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In summary hubs reside at the physical layer of the OSI model.
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They are not intelligent.
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They do not understand the frames that they are repeating
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they simply amplify the signal that they receive out of all other ports
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except the ports on which it was received.
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Hubs were good for their time, but in general today have been replaced by switches.
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Now there's always an exception to the rule.
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Wireless networks act like hubs, if you have a 54 Mbps wireless network
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be careful, it�s not 54 Mbps dedicated or if you have a 200 Mbps wireless network
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be careful it�s shared between all devices on the wireless network
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So you need to divide the speed of your wireless network
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by the devices connected to a access point.
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Wireless network also have other issues which reduce the throughput even more
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but the moral of the story is wireless networks operate as hubs.
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Hubs are shared devices that would good for their time
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but are very slow when compared to today's switches.
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So overtime hubs were replaced by bridges
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and bridges in turn have been receives by switches.
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A bridge is a layer 2 device, in other words it resides at the data link layer
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of the OSI model, bridges are more intelligent than hubs.
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They use something called a MAC address table
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to learn where devices are in the topology.
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So rather than simply just repeating the signal
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and sending traffic out of all ports without understanding it.
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Bridges maintain a table with a list of MAC addresses learned in this topology.
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So in our sample network we have 4 devices A, B, C and D
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and the hub is being replaced by a bridge.
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The topology is still a star topology.
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So the main change here, is the hub has been replaced with the bridge.
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Bridges store Mac address in the Mac address table
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and that in turn is stored in software.
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Bridges are therefore very slow in comparison to modern day devices like switches.
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Switches and bridges operate in a very similar way, but bridges do the processing
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and software where as switches do the processing and hardware.
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Switches use something called an ASIC or Application Specific Integrated Circuit
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which allows for high throughput, very quick table lookups and forwarding of traffic
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often at line rate, in other words switches don�t slow the traffic down.
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Bridges were the predecessors to switches and did things in software.
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They were a lot slower, but from a forwarding point of view bridges and switches
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forward traffic on a layer 2 segment in the same way
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except switches do it in hardware and bridges do it in software.
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So what does a bridge do when it receives a frame?
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So in the similar way to the previous example.
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Host A is sending traffic to host C, the source MAC address in the frame is A
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the destination address in the frame is C, when the bridge boots up
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its MAC address table is empty
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in other words its do not contain dynamically learned MAC addresses.
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MAC addresses can be statically configured by an administrator
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but in this example let�s assume that Mac address are gonna be learned dynamically.
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So at the moment the table is empty, when a frame arrives on port 1
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on the bridge sent by host A, the bridge now knows that host A is connected to port 1
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and can add MAC address A to its MAC address table
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and essentially creates a mapping saying that MAC address A can be found on port 1
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So it�s now learned where A is in the topology.
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However it doesn't know where C is in the topology
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because that information is not in its MAC address table yet.
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In other words because it doesn�t know where C is, its gonna send the frame out of all
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ports except the port on which would was received to ensure that C receives the frame.
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Now because the frame is sent out of all ports, both B and D receive a copy of the
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frame but they will drop it because the frame is not destined to them.
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So in other words the network interface cards on NICs on Pc's B and D
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will read the destination MAC address and see that it's destined to C
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and not themselves and therefore drop the frame.
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The network interface card on host C will receive the frame, strip the layer 2 headers
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and pass the information to high layer protocols
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and it does that because the destination MAC address on the frame is C.
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