1 00:00:00,000 --> 00:00:05,000 In summary hubs reside at the physical layer of the OSI model. 2 00:00:05,000 --> 00:00:07,000 They are not intelligent. 3 00:00:07,000 --> 00:00:10,000 They do not understand the frames that they are repeating 4 00:00:10,000 --> 00:00:15,000 they simply amplify the signal that they receive out of all other ports 5 00:00:15,000 --> 00:00:18,000 except the ports on which it was received. 6 00:00:18,000 --> 00:00:22,000 Hubs were good for their time, but in general today have been replaced by switches. 7 00:00:22,000 --> 00:00:25,000 Now there's always an exception to the rule. 8 00:00:25,000 --> 00:00:31,000 Wireless networks act like hubs, if you have a 54 Mbps wireless network 9 00:00:31,000 --> 00:00:39,000 be careful, it’s not 54 Mbps dedicated or if you have a 200 Mbps wireless network 10 00:00:39,000 --> 00:00:44,000 be careful it’s shared between all devices on the wireless network 11 00:00:44,000 --> 00:00:49,000 So you need to divide the speed of your wireless network 12 00:00:49,000 --> 00:00:52,000 by the devices connected to a access point. 13 00:00:52,000 --> 00:00:57,000 Wireless network also have other issues which reduce the throughput even more 14 00:00:57,000 --> 00:01:02,000 but the moral of the story is wireless networks operate as hubs. 15 00:01:02,000 --> 00:01:05,000 Hubs are shared devices that would good for their time 16 00:01:05,000 --> 00:01:08,000 but are very slow when compared to today's switches. 17 00:01:08,000 --> 00:01:11,000 So overtime hubs were replaced by bridges 18 00:01:11,000 --> 00:01:15,000 and bridges in turn have been receives by switches. 19 00:01:15,000 --> 00:01:21,000 A bridge is a layer 2 device, in other words it resides at the data link layer 20 00:01:21,000 --> 00:01:25,000 of the OSI model, bridges are more intelligent than hubs. 21 00:01:25,000 --> 00:01:29,000 They use something called a MAC address table 22 00:01:29,000 --> 00:01:32,000 to learn where devices are in the topology. 23 00:01:32,000 --> 00:01:36,000 So rather than simply just repeating the signal 24 00:01:36,000 --> 00:01:40,000 and sending traffic out of all ports without understanding it. 25 00:01:40,000 --> 00:01:46,000 Bridges maintain a table with a list of MAC addresses learned in this topology. 26 00:01:46,000 --> 00:01:50,000 So in our sample network we have 4 devices A, B, C and D 27 00:01:50,000 --> 00:01:53,000 and the hub is being replaced by a bridge. 28 00:01:53,000 --> 00:01:56,000 The topology is still a star topology. 29 00:01:56,000 --> 00:02:00,000 So the main change here, is the hub has been replaced with the bridge. 30 00:02:00,000 --> 00:02:04,000 Bridges store Mac address in the Mac address table 31 00:02:04,000 --> 00:02:06,000 and that in turn is stored in software. 32 00:02:06,000 --> 00:02:11,000 Bridges are therefore very slow in comparison to modern day devices like switches. 33 00:02:11,000 --> 00:02:16,000 Switches and bridges operate in a very similar way, but bridges do the processing 34 00:02:16,000 --> 00:02:21,000 and software where as switches do the processing and hardware. 35 00:02:21,000 --> 00:02:27,000 Switches use something called an ASIC or Application Specific Integrated Circuit 36 00:02:27,000 --> 00:02:32,000 which allows for high throughput, very quick table lookups and forwarding of traffic 37 00:02:32,000 --> 00:02:36,000 often at line rate, in other words switches don’t slow the traffic down. 38 00:02:36,000 --> 00:02:41,000 Bridges were the predecessors to switches and did things in software. 39 00:02:41,000 --> 00:02:46,000 They were a lot slower, but from a forwarding point of view bridges and switches 40 00:02:46,000 --> 00:02:50,000 forward traffic on a layer 2 segment in the same way 41 00:02:50,000 --> 00:02:54,000 except switches do it in hardware and bridges do it in software. 42 00:02:54,000 --> 00:02:57,000 So what does a bridge do when it receives a frame? 43 00:02:57,000 --> 00:02:59,000 So in the similar way to the previous example. 44 00:02:59,000 --> 00:03:04,000 Host A is sending traffic to host C, the source MAC address in the frame is A 45 00:03:04,000 --> 00:03:08,000 the destination address in the frame is C, when the bridge boots up 46 00:03:08,000 --> 00:03:10,000 its MAC address table is empty 47 00:03:10,000 --> 00:03:14,000 in other words its do not contain dynamically learned MAC addresses. 48 00:03:14,000 --> 00:03:18,000 MAC addresses can be statically configured by an administrator 49 00:03:18,000 --> 00:03:22,000 but in this example let’s assume that Mac address are gonna be learned dynamically. 50 00:03:22,000 --> 00:03:27,000 So at the moment the table is empty, when a frame arrives on port 1 51 00:03:27,000 --> 00:03:34,000 on the bridge sent by host A, the bridge now knows that host A is connected to port 1 52 00:03:34,000 --> 00:03:38,000 and can add MAC address A to its MAC address table 53 00:03:38,000 --> 00:03:43,000 and essentially creates a mapping saying that MAC address A can be found on port 1 54 00:03:43,000 --> 00:03:47,000 So it’s now learned where A is in the topology. 55 00:03:47,000 --> 00:03:50,000 However it doesn't know where C is in the topology 56 00:03:50,000 --> 00:03:53,000 because that information is not in its MAC address table yet. 57 00:03:53,000 --> 00:03:57,000 In other words because it doesn’t know where C is, its gonna send the frame out of all 58 00:03:57,000 --> 00:04:03,000 ports except the port on which would was received to ensure that C receives the frame. 59 00:04:03,000 --> 00:04:07,000 Now because the frame is sent out of all ports, both B and D receive a copy of the 60 00:04:07,000 --> 00:04:11,000 frame but they will drop it because the frame is not destined to them. 61 00:04:11,000 --> 00:04:16,000 So in other words the network interface cards on NICs on Pc's B and D 62 00:04:16,000 --> 00:04:21,000 will read the destination MAC address and see that it's destined to C 63 00:04:21,000 --> 00:04:24,000 and not themselves and therefore drop the frame. 64 00:04:24,000 --> 00:04:30,000 The network interface card on host C will receive the frame, strip the layer 2 headers 65 00:04:30,000 --> 00:04:33,000 and pass the information to high layer protocols 66 00:04:33,000 --> 00:04:38,000 and it does that because the destination MAC address on the frame is C.