,150 and what I've done is simply disable IP routing on switch 3 and switch 4. 8 00:00:56,440 --> 00:01:05,220 So we have four switches in the topology and two IOS V routers, router 1 and router 2 acting as PCs. 9 00:01:05,260 --> 00:01:10,250 This is a very simple topology where we have a core and an access Layer. 10 00:01:10,540 --> 00:01:15,700 Typically you would connect the core to the access layer using cross-connects. 11 00:01:15,940 --> 00:01:18,590 I'm not gonna do that in this trouble shooting video. 12 00:01:18,610 --> 00:01:26,770 Have a look in my CCNA course which is linked below for a more complex topology running HSRP optimized 13 00:01:26,830 --> 00:01:33,460 spanning-tree redundancy between the core and the access layer and multiple other options that this 14 00:01:33,460 --> 00:01:37,400 network will suffice for our basic troubleshooting. 15 00:01:37,410 --> 00:01:44,670 So now let's assume that a new hire has made some configuration changes on the network and users are 16 00:01:44,670 --> 00:01:48,410 complaining about a network connectivity issues. 17 00:01:49,070 --> 00:01:58,130 You've been told that this user 10 121 in VLAN 2 is unable to ping this user that has a PC 18 00:01:58,130 --> 00:02:04,670 with IP address 10.1.3.2 in VLAN 3 you've been told that switch 3 19 00:02:04,670 --> 00:02:08,000 is connected to switch 1 as follows. 20 00:02:08,000 --> 00:02:14,920 Switch 1 is connected to switch 2 and switch 2 is connected to switch 4 but as Ronald Reagan 21 00:02:14,920 --> 00:02:17,960 said "Trust but verify" 22 00:02:18,370 --> 00:02:24,520 and what I like to say is don't trust anything that you've been told about a network until you've checked 23 00:02:24,520 --> 00:02:25,980 it yourself. 24 00:02:26,050 --> 00:02:37,300 So in our topology router 1 is acting as PC 1, show IP interface brief shows us the IP address on 25 00:02:37,510 --> 00:02:44,760 the gigabit interface that looks right, the IP address looks right, subnet mask looks right. 26 00:02:44,940 --> 00:02:58,490 Let's see if we can ping 10.1.3.2 which is the IP address of router 2 acting as PC 2. 27 00:02:58,540 --> 00:03:00,530 Okay so the ping is failing. 28 00:03:00,640 --> 00:03:03,370 We don't seem to be able to ping that device. 29 00:03:03,610 --> 00:03:05,240 So that piece of information is true. 30 00:03:05,350 --> 00:03:13,960 Show IP route shows us no routes in the routing table but it shows us that the default gateway is set 31 00:03:13,960 --> 00:03:22,020 to 10.1.2.254. 32 00:03:22,020 --> 00:03:29,660 Now that's because IP routing has been disabled on this router. In this topology, we've got router 1 acting as 33 00:03:29,660 --> 00:03:31,010 a PC. 34 00:03:31,010 --> 00:03:36,070 So when IP routing is disabled this is what the routing table looks like. 35 00:03:37,190 --> 00:03:41,610 The default gateway has been configured on the PC so that's good. 36 00:03:41,660 --> 00:03:45,720 Let's check if we can ping the default gateway, Yes we can. 37 00:03:46,660 --> 00:03:50,170 Now where is the default gateway. 38 00:03:50,230 --> 00:03:53,040 We could try and telnet to the default gateway 39 00:03:56,590 --> 00:04:00,470 and let's try and telnet to the right IP address. 40 00:04:00,520 --> 00:04:06,370 So the device is there but we're told that a password is required but none is set. 41 00:04:06,460 --> 00:04:11,980 So hopefully the default gateway and this typologies either switch one or switch to which all of the 42 00:04:12,100 --> 00:04:21,720 core layer 3 switches. So here's switch 1 show IP interface brief we can see that VLAN 1, VLAN 2 and VLAN 43 00:04:21,730 --> 00:04:24,440 3 have an IP address configure 44 00:04:24,550 --> 00:04:27,950 and that's the IP address on VLAN 2. 45 00:04:28,390 --> 00:04:32,950 So these should be the device that the PC is pinging. 46 00:04:33,040 --> 00:04:40,560 I'll do a debug IP ICMP on that switch and get the PC to ping the default gateway. 47 00:04:40,570 --> 00:04:45,640 Now you need to be careful with debugging in a real world environment. 48 00:04:45,740 --> 00:04:52,360 Here because we're studying for the CCNA we can simply enable any debug command because that'll help you 49 00:04:52,360 --> 00:04:54,680 learn the debugs that are available. 50 00:04:54,880 --> 00:05:02,570 But in the real world be careful with simply enabling a debug especially on a core device. 51 00:05:02,620 --> 00:05:09,680 You may get so much output on the console that you can't read anything or in a very bad situation 52 00:05:09,700 --> 00:05:13,260 the device may fall over so we don't want that. 53 00:05:13,700 --> 00:05:16,610 So in the real world be careful with debugging. 54 00:05:17,060 --> 00:05:24,100 Okay, so the pings do arrive on this default gateway so that's good. 55 00:05:25,280 --> 00:05:30,430 I'm gonna turn off IP domain look up in the lab here to make things happen quicker 56 00:05:31,190 --> 00:05:42,150 and then what we'll do is trace 10.1.3.2 which should be PC 2 in our topology. 57 00:05:42,220 --> 00:05:43,430 Let's see how far it gets. 58 00:05:43,450 --> 00:05:51,360 So it gets to the default gateway which we've now determined the switch 1. 59 00:05:51,360 --> 00:06:00,030 So what we should do is update the documentation so we could say that these are the IP addresses on 60 00:06:00,030 --> 00:06:03,740 the VLANs of that switch. 61 00:06:06,310 --> 00:06:13,300 That once again can be seen on switch 1 by using the show IP interface brief command 62 00:06:13,560 --> 00:06:15,060 and there they are once again. 63 00:06:19,030 --> 00:06:27,270 The trace route shows us that the traffic gets to the default gateway but then doesn't go anywhere else. 64 00:06:28,590 --> 00:06:34,360 Let's check if PC 2 can ping its default gateway. 65 00:06:34,460 --> 00:06:43,250 Now a test I like to do is to make sure that the local device can ping the other VLAN of the default 66 00:06:43,250 --> 00:06:44,240 gateway. 67 00:06:44,240 --> 00:06:56,020 So in other words this PC is in VLAN 2 it's in subnet 10 120 and I'm checking that it could ping 68 00:06:56,500 --> 00:07:02,200 the subnet of the device that we're trying to go to but this is the IP address of the default gateway in 69 00:07:02,220 --> 00:07:03,840 that VLAN. 70 00:07:03,850 --> 00:07:11,150 In other words, we've proven that inter VLAN routing is working on the layer 3 switch but now let's 71 00:07:11,150 --> 00:07:17,720 check the connectivity of PC 2, can it get to its default gateway? 72 00:07:17,780 --> 00:07:28,020 So this is router 2 acting as PC 2 show IP interface brief IP address looks right per the diagram subnet 73 00:07:28,020 --> 00:07:32,670 mask looks right now that information is once again not shown in the diagram. 74 00:07:32,800 --> 00:07:42,630 So we'd wanna add that somewhere and you may wanna add that on individual devices or make a note 75 00:07:42,630 --> 00:07:51,450 somewhere that the subnets have a /24 mask, show IP route shows us the default gateway of PC 2. 76 00:07:51,600 --> 00:07:57,050 So can it ping its default gateway? 77 00:07:57,090 --> 00:07:58,870 No, it can't. 78 00:07:58,930 --> 00:08:04,250 So there's something wrong either here or here 79 00:08:05,480 --> 00:08:15,390 or here. Let's have a look at switch 4, switch 4 is the local access switch, show IP interface brief. 80 00:08:15,480 --> 00:08:22,260 It has an IP address of 10 1114, can it ping switch 1 in VLAN 1? 81 00:08:22,290 --> 00:08:31,230 Yes, it can, show IP route default gateway is 10.1.1.254, can it ping 10. 82 00:08:31,230 --> 00:08:40,659 1.3.254? Yes it can, can it ping the PC in VLAN 3? 83 00:08:40,789 --> 00:08:42,870 No it can't. 84 00:08:42,880 --> 00:08:49,210 So this switch could ping the default gateway inter-VLAN routing is working on the default gateway but 85 00:08:49,240 --> 00:08:57,030 it can't ping this PC. Traffic from the switch to this PC you'd have to traverse to here to get to the default 86 00:08:57,030 --> 00:08:59,580 gateway and then it would have to come back again. 87 00:09:00,940 --> 00:09:09,460 Let's confirm the ports on switch 4, so firstly show IP interface brief, let's confirm that interfaces 88 00:09:09,460 --> 00:09:09,970 are up. 89 00:09:09,980 --> 00:09:14,360 All interfaces look up including gigabit 01 90 00:09:17,610 --> 00:09:21,340 that interface is up gigabit 00 is up. 91 00:09:21,490 --> 00:09:26,310 Which is the link to the core network pings did work previously so, 92 00:09:26,530 --> 00:09:30,930 so we have an indication that those interfaces are already up and working. 93 00:09:31,090 --> 00:09:38,400 Interface trunk is a trunk to the core on gigabit 0 that looks good. 94 00:09:38,400 --> 00:09:46,300 Let's have a look at the gigabit 01 interface, so show interface gigabit 01 switch port this interface 95 00:09:46,300 --> 00:09:51,430 should be in VLAN 3. Can you see the problem in this out 96 00:09:51,430 --> 00:09:54,770 put? Interface name is gigabit 97 00:09:54,770 --> 00:10:00,290 01 it's enabled us to switch port administer of mode is dynamic auto. 98 00:10:00,630 --> 00:10:08,820 It's currently acting as a static access port so dynamic trunk protocol or DTP did not negotiate to 99 00:10:08,820 --> 00:10:10,080 form a trunk. 100 00:10:10,170 --> 00:10:16,030 So DTP is on but there wasn't a switch to negotiate trunking with. 101 00:10:16,260 --> 00:10:27,190 Notice the problem access mode VLAN is 1, show run interface gigabit 01 this port is 102 00:10:27,200 --> 00:10:29,500 not in VLAN 3. 103 00:10:32,500 --> 00:10:33,290 Interface gigabit 104 00:10:33,330 --> 00:10:37,530 01 is currently in VLAN 1. 105 00:10:37,540 --> 00:10:49,040 So switch port access VLAN 3 show interface gigabit 01 switch port, port is now an access port in 106 00:10:49,040 --> 00:10:52,900 VLAN 3 show run interface gigabit 01, 107 00:10:52,920 --> 00:10:54,920 there's our configuration. 108 00:10:55,470 --> 00:11:02,400 Let's check if the local switch can ping that PC, at the moment it still can't. 109 00:11:02,400 --> 00:11:10,190 It may take a while for spanning tree and other protocols to converge. 110 00:11:10,370 --> 00:11:17,330 So you may just need to wait a bit before you assume that there's a problem but spanning tree at this point 111 00:11:17,330 --> 00:11:18,160 looks good. 112 00:11:18,260 --> 00:11:22,200 So show CDP neighbors. 113 00:11:22,370 --> 00:11:35,220 We can see that router 2 is connected to gigabit 01 using gigabit 00, show interface gigabit 114 00:11:35,220 --> 00:11:41,670 01 switch port confirms that this port is in VLAN 3. 115 00:11:41,750 --> 00:11:50,070 So let's go back onto the PC and check if it can ping its default gateway. 116 00:11:50,120 --> 00:11:51,120 It can now. 117 00:11:51,350 --> 00:11:57,380 So once again you may just have to wait a little bit before you assume that your change hasn't made any 118 00:11:57,380 --> 00:11:58,110 difference. 119 00:11:59,800 --> 00:12:05,690 Can it ping router 1 acting as PC 1? 120 00:12:05,740 --> 00:12:07,390 Yes, it can. 121 00:12:07,420 --> 00:12:14,460 So it looks like we've solved the problem. 122 00:12:14,490 --> 00:12:20,660 We could trace to the PC which didn't work before that now works. 123 00:12:20,680 --> 00:12:28,450 So previously when we traced to 10 132 it failed at the default gateway 124 00:12:28,570 --> 00:12:35,650 but now it's working and we could do a ping to prove it 125 00:12:38,370 --> 00:12:43,550 and do a debug on this side to prove that the traffic arrives. 126 00:12:43,750 --> 00:12:47,720 So that was an example of how to troubleshoot a local area network. 127 00:12:47,770 --> 00:12:53,880 It can sometimes be complicated to troubleshoot a layer 2 issues but remember to check your interfaces 128 00:12:53,890 --> 00:12:57,900 check your VLANs, check your encapsulations. In other videos 129 00:12:57,910 --> 00:13:01,240 I will show you all the problems that you may encounter. 130 00:13:01,660 --> 00:13:04,390 I need to troubleshoot in this topology. 131 00:13:04,720 --> 00:13:06,350 I hope you enjoyed this video. 132 00:13:06,340 --> 00:13:10,610 If you did please like it and please subscribe to my YouTube channel. 133 00:13:10,900 --> 00:13:12,310 I wish you all the very best.