Transcript LAN protocols - University of Northampton

Switches
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In this lecture, we will learn about
Collision Domain and Microsegmentation
Switches – a layer two device
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MAC address Table
Frame Filtering
Frame Forwarding
Frame Buffering
Full Duplexing
Collision domain in switches
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A collision domain is a logical area in a computer network in which data
packets can collide with one another. A collision occurs when two or more
devices attempt to send a signal along the same transmission channel (e.g.,
the same section of copper wire or optical fiber) simultaneously.
Disadvantages of data collisions are:
◦ Decreased network efficiency
◦ Packet Loss.
◦ Slow Performance of the network.
◦ More bandwidth utilization
Network devices are used to reduce the size of network segment or
Collision domain.
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Layer 3 device: Router
Layer 2 device: Switch and bridge
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Switches reduce the size of collision domain
Microsegmentation is the dividing of a network
through the use of network switches such that only two
nodes coexist within each collision domain.
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router
switch
hub
Collision Domains
switch
hub
hub
hub
Collision Domains
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Layer 2 Multiport device
◦ E.g., 12 ports, 24 ports, 48
ports
Switches are specified by
number of ports and
bandwidth supported
◦ 10/100 Mbps ports
 connecting computers in a
network.
Port
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Switches associate MAC
addresses with port numbers
and store them in its memory
in a table called MAC
Address Table
Whenever a frame enters a
switch, it will add Source
Address (1111) against port
number (2) through which it
came in is in it’s table.
This table for catalyst switches
is called CAM (Content
Addressable Memory) table
The CAM table times out
every 5 minutes
switch
1111
3333
2222
Source Address Table
Port Source MAC Add.
2
1111
5
2222
7
3333
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Switches output a frame on a
specific destination port .
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Switch examines the MAC address
table for the Destination MAC
address.
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If it finds a match, it filters the
frame by only sending it out on that
port.
3333
1111
Source Address Table
Port Source MAC Add. Port Source MAC Add.
2
1111
switch
◦ If no match is found, it floods it
out on all the ports.
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1111
3333
Sample MAC addresses
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PC 3333 wants to send data back
to 1111.
The switch now has an entry for
the Source Address (3333)
◦ It does NOT add it.
It checks the Destination
Address, it has it in its table
As both MAC addresses are in the
switch’s table, information
exchanged between 1111 and 3333
can be filtered out the appropriate
port (port 8) and not Flooded
MAC Address Table
Port Source MAC Address.
2
1111
8 3333
switch
1111
3333
2222
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3333
1111
1111
3333
3333
2222
2222
when two devices (1111 and 2222) send to
same destination (3333)?
The switch buffers the frames and sends
them out on port number 7 one at a time.
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Full-duplex allows simultaneous (send and receive at the same
time) communication between a pair of devices.
Both ends of the link must be in full-duplex mode.
Most switches operate at either full-duplex but can operate in halfduplex.
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For every frame that enters a switch…
 Learning Stage (Building/Updating of MAC table)
◦ Examines Source MAC Address:
 If Source MAC Address is in the MAC table, update 5 minute timer
 If Source MAC Address is NOT in the MAC table, add Source MAC Address
and incoming port number to MAC table
 Forwarding Stage (Flood or Filter)
◦ Examines Destination MAC Address:
 If Destination MAC Address is in the MAC table, forward the frame only out
that port (Filter),
 If Destination MAC Address is NOT in the MAC table, forward the frame only
out all ports except incoming port (Flood)
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Switches have multiple collision domains.
To communicate with all collision domains, broadcast
and multicast frames are used at Layer 2.
Every NIC must recognize this address
Since broadcasts generate excess traffic in the
network, broadcasts must be controlled.
Devices such as router which operate at Layer 3
control broadcasts.
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Five new terms that you learnt in this session
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Find information about switches available in the lab
◦ Cisco 2950 switches
◦ Cisco 3560 switches
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Compare their features
 Hardware options possible
 Memory –RAM/Flash
 IOS images supported
 Applications of switches
 Find the differences between Layer 2 /Layer 3 Switches
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