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The Switch
Steps of Switch Functioning
Switching Methods
The Bridge
A
switch is a layer 2 network device that forwards frames using MAC addresses in
the header of frames. It is used to improve network performance by:-
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segmenting the network and creating separate collision domains.
•
reducing competition for bandwidth.
In
a switch frame forwarding is handled by specialized hardware called
"Application Specific Integrated Circuit" (ASIC). ASIC
technology allows a silicon chip to be programmed to perform specific
functions much faster than that of a chip programmed by software.
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Learning
When switch starts, the MAC address table has no entry. When a
node transmits data on its wire the MAC address of the node is
learned by Switch Port connected to that node. In this way all
the MAC addresses are learned by respective ports and these
entries remain in the cache for a specific time. If during this
specific time no new frame arrives from a node MAC address entry
for that node is dropped from cache.
Forwarding & Filtering
When
a MAC address for a port is learnt, packets addressed to that MAC
address are forwarded only to the port associated with it, using one of
the Switching Methods.
Loop Avoidance
Switches
and Bridges use Spanning Tree Protocol (STP), specified by IEEE 802.1d,
to prevent loops.
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Switching Methods
Store
& Forward: In this method complete frame is
received by the switch. CRC, source address and destination address are
checked. This method has following features:-
•
Highest latency (delay in forwarding of frame) but may vary depending
upon the length of frame.
•
Highest error checking.
•
Lowest frame forwarding speed.
Catalyst
500 switch uses this method. Cut
Through: In this method forwarding starts as soon as
destination address of the frame is received in header. Also known as
WIRE SPEED. This method has following features:-
•
Lowest latency.
•
Lowest error checking.
•
Highest frame forwarding speed.
Fragment
Free (Modified Cut Through): In this method forwarding
starts as soon as first 64 bytes of the frame are received as
fragmentation occurs usually in first 64 bytes. This method has
following features:-
•
Latency approx 60m Sec.
•
Sufficient error checking.
•
Moderate frame forwarding speed.
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It
is a layer 2 device used to connect different network types or networks
of the same type. Packets having destination address on the same network
segment are dropped. Bridges use "Store and Forward" method to
inspect the whole packet.
Advantages:
Using a bridge to segment network can provide:-
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Reliability.
•
Manageability.
•
Scalability.
Disadvantages:
•
A bridge cannot filter out broadcast
traffic.
•
It introduces 20 to 30 %
latency.
•
Only 2 networks can be linked with a
bridge.
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