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As more nodes are added to an Ethernet
physical segment, contention for the media increases. Ethernet is a
shared media, which means only one node can transmit data at a time.
The addition of more nodes increases the demands on the available
bandwidth and places additional loads on the media. By increasing the
number of nodes on a single segment, the probability of collisions
increases, resulting in more retransmissions. A solution to the
problem is to break the large segment into parts and separate it into
isolated collision domains.
To accomplish this a bridge keeps a table
of MAC addresses and the associated ports. The bridge then forwards or
discards frames based on the table entries. The following steps
illustrate the operation of a bridge:
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The bridge has just been started so the
bridge table is empty. The bridge just waits for traffic on the
segment. When traffic is detected, it is processed by the bridge.
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Host A is pinging Host B. Since the data
is transmitted on the entire collision domain segment, both the bridge
and Host B process the packet.
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The bridge adds the source address of
the frame to its bridge table. Since the address was in the source
address field and the frame was received on port 1, the frame must be
associated with port 1 in the table.
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The destination address of the frame is
checked against the bridge table. Since the address is not in the
table, even though it is on the same collision domain, the frame is
forwarded to the other segment. The address of Host B has not been
recorded yet as only the source address of a frame is recorded.
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Host B processes the ping request and
transmits a ping reply back to Host A. The data is transmitted over
the whole collision domain. Both Host A and the bridge receive the
frame and process it.
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The bridge adds the source address of
the frame to its bridge table. Since the source address was not in the
bridge table and was received on port 1, the source address of the
frame must be associated with port 1in the table. The destination
address of the frame is checked against the bridge table to see if its
entry is there. Since the address is in the table, the port assignment
is checked. The address of Host A is associated with the port the
frame came in on, so the frame is not forwarded.
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Host A is now going to ping Host C.
Since the data is transmitted on the entire collision domain segment,
both the bridge and Host B process the frame. Host B discards the
frame as it was not the intended destination.
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The bridge adds the source address of
the frame to its bridge table. Since the address is already entered
into the bridge table the entry is just renewed.
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The destination address of the frame
is checked against the bridge table to see if its entry is there.
Since the address is not in the table, the frame is forwarded to the
other segment. The address of Host C has not been recorded yet as only
the source address of a frame is recorded.
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Host C processes the ping request and
transmits a ping reply back to Host A. The data is transmitted over
the whole collision domain. Both Host D and the bridge receive the
frame and process it. Host D discards the frame, as it was not the
intended destination.
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The bridge adds the source address of the frame to its bridge
table. Since the address was in the source address field and the frame
was received on port 2, the frame must be associated with port 2 in
the table.
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The destination address of the frame is checked against the
bridge table to see if its entry is present. The address is in the
table but it is associated with port 1, so the frame is forwarded to
the other segment.
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When Host D transmits data, its MAC address will also be recorded
in the bridge table. This is how the bridge controls traffic between
to collision domains.
These are the steps that a bridge uses to forward and discard frames
that are received on any of its ports.
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