Original subtitles

In summary hubs reside at the physical layer of the OSI model.

They are not intelligent.

They do not understand the frames that they are repeating

they simply amplify the signal that they receive out of all other ports

except the ports on which it was received.

Hubs were good for their time, but in general today have been replaced by switches.

Now there's always an exception to the rule.

Wireless networks act like hubs, if you have a 54 Mbps wireless network

be careful, it�s not 54 Mbps dedicated or if you have a 200 Mbps wireless network 10

be careful it�s shared between all devices on the wireless network 11

So you need to divide the speed of your wireless network 12

by the devices connected to a access point. 13

Wireless network also have other issues which reduce the throughput even more 14

but the moral of the story is wireless networks operate as hubs. 15

Hubs are shared devices that would good for their time 16

but are very slow when compared to today's switches. 17

So overtime hubs were replaced by bridges 18

and bridges in turn have been receives by switches. 19

A bridge is a layer 2 device, in other words it resides at the data link layer 20

of the OSI model, bridges are more intelligent than hubs. 21

They use something called a MAC address table 22

to learn where devices are in the topology. 23

So rather than simply just repeating the signal 24

and sending traffic out of all ports without understanding it. 25

Bridges maintain a table with a list of MAC addresses learned in this topology. 26

So in our sample network we have 4 devices A, B, C and D 27

and the hub is being replaced by a bridge. 28

The topology is still a star topology. 29

So the main change here, is the hub has been replaced with the bridge. 30

Bridges store Mac address in the Mac address table 31

and that in turn is stored in software. 32

Bridges are therefore very slow in comparison to modern day devices like switches. 33

Switches and bridges operate in a very similar way, but bridges do the processing 34

and software where as switches do the processing and hardware. 35

Switches use something called an ASIC or Application Specific Integrated Circuit 36

which allows for high throughput, very quick table lookups and forwarding of traffic 37

often at line rate, in other words switches don�t slow the traffic down. 38

Bridges were the predecessors to switches and did things in software. 39

They were a lot slower, but from a forwarding point of view bridges and switches 40

forward traffic on a layer 2 segment in the same way 41

except switches do it in hardware and bridges do it in software. 42

So what does a bridge do when it receives a frame? 43

So in the similar way to the previous example. 44

Host A is sending traffic to host C, the source MAC address in the frame is A 45

the destination address in the frame is C, when the bridge boots up 46

its MAC address table is empty 47

in other words its do not contain dynamically learned MAC addresses. 48

MAC addresses can be statically configured by an administrator 49

but in this example let�s assume that Mac address are gonna be learned dynamically. 50

So at the moment the table is empty, when a frame arrives on port 1 51

on the bridge sent by host A, the bridge now knows that host A is connected to port 1 52

and can add MAC address A to its MAC address table 53

and essentially creates a mapping saying that MAC address A can be found on port 1 54

So it�s now learned where A is in the topology. 55

However it doesn't know where C is in the topology 56

because that information is not in its MAC address table yet. 57

In other words because it doesn�t know where C is, its gonna send the frame out of all 58

ports except the port on which would was received to ensure that C receives the frame. 59

Now because the frame is sent out of all ports, both B and D receive a copy of the 60

frame but they will drop it because the frame is not destined to them. 61

So in other words the network interface cards on NICs on Pc's B and D 62

will read the destination MAC address and see that it's destined to C 63

and not themselves and therefore drop the frame. 64

The network interface card on host C will receive the frame, strip the layer 2 headers 65

and pass the information to high layer protocols 66

and it does that because the destination MAC address on the frame is C.