WEBVTT 1 00:00.000 --> 00:05.400 When it comes to layer 2 forwarding, we are going to rely on MAC addresses. 2 00:05.400 --> 00:10.320 And specifically, we're going to look at a MAC address as it comes into our 3 00:10.320 --> 00:11.000 switch here. 4 00:11.000 --> 00:15.000 And we're going to pay attention to the destination MAC address. 5 00:15.000 --> 00:20.000 And the idea with a switch, especially when we compare it to layer 1 hubs, 6 00:20.000 --> 00:24.000 is that we want to intelligently forward it out a single interface. 7 00:24.000 --> 00:27.000 And so, yeah, we might have to flood a packet out if needed, 8 00:27.000 --> 00:32.000 but if we can send it out a single interface, then that would be ideal. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 8 00:27.000 --> 00:32.000 but if we can send it out a single interface, then that would be ideal. 9 00:32.000 --> 00:37.000 And so, what we're going to do is we're going to check the CAM table. 10 00:37.000 --> 00:43.100 And this CAM table is going to contain the very important MAC to interface m 11 00:43.100 --> 00:44.000 appings, 12 00:44.000 --> 00:46.000 meaning that I can check this destination MAC. 13 00:46.000 --> 00:50.000 And maybe it's something like 0.1.2. 14 00:50.000 --> 00:58.000 And it is an extremely abbreviated MAC address here, and this maps to gig 0/0. 15 00:58.000 --> 01:04.000 And so, because I have this mapping, I can now look at this destination MAC, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 15 00:58.000 --> 01:04.000 And so, because I have this mapping, I can now look at this destination MAC, 16 01:04.000 --> 01:07.000 say, okay, well, in order to get it to its destination, 17 01:07.000 --> 01:11.000 I'm going to forward it out gig 0/0 and send it over there. 18 01:11.000 --> 01:16.000 Naturally, if we don't have an entry, then we are still going to have to flood 19 01:16.000 --> 01:17.000 that frame 20 01:17.000 --> 01:22.000 out all of the interfaces in order to get it to its destination. 21 01:22.000 --> 01:25.000 And hopefully, at some point, we do learn the MAC address. 22 01:25.000 --> 01:28.460 Layer 2, from a control plane perspective, we don't dynamically share MAC 23 01:28.460 --> 01:30.000 addresses everywhere. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 23 01:28.460 --> 01:30.000 addresses everywhere. 24 01:30.000 --> 01:33.000 Instead, we use the concept of flood and learn. 25 01:33.000 --> 01:37.590 And again, we start with flooding, and we hope that we eventually learn the MAC 26 01:37.590 --> 01:38.000 address 27 01:38.000 --> 01:41.000 maybe when return traffic comes through. 28 01:41.000 --> 01:45.000 And so, that's probably the extent of our CCNA level studies 29 01:45.000 --> 01:48.000 when we were initially diving into the world of radigant switching. 30 01:48.000 --> 01:53.000 We probably stopped there understanding these two important concepts will get 31 01:53.000 --> 01:53.000 us a long way, 32 01:53.000 --> 01:56.000 as far as, again, that level of study. 33 01:56.000 --> 01:58.000 But now we're going to go a little bit deeper. 34 01:58.000 --> 02:01.000 We need to know, for example, what is this cam table? WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 34 01:58.000 --> 02:01.000 We need to know, for example, what is this cam table? 35 02:01.000 --> 02:04.000 What is cam and why are we using it here? 36 02:04.000 --> 02:09.110 In order to understand what content addressable memory is, which is what cam 37 02:09.110 --> 02:10.000 stands for, 38 02:10.000 --> 02:13.770 we need to start off by taking a look at what we might consider to be a little 39 02:13.770 --> 02:14.000 bit more of a 40 02:14.000 --> 02:16.000 normal memory situation. 41 02:16.000 --> 02:21.000 When we're looking at memory lookups, our standard DRAM type of lookups, 42 02:21.000 --> 02:27.280 or maybe running on a PC or a laptop, we might be using something like DDR 43 02:27.280 --> 02:28.000 memory. 44 02:28.000 --> 02:34.000 But either way, the concept of memory typically involves providing an index WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 44 02:28.000 --> 02:34.000 But either way, the concept of memory typically involves providing an index 45 02:34.000 --> 02:38.000 and exchanging an index for a value. 46 02:38.000 --> 02:43.080 What this essentially equates to is, you can almost think of it as forming a 47 02:43.080 --> 02:44.000 spreadsheet, 48 02:44.000 --> 02:50.000 where we have a bunch of cells of information, and in each one of these cells, 49 02:50.000 --> 02:53.000 we have data. 50 02:53.000 --> 02:58.000 And that data is stored with a reference ID here that is the index. 51 02:58.000 --> 03:01.000 So it's data at a specific index. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 51 02:58.000 --> 03:01.000 So it's data at a specific index. 52 03:01.000 --> 03:05.760 So every one of these cells, if we want to keep calling them, that is going to 53 03:05.760 --> 03:06.000 have an index. 54 03:06.000 --> 03:10.000 So again, think of it from a spreadsheet perspective, it would be like saying, 55 03:10.000 --> 03:12.000 "Hey, what's at A1?" 56 03:12.000 --> 03:17.490 And we look at A1, we've provided the index, and we're going to get some value 57 03:17.490 --> 03:19.000 out of that. 58 03:19.000 --> 03:21.000 That is how a lot of our memory works. 59 03:21.000 --> 03:23.000 It's how PC store information in memory. 60 03:23.000 --> 03:28.000 We're taking data, we're putting it into a spot in the memory table. 61 03:28.000 --> 03:32.300 We're going to remember what that index is, or released report back to the WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 61 03:28.000 --> 03:32.300 We're going to remember what that index is, or released report back to the 62 03:32.300 --> 03:33.000 application 63 03:33.000 --> 03:38.000 that's storing that data, the index, and then this is how we come back and say, 64 03:38.000 --> 03:41.000 "Okay, I need that value back computer CPU. 65 03:41.000 --> 03:44.000 Please give it to me. Here's the index." 66 03:44.000 --> 03:48.690 So that's how standard memory works, and it's important to recognize that it's 67 03:48.690 --> 03:50.000 based on the idea 68 03:50.000 --> 03:53.000 that I know where the value is. 69 03:53.000 --> 03:56.000 And because of that, the index must be known. 70 03:56.000 --> 04:01.450 So now let's talk about a different type of memory, which is that content-add WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 70 03:56.000 --> 04:01.450 So now let's talk about a different type of memory, which is that content-add 71 04:01.450 --> 04:02.000 ressable memory. 72 04:02.000 --> 04:09.000 So content-addressable memory, or CAM, is still going to store information, 73 04:09.000 --> 04:12.000 but it's going to be referenced in a different way. 74 04:12.000 --> 04:17.000 We are going to reference it by content rather than by index. 75 04:17.000 --> 04:22.000 So the idea here is rather than performing this operation, I provide an index 76 04:22.000 --> 04:23.000 and get a value, 77 04:23.000 --> 04:30.000 I'm going to provide content up front, and I'm still going to get a value back, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 77 04:23.000 --> 04:30.000 I'm going to provide content up front, and I'm still going to get a value back, 78 04:30.000 --> 04:33.000 but in this case it's going to be a different value. 79 04:33.000 --> 04:38.000 We can probably already see how this ties well into this paradigm here, 80 04:38.000 --> 04:42.540 where I provided a MAC address that would be the content, and I get a value 81 04:42.540 --> 04:43.000 back, 82 04:43.000 --> 04:45.000 I'm getting an interface back. 83 04:45.000 --> 04:47.000 And that's exactly what it is. 84 04:47.000 --> 04:51.000 We're providing something up front and getting a different value back. 85 04:51.000 --> 04:56.000 So in the world of programming, this is sort of like a two-dimensional array. 86 04:56.000 --> 05:01.530 We've ever used that, or the concept of key value pairs, which we might WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 86 04:56.000 --> 05:01.530 We've ever used that, or the concept of key value pairs, which we might 87 05:01.530 --> 05:03.000 experience a little bit more, 88 05:03.000 --> 05:07.000 especially as we continue to go through Encore, and we think about scripting, 89 05:07.000 --> 05:11.000 such as with JSON dictionaries, for example, these kinds of things. 90 05:11.000 --> 05:14.000 We don't have experience with programming, that's perfectly fine, 91 05:14.000 --> 05:19.150 but this is the same concept where I'm not necessarily putting things in a 92 05:19.150 --> 05:20.000 specific order. 93 05:20.000 --> 05:25.870 There is no order to the information that's stored inside of CAM, whereas up 94 05:25.870 --> 05:27.000 here we do have an order. 95 05:27.000 --> 05:33.000 We do have index values assigned to all of these different data points, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 95 05:27.000 --> 05:33.000 We do have index values assigned to all of these different data points, 96 05:33.000 --> 05:38.000 and so it's structured in a way that I can provide an index and get value back, 97 05:38.000 --> 05:41.000 whereas with CAM that's not really what we're doing here. 98 05:41.000 --> 05:43.000 So we're providing content. 99 05:43.000 --> 05:48.060 This content has a few limitations to what we can provide and what we're trying 100 05:48.060 --> 05:49.000 to match against. 101 05:49.000 --> 05:56.030 First of all, this is a binary operation, meaning that I can only match zeros 102 05:56.030 --> 05:58.000 and ones. 103 05:58.000 --> 06:03.000 Well, Jeff, what else do we match in the world of networking and with computers WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 103 05:58.000 --> 06:03.000 Well, Jeff, what else do we match in the world of networking and with computers 104 06:03.000 --> 06:03.000 ? 105 06:03.000 --> 06:05.000 Isn't it always zeros and ones? 106 06:05.000 --> 06:08.440 I don't hold that thought because we're going to find that there is a non- 107 06:08.440 --> 06:10.000 binary content addressable memory 108 06:10.000 --> 06:15.250 called a ternary CAM, or a T-cam, coming up, and we'll take a look at that 109 06:15.250 --> 06:16.000 momentarily. 110 06:16.000 --> 06:20.630 So from just staying within the context of CAM, though, we're matching zeros or 111 06:20.630 --> 06:21.000 ones, 112 06:21.000 --> 06:27.000 and it must be an exact match. 113 06:27.000 --> 06:31.000 In other words, we're not using the concept of masks here. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 113 06:27.000 --> 06:31.000 In other words, we're not using the concept of masks here. 114 06:31.000 --> 06:36.000 Ever noticed that MAC addresses, and we don't use masks with that? 115 06:36.000 --> 06:37.000 We don't have that concept. 116 06:37.000 --> 06:41.000 We're not searching against multiple MAC addresses at a time. 117 06:41.000 --> 06:43.000 Instead, we're looking for an exact match. 118 06:43.000 --> 06:49.560 I have an exact destination Mac showing up on the switch, and yeah, we're not 119 06:49.560 --> 06:51.000 really organizing our networks 120 06:51.000 --> 06:53.000 by MAC address subnets. 121 06:53.000 --> 06:57.110 If we're going to think of it that way instead, this specific MAC address is 122 06:57.110 --> 06:58.000 out this interface 123 06:58.000 --> 07:02.490 and this specific MAC address is out this interface, and that works well from a WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 123 06:58.000 --> 07:02.490 and this specific MAC address is out this interface, and that works well from a 124 07:02.490 --> 07:04.000 layer 2 perspective 125 07:04.000 --> 07:07.700 because scaling-wise, we're not usually talking about more than a few hundred 126 07:07.700 --> 07:10.000 hosts in a given broadcast domain. 127 07:10.000 --> 07:13.000 But think about it from a network perspective. 128 07:13.000 --> 07:20.020 If I had to have exact entries in my routing table, for every single IP address 129 07:20.020 --> 07:21.000 that's out there 130 07:21.000 --> 07:22.000 wouldn't work very well. 131 07:22.000 --> 07:26.000 And so that's where layer 2 and layer 3 are structured very different. 132 07:26.000 --> 07:32.520 And once again, why content addressable memory really does fit the layer 2 WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 132 07:26.000 --> 07:32.520 And once again, why content addressable memory really does fit the layer 2 133 07:32.520 --> 07:34.000 paradigm. 134 07:34.000 --> 07:39.490 So from a switch operation perspective, we think about how exactly we're going 135 07:39.490 --> 07:43.000 to use CAM and perform layer 2 lookups. 136 07:43.000 --> 07:45.000 And there's a few things to keep in mind. 137 07:45.000 --> 07:49.280 So first of all, once again, our content here is going to be MAC addresses, so 138 07:49.280 --> 07:54.000 we know that we map MAC addresses to interfaces. 139 07:54.000 --> 07:57.480 And so when I want to look up what to do with a packet, I submit the MAC 140 07:57.480 --> 08:01.000 address and I get the interface back. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 140 07:57.480 --> 08:01.000 address and I get the interface back. 141 08:01.000 --> 08:06.010 Once again, content is the MAC address and the interface is going to be the 142 08:06.010 --> 08:08.000 value that gets returned. 143 08:08.000 --> 08:15.000 Furthermore, we're going to create a table like this per VLAN. 144 08:15.000 --> 08:21.040 Now this is important because if we think about it, we just said that this is a 145 08:21.040 --> 08:23.000 binary match, it must be exact, 146 08:23.000 --> 08:27.000 and I can only submit my content and get a single value back. 147 08:27.000 --> 08:32.080 Meaning I can't submit my content and get a VLAN ID back, nor can I submit WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 147 08:27.000 --> 08:32.080 Meaning I can't submit my content and get a VLAN ID back, nor can I submit 148 08:32.080 --> 08:35.000 content with a VLAN ID to get the value back, 149 08:35.000 --> 08:37.000 which is most likely how that would work. 150 08:37.000 --> 08:42.000 Instead, we will just simply have a different table per VLAN. 151 08:42.000 --> 08:45.000 It isn't a third component. 152 08:45.000 --> 08:48.000 Now next, remember how we learn MAC addresses. 153 08:48.000 --> 08:52.000 Remember, we are going to perform MAC address learning. 154 08:52.000 --> 08:58.740 So learning MACs, when we see those MAC addresses show up, that is how we're 155 08:58.740 --> 09:02.000 going to populate the CAM table. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 155 08:58.740 --> 09:02.000 going to populate the CAM table. 156 09:02.000 --> 09:10.000 And lastly, if we have no entry, then this is going to result in flooding. 157 09:10.000 --> 09:14.000 So some of this is going to be review just from a CCNA perspective. 158 09:14.000 --> 09:17.780 We should understand the basic operation of layer two switch, but as we kind of 159 09:17.780 --> 09:19.000 tie all of this together, 160 09:19.000 --> 09:23.000 and now understand a little bit better of what CAM is and why we use it, 161 09:23.000 --> 09:27.800 and how exactly it works well for layer two, but might not work well for layer 162 09:27.800 --> 09:29.000 three lookups and such, 163 09:29.000 --> 09:32.000 and now we start to see all of this come together. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 163 09:29.000 --> 09:32.000 and now we start to see all of this come together.