All language subtitles for 17. How do switches forward traffic

So here�s our topology which is very similar to the previous topology

but notice the bridge has been replaced by a switch.

Switches are once again layer 2 devices or data link layer devices

and also have a MAC address table in the similar way to a bridge.

The network topology is also a star topology where devices are cabled directly

to ports on the switch, as an analogy, think of the switch as a bridge

but it�s much more powerful and quicker.

If you had a problem in a bridge environment

and you replace the bridge with a switch, you would still have the same problems
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but they would occur a lot quicker.
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Bridge problem are not solved by switches. Switches simply increase the performance.
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So here�s our sample topology once again,
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but in this case we have replaced the bridge with the switch.
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How will traffic flow in this example?
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So we once again using easy to read MAC addresses ike A, B, C and D
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rather than full 48 bit MAC addresses and we're doing that to simplify these examples.
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So if A sends a frame to C and the frame arrived at the switch on port 1.
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What would the switch do with the frame?
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Now in this example let�s assume that the switch is just started up.
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So the MAC address table is empty, it hasn�t learnt where devices are on the topology.
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Now the switch just like a bridge will flood the frame out of all ports
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because it doesn�t know where C is
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when the frame arrives at the switch to an unknown destination MAC address
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that frame is flooded out of all ports except the port on which the frame arrived.
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However just like a bridge the switch doesn�t just flood the frame out of all ports
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but it also learns where devices are in the topology.
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So because the frames was received on port 1
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and the source MAC address in the frame is A
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that information is written to the MAC address table of the switch.
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The switch now knows that MAC address A can be found on port 1.
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When C replies to A, the frame would be received on port 3 of the switch.
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And the switch would update its MAC address table with that information
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in other words the switch knows that MAC address C can be found on port 3
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but in this case because it knows where the destination MAC address is
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in other words A, it will only send the traffic out of port 1 and that�s because A
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is found in the MAC address table as being available out of port 1.
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The switch doesn�t flood the frame out of all ports.
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So in the same way as a bridge all subsequent frames between A and C are forwarded
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only out of those 2 ports, when A sends another frame to C
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the frame is only sent out of port 3
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because the switch knows that MAC address C can be found on port 3.
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when C replies sending traffic to a destination MAC address of A.
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the switch only forwards that traffic out of port 1 because that is learnt
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that MAC address A can be found on port 1.
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So all traffic between these 2 devices will only flow between port 1 and port 3.
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Traffic is not sent out of port 2 or port 4 in a similar way to how a bridge operates.
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In the same way as a bridge
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each interface on the switch is a separate collision domain.
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So if a collision took place on this hub
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it would not affect other ports on the switch.
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Each port on a switch is a separate collision domain.
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They are therefore 4 collision domains in this topology.
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A hub once again is a single collision domain.
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So this interface is a single collision domain
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separated from the other interfaces on the switch.
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A switch however, will flood broadcast and multicast traffic by default.
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So this is a single broadcast domain.
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If A sends a broadcast that broadcast will be flooded out of all ports
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and will be received by all devices in the topology.
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This is very similar to the way bridges operated.
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You once again have the same issues in a switch environment
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as you would have in a bridge environment.
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But switches operate at much higher speeds and support a greater number of ports.
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So typically you wouldn�t have only 4 ports on the switch.
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But in this example we have 4 collision domains and the single broadcast domain.
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Now the reason why a broadcast is flooded out of all ports except
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the ingress port is not a broadcast address consist of 8 hexadecimal Fs at layer 2.
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So when a switch receives the frame with a destination address of 8 Fs it will flood
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that frame out of all ports because this address of 8 Fs at layer 2 indicates everyone.
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In other words the switch will flood this out of all ports
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except the port on which it received.
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So in this example, it was received from A and that frame is then flooded everywhere
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because of broadcast is supposed to go to everyone at layer 2.
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That�s what a broadcast is designed to do.
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Broadcast addresses also indicate all devices rather than
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a single device so the MAC address table is never populated with the broadcast address.
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This information is not written to the MAC address table
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as the Unicast MAC address would have been.
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Broadcast addresses are not associated with specific
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or an individual ports on which the broadcast is received
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it is always flooded out of all ports except the port on which it's received.
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Now as always there are exceptions and we'll talk more about those exceptions later.
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There are some major advantages to using switches over hubs and bridges.
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The first advantage is that, switches can support many ports
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and some switches can support 100 of ports.
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The second advantage is that switches can operate at wire speed.
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So as I've mentioned previously the switch will not slow frames down.
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The switch can physically move a frame from one port to another port
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without slowing the frame down.
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Some switches have backplanes that operate a terabits per second.
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In other words a very, very fast backplanes in comparison to interface speeds.
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So the back plain of the switch is operating at the much greater speed
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than the physical ports. So what does that mean?
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The switch can move traffic from 1 port to another port
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faster or quicker than it can received traffic on an interface or port.
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So traffic from A to D is not slowdown by the switch.
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Another major advantage of switches over hubs is that
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each device is directly connected to a switch port.
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So A is connected to port 1, B to port 2, C to port 3, D to port 4.
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Each device is individually cabled to port on the switch.