1 00:00:08,960 --> 00:00:14,550 This is one of multiple Ethernet troubleshooting videos. In this topology 2 00:00:14,560 --> 00:00:20,870 we've been told that router 1 on the top left is not able to ping some of the other routers in the 3 00:00:20,870 --> 00:00:29,840 topology. So let's verify that on router 1 show IP interface brief, IP address of router 1 is 10 4 00:00:29,840 --> 00:00:40,750 .1.1.1 router 2s IP address is 10.1.1.2. 5 00:00:40,790 --> 00:00:50,640 So, from router 1 can we ping router 2? pings are failing. Let's confirm the IP address information 6 00:00:50,650 --> 00:00:58,340 so show IP interface gigabit 00 IP address is 10 112/24. 7 00:00:58,760 --> 00:00:59,870 Let's do the same over here 8 00:00:59,870 --> 00:01:09,950 make sure that the subnet mask is correct, IP address is 10 111/24 this side it's 10 112/ 9 00:01:09,950 --> 00:01:10,790 24. 10 00:01:11,030 --> 00:01:12,620 So that looks good, 11 00:01:13,640 --> 00:01:19,620 doesn't look like they'll be any other reason why we can't ping to router 2. 12 00:01:20,210 --> 00:01:23,160 What we'll do on router 1 is do a debug IP packet. 13 00:01:23,330 --> 00:01:27,310 Be careful doing these commands in the real world. 14 00:01:27,920 --> 00:01:31,700 You may have to use an access list on that debug. 15 00:01:32,030 --> 00:01:37,100 Otherwise, you could kill your router with the amount of output that's generated. 16 00:01:37,100 --> 00:01:42,100 Okay, so we see messages such as sending full packet. 17 00:01:42,530 --> 00:01:51,680 So it looks like router 1 is forwarding the packet out of the ethernet interface onto the network, debug IP 18 00:01:51,680 --> 00:01:52,980 packet on this side. 19 00:01:54,020 --> 00:02:03,210 What I'll do now is repeat one ping only so only send one packet onto the network. 20 00:02:03,250 --> 00:02:07,600 It doesn't look like anything's arriving on this side. 21 00:02:07,660 --> 00:02:14,980 If you can use Wireshark to help you capture traffic. So I'm gonna capture traffic between the switch 22 00:02:15,040 --> 00:02:23,740 and router 2 and I'll do that ping again 23 00:02:23,940 --> 00:02:29,910 but I don't see any ICMP traffic in Wireshark. 24 00:02:32,840 --> 00:02:39,670 We see spanning-tree information but we don't see any other information in Wireshark we see for instance 25 00:02:39,670 --> 00:02:43,310 a loop from a device with a MAC address 26 00:02:43,330 --> 00:02:47,550 we see some spanning tree we don't see any other traffic. 27 00:02:52,410 --> 00:02:55,800 So let's see if the traffic is actually getting to the switch. 28 00:02:57,720 --> 00:02:59,360 I'll do that ping again. 29 00:02:59,710 --> 00:03:11,070 Okay, so there's the ICMP message sent from router 1 to router 2 the fact that we see ICMP means that 30 00:03:11,070 --> 00:03:17,940 the router has learnt the MAC address of router 2, source MAC address is zeros ending in one destination 31 00:03:17,940 --> 00:03:21,830 MAC address is zeros ending in a two. 32 00:03:21,850 --> 00:03:23,710 So it looks like ARP worked 33 00:03:23,800 --> 00:03:25,920 but the pings are failing. 34 00:03:26,200 --> 00:03:35,560 Let's confirm the MAC address on router 1 MAC address is set to this, burnt in MAC address is this but 35 00:03:35,600 --> 00:03:38,670 the MAC address has been changed to that value. 36 00:03:39,200 --> 00:03:46,650 What about on router 2? MAC address is this burnt-in MAC address is that. 37 00:03:46,900 --> 00:03:53,840 So it looks like the traffic from router 1 to router 2 gets sent on this link but doesn't arrive at 38 00:03:53,870 --> 00:03:57,260 router 2. What I'll do is do a broadcast 39 00:03:57,290 --> 00:04:03,240 again you need to be careful with doing this kind of stuff in the real world but 40 00:04:05,840 --> 00:04:08,960 because this is a lab I'm gonna show you various options. 41 00:04:09,230 --> 00:04:14,340 So notice when I send a broadcast traffic it does arrive at router 2. 42 00:04:14,450 --> 00:04:22,490 So let me un all that and only do a debug IP ICMP not a debug packet because that generates a lot of traffic. 43 00:04:22,490 --> 00:04:27,010 I'll un all over here. 44 00:04:28,560 --> 00:04:33,730 In other words, turn off all debugging and only send one broadcast. 45 00:04:34,140 --> 00:04:44,280 So the broadcast gets to router 2, so here we got a reply to the broadcast it looks like but it doesn't 46 00:04:44,280 --> 00:04:48,160 look like unicast traffic works in the network. 47 00:04:49,620 --> 00:04:54,500 So broadcast is sent it gets to router 2 48 00:04:54,900 --> 00:05:00,050 but a unicast doesn't seem to get to router 2. 49 00:05:00,420 --> 00:05:03,780 So let's have a look at that switch and see if there's a problem on the switch. 50 00:05:05,920 --> 00:05:09,900 So on the switch show MAC address table. 51 00:05:10,280 --> 00:05:10,610 Okay, 52 00:05:10,610 --> 00:05:19,060 Can you see a problem in the output? What looks wrong in the output? 53 00:05:19,120 --> 00:05:25,760 Notice here we've got a static MAC address for MAC address 2 going out of gigabit 54 00:05:25,800 --> 00:05:27,670 2 and gigabit 3 55 00:05:27,970 --> 00:05:34,680 but switch 2, show interface gigabit 56 00:05:34,770 --> 00:05:40,660 0 shows us that has a MAC address of two. 57 00:05:41,070 --> 00:05:49,310 So this static MAC address is a problem show run pipe include MAC. 58 00:05:49,470 --> 00:05:53,400 We've got this entry in the running-config which shouldn't be there. 59 00:05:53,420 --> 00:05:56,190 So no, I'll paste that in. 60 00:05:57,800 --> 00:06:03,120 So no MAC address show MAC Address table. 61 00:06:03,140 --> 00:06:09,900 Now we only have dynamic MAC addresses. I'll do a ping from router 1 to router 2 that succeeds 62 00:06:09,910 --> 00:06:13,850 now because the MAC address has been learnt properly. 63 00:06:13,850 --> 00:06:20,120 Be careful with static MAC addresses in your MAC address table that will override your dynamic entries 64 00:06:20,480 --> 00:06:24,500 and could send traffic to the wrong port in your topology. 65 00:06:25,330 --> 00:06:31,400 Okay, so can we ping router 3 which has an IP address of 10 113? 66 00:06:31,480 --> 00:06:38,680 No we can't, show IP interface gigabit 00. 67 00:06:38,780 --> 00:06:48,610 So this is the IP address of router 3 debug IP ICMP. 68 00:06:48,710 --> 00:06:51,200 Let's do a ping again. 69 00:06:52,170 --> 00:06:55,180 It doesn't look like the traffic's arriving at router 3. 70 00:06:55,350 --> 00:06:57,470 Let's have a look at the MAC address table. 71 00:06:57,990 --> 00:07:07,710 I don't see a MAC address for router 3, let's ping router 1 from router 3. 72 00:07:07,740 --> 00:07:11,180 Look at this output and see if you see a problem. 73 00:07:13,340 --> 00:07:20,060 So show MAC address table, showed us previously that the MAC address ending in one was connected to Gigabit 74 00:07:20,070 --> 00:07:22,490 00 on the switch. 75 00:07:22,490 --> 00:07:31,370 In other words over here but now it looks like that MAC address is on gigabit 02 this port here. 76 00:07:31,500 --> 00:07:36,160 So I'll do a ping from router 1 again and let's have a look at the MAC address table. 77 00:07:36,180 --> 00:07:45,570 Notice that MAC address is now shown on gigabit 00, ping is finished so let's do a ping from this side. 78 00:07:45,570 --> 00:07:48,580 Notice the MAC address has moved to gigabit 79 00:07:48,580 --> 00:07:50,000 02. 80 00:07:50,370 --> 00:07:53,980 So we're getting MAC addresses flipping on the switch. 81 00:07:54,390 --> 00:07:57,570 Let's confirm what the MAC addresses are on these routers. 82 00:07:57,900 --> 00:08:00,320 So show interface gigabit 00. 83 00:08:00,690 --> 00:08:04,040 That's the MAC address of router 1. 84 00:08:04,080 --> 00:08:05,810 What does it look like on router 3? 85 00:08:05,850 --> 00:08:08,260 So show interface 86 00:08:08,260 --> 00:08:14,130 gigabit 00 this is the routers interface. 87 00:08:14,160 --> 00:08:15,530 Notice the MAC address. 88 00:08:15,780 --> 00:08:18,820 The routers have duplicate MAC addresses. 89 00:08:18,900 --> 00:08:28,280 That's not gonna work in an Ethernet environment, so let's configure the MAC address as follows. 90 00:08:29,100 --> 00:08:40,419 And let's see if router 1 can ping router 3 now, notice the ping succeeds and that's because the 91 00:08:40,419 --> 00:08:45,360 devices have different MAC addresses now available on different ports. 92 00:08:45,430 --> 00:08:52,930 Which is correct per our diagram ethernet breaks if you have the same MAC addresses on the same layer 93 00:08:52,960 --> 00:08:57,450 2 segment. MAC addresses are meant to be unique 94 00:08:57,640 --> 00:09:04,150 but in the real world, you can encounter duplicate MAC addresses. Check your MAC address tables to see 95 00:09:04,150 --> 00:09:07,630 if MAC addresses are flipping from one port to another. 96 00:09:07,630 --> 00:09:09,090 Hope you enjoy this video. 97 00:09:09,220 --> 00:09:15,090 If it's been of benefit to you please like it and please subscribe to my YouTube channel. 98 00:09:15,100 --> 00:09:16,780 I wish you all the very best.