In a similar way to our previous example, let’s assume that C is replies to A.
So C sends a frame to the bridge, the bridge will read the source MAC address on the
frame and then update its MAC address table with that information.
So the bridge now knows that C is on port 3, as well as knowing that A is on port 1
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 1
so it only forwards the frame out of port 1.
The frame from C therefore only goes out of port 1
its not send out of port 2 or port 4 because the bridge knows that A is on port 1.
So what does this mean, all subsequent frames from A and C will only use port 1 and 3
in other words if A sends another frame to C it will only go out of port 3
this is because the MAC addresses of A and C are in the MAC address table
and the bridge will forward traffic base 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 3 will go out of port 1
and frames from A to c arriving on port 1 will be sent out of port 3.
therefore A and C can have a conversation independently of B and D.
B and D are no longer receiving frames sent between A and c.
the frames between A and c are contained between ports 1 and 3 .
No bandwidth is used on port 2 and 4 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.
Overtime the bridge will learn where all MAC address are, so the bridge will learn
that A is on port 1, B is on port 2, C is on port 3 and D is on port 4.
That means that overtime B and D can have a conversation independently of A and C.
the 2 conversations do not affect each other.
Frames from each conversation do not interfere with the other conversation.
Therefore B and D 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 1 will not affect port 3.
Each interface in a bridge is a separate collision domain.
So in this example we have 1234 collision domains.
If A and B were having a conversation and a collision took place on port 3.
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 4 of the bridge.
A hub is a single collision domain. So any collisions that takes 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 D
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 this devices D and E are in a single collision domain the bandwidth
and throughput that they have is lower than 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're 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 a most cases it’s not.
In networking we typically want to restrict all contain broadcast traffic.
When there are too many broadcast 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 you 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 2 device in the OSI 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 add those MAC addresses
to the MAC address table and can then make intelligent decision 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 repeat signals
out of all ports except the ports on 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 has unlearned where the destination MAC address is.
It will also flood broadcast out of all ports.
So each port on a bridge is a separate collision domain
but a bridge is still a single broadcast domain.