1 00:00:11,240 --> 00:00:16,410 Okay, for this question-and-answer vlog I got a question from Remus. 2 00:00:16,860 --> 00:00:23,370 He asks, What is the use of giving a default gateway to a switch using the IP default gateway command? 3 00:00:33,570 --> 00:00:42,980 Okay Remus, here's the answer to the question. In this GNS3 topology I've got a router connected to a layer 3 switch which in turn is connected 4 00:00:43,010 --> 00:00:47,650 to a layer 2 switch, a layer 3 switch has IP routing enabled. 5 00:00:47,660 --> 00:00:53,870 In other words, it will route from VLAN 1 to VLAN 2 and back again, switch 2 is a layer 2 switch. 6 00:00:53,870 --> 00:00:56,560 In other words, it doesn't have IP routing enabled. 7 00:00:56,650 --> 00:01:06,920 So for the CCNA, this could be a 2950 switch or 2960 and this could be a 3750 or 3550 as 8 00:01:06,920 --> 00:01:08,140 an example. 9 00:01:08,150 --> 00:01:15,130 So on router 1, is router 1 able to ping switch 2? Answer is 10 00:01:15,130 --> 00:01:20,010 no, the router is not able to ping switch 2. 11 00:01:20,170 --> 00:01:22,920 What about trace and to speed things up I'm gonna type 12 00:01:22,930 --> 00:01:24,110 no IP domain 13 00:01:25,430 --> 00:01:31,500 look-up. So traceroute to 10.1.1.1 14 00:01:31,590 --> 00:01:39,540 Okay so the traffic gets to switch 1 so it goes from router 1 to switch 1 10.1.2.2 15 00:01:39,540 --> 00:01:40,300 54 16 00:01:40,530 --> 00:01:48,990 and then the trace fails. So can i ping 10.1.254? 17 00:01:49,030 --> 00:01:52,150 Yes, I can and I would have known that from the trace 18 00:01:52,510 --> 00:01:55,310 but what I like to do is check the following. 19 00:01:55,330 --> 00:01:59,180 Can router 1 ping this IP address on the other side 20 00:01:59,360 --> 00:02:00,470 on switch 1. 21 00:02:00,850 --> 00:02:01,790 Yes, it can. 22 00:02:02,080 --> 00:02:05,000 So that means that the problem is not here. 23 00:02:05,050 --> 00:02:13,210 The problem is somewhere on this side. So on switch, 1 can we ping switch 2? 24 00:02:13,300 --> 00:02:14,690 The answer is yes. 25 00:02:14,950 --> 00:02:22,930 On switch 2 can it ping switch 1 on VLAN 1? 26 00:02:22,990 --> 00:02:24,040 Yes it can. 27 00:02:24,280 --> 00:02:33,970 But can it ping across switch 1? Remember 1 was able to ping this IP address in VLAN 1. So 28 00:02:34,020 --> 00:02:39,800 switch 1 is doing inter VLAN routing but can switch 2 ping switch 1 IP address in VLAN 2? 29 00:02:40,150 --> 00:02:47,690 and it looks like it can't this is a very typical scenario where 30 00:02:47,800 --> 00:02:54,890 you can't ping or telnet to a switch in a remote subnet. 31 00:02:54,920 --> 00:03:01,050 Now on the switch when we use the command show IP route notice no default gateway has been set. 32 00:03:01,080 --> 00:03:04,420 This means that IP writing has been disabled on the switch. 33 00:03:04,770 --> 00:03:13,840 So what I need to do is type IP default gateway and point the switch to its default gateway. So show 34 00:03:13,840 --> 00:03:14,690 IP route 35 00:03:15,460 --> 00:03:20,920 notice default gateway set I'll do that command again there's the default gateway. 36 00:03:20,920 --> 00:03:26,540 So on router 1 can we ping switch 2? Answer is yes. 37 00:03:26,560 --> 00:03:30,740 So you need to configure a default gateway on a layer 2 switch. 38 00:03:30,760 --> 00:03:36,580 In other words, a switch that hasn't got IP routing enabled to allow that switch to send traffic to remote 39 00:03:36,700 --> 00:03:43,900 subnets. Traffic from router 1 could get to switch 2 but won't be able to get back again if switch 40 00:03:43,900 --> 00:03:48,140 2 doesn't have a default gateway configured. 41 00:03:48,150 --> 00:03:53,680 Now, this can get confusing if the switch is a Layer 3 switch or supports Layer 3 functionality. 42 00:03:53,790 --> 00:04:01,080 Notice at the moment once again IP routing is disabled because we don't see a routing table here. 43 00:04:01,200 --> 00:04:06,520 Show run pipe include IP route 44 00:04:06,630 --> 00:04:07,500 Notice this command 45 00:04:07,520 --> 00:04:11,460 no IP routing, IP routing was disabled on this switch. 46 00:04:11,750 --> 00:04:13,960 That's a default on all the switches. 47 00:04:13,970 --> 00:04:19,200 So IP routing, let's enable that do the show IP route 48 00:04:19,200 --> 00:04:22,830 command again notice the output is very different. 49 00:04:22,830 --> 00:04:26,880 Notice also that the gateway of last resort is not set. 50 00:04:26,890 --> 00:04:38,600 So can router 1 ping switch 2? Answer is no, show run pipe include default. 51 00:04:38,830 --> 00:04:46,560 We still have the IP default gateway command configured on the switch but because IP routing is enabled 52 00:04:46,880 --> 00:04:49,050 that command is ignored. 53 00:04:49,050 --> 00:04:56,320 So you only use the IP default gateway command when IP routing is disabled. So let's do that again, 54 00:04:56,490 --> 00:04:58,110 No IP routing 55 00:05:01,160 --> 00:05:03,350 pings succeeds, IP routing 56 00:05:06,120 --> 00:05:07,700 ping fail. 57 00:05:08,010 --> 00:05:11,490 So an IP routing is enabled like this. 58 00:05:11,550 --> 00:05:19,750 What you need to do is you need to create a default route to your default gateway. So something like that 59 00:05:21,110 --> 00:05:24,260 and now routing works. 60 00:05:24,260 --> 00:05:31,360 So here's the thing to remember if IP routing is enabled on a switch you must use a static default route 61 00:05:31,520 --> 00:05:36,080 like this or run a routing protocol. If IP routing is disabled 62 00:05:36,080 --> 00:05:42,880 He used the IP default gateway command to allow a switch to send traffic to a remote subnet. Remus 63 00:05:42,890 --> 00:05:46,670 I'm hoping that answers your question and I hope that helps everyone else. 64 00:05:46,670 --> 00:05:49,400 Don't forget to ask questions in the comments below. 65 00:05:49,790 --> 00:05:53,380 Please also look at the video and please subscribe. 66 00:05:53,690 --> 00:05:55,520 I want to wish you all the very best.