In a similar way to our previous example.
Let's assume that C replies to A so C sends a frame to the bridge.
The bridge will read the source Mac address in the frame and then update its Mac address table with
that information.
So the bridge now knows that C is on Port three.
As well as knowing that A is on port one because it learnt that from the previous frame and now unlike
a hub, the bridge does not forward the frame out of all ports.
The destination address in the frame is a the bridge knows that Mac address A is on port one so it only
forwards the frame out of port one.
The frame from C therefore only goes out of port one.
It's not sent out of port two or port four because the bridge knows that A is on port one.
So what does this mean?
All subsequent frames from A and C will only use port one and three.
In other words, if A sends another frame to see, it'll only go out a port three.
This is because the MAC addresses of A and C are in the MAC address table and the bridge will forward
traffic based on entries in the MAC address table.
B and D are no longer receiving frames between A and C frames from C to A.
Arriving on port three will go out port one and frames from A to C.
Arriving on port one will be sent out of port three.
Therefore, A and C can have a conversation independently of bad bad D on no longer receiving frames
sent between A and C.
The frames between A and C are contained between ports one and three.
No bandwidth is used on port two and four.
When traffic is sent between A and C, devices, B and D do not receive any frames sent between A and
C and therefore avoid unnecessary processing of frames not destined to themselves.
Bandwidth is being conserved, devices are not unnecessarily processing, traffic not destined to them,
and thus bridges have major advantages over hubs.
Over time, the bridge will learn where all Mac addresses are.
So the bridge will learn that A is in port one, B's on port two, sees on port three and DD is on port
four.
That means that over time, B and D can have a conversation independently of A and C.
The two conversations do not affect each other.
Frames from each conversation do not interfere with the other conversation.
Therefore, B and C can communicate at the same time as A and C.
Now continuing with the advantages of bridges, each port is a different collision domain, so a collision
on port one will not affect Port three.
Each interface on a bridge is a separate collision domain.
So in this example, we have one, two, three, four collision domains.
If A and B were having a conversation and a collision took place on Port three, it will not affect
A and B, they wouldn't even realize that there was a collision in the network.
Now, in this topology, we have a hub connected to port four of the bridge.
A hub is a single collision domain, so any collisions that take place on the hub will affect devices
connected to the hub, but will not affect other devices elsewhere in the topology.
So if there was a collision on the hub, it would affect host E and host DX, but it would not affect
host A, C and B.
The problem with collisions is that if a collision takes place, the devices have to back off for a
random period of time and then they need to try and access the network again.
So if these devices D and E are in a single collision domain, the bandwidth and throughput that they
have is lower.
Then these devices, which are in a separate collision domain by themselves A, C, and B have a dedicated
link.
They are on a single broadcast domain and single collision domain.
D and E however, are sharing bandwidth because they connected to a hub host A, C and B are on separate
collision domains.
Now it's important to remember that a bridge is still a single broadcast domain.
So if a sent a broadcast, it would be received by everyone in this topology.
All devices will receive the broadcast, and in some cases that's a good thing, but in most cases it's
not.
In networking, we typically want to restrict or contain broadcast traffic.
When there are too many broadcasts in the network, it can slow down all devices on the network and
in the worst cases it will bring your network to its knees.
In other words, your network will just break and not function if you have what's called a broadcast
storm.
Bridges once again process information in software rather than in hardware, and therefore tend to be
slow in comparison to devices such as switches, which process frames in hardware.
The number of ports on a bridge is also limited when compared to switches.
In today's environments, switches have essentially replaced bridges, but it's good for you to realize
that a bridge and a switch operate in a very similar way.
So in summary, a bridge is a layer two device in the OSA model.
In other words, it operates at the data link layer.
It's more intelligent than a hub because it has a mac address table and it learns where Mac addresses
are and then adds those MAC addresses to the MAC address table and can then make intelligent decisions
on where to forward traffic based on the information learned and contained in the Mac address table.
A hub is a physical device that simply repeats signals out of all ports, except the ports in which
the traffic was received.
A bridge will flood a frame out of all ports when it doesn't know where to send the frame.
In other words, it hasn't learned where the destination Mac address is.
It will also flood broadcasts out of all ports.
So each port on a bridge is a separate collision domain.
But a bridge is still a single broadcast domain.
Switches are very similar to bridges as they both reside at layer two or the data link layer of the
same model.
The big advantage of switching when compared to bridging is that processing can be done in hardware
using what are called ASICs or application specific integrated circuits.
The number of ports supported by switches is also a lot higher.
Hundreds of ports are supported on certain switches, whereas with bridges you were limited to a few
ports.
Switches are able to do this because processing is done in hardware and in actual fact, these days
processing is done at wire speed, which means that there is no degradation in performance between two
devices when they are connected via a switch.
In other words, switches can move traffic from one port to another port at the same speed as if they
weren't there.
They can process and switch frames from one port to another port without slowing the frame down.
So here's a quick comparison between switches and bridges switches process in hardware using a six bridges
process in software and are therefore a lot slower.
Switches support many ports bridges are limited in the number of ports that they support.
Bridges have been replaced by switches in today's networks.