connected to each switch port. When two connected
hosts want to communicate with each other, the switch looks up
the switching table and establishes a virtual connection
between the ports. The virtual circuit is maintained until the
session is terminated. In Figure , Host B and Host C want to
communicate with each other. The switch creates the virtual
connection which is referred to as a microsegment. The
microsegment behaves as if the network has only two hosts, one
host sending and one receiving providing maximum utilization of
the available bandwidth. Switches reduce collisions and
increase bandwidth on network segments because they provide
dedicated bandwidth to each network segment.
Content
4.3 Switch Operation 4.3.9
Switches and broadcast domains Communication in a network
occurs in three ways. The most common way of communication is
by unicast transmissions. In a unicast transmission, one
transmitter tries to reach one receiver.Another way to
communicate is known as a multicast transmission. Multicast
transmission occurs when one transmitter tries to reach only a
subset, or a group, of the entire segment. The final way to
communicate is by broadcasting. Broadcasting is when one
transmitter tries to reach all the receivers in the network.
The server station sends out one message and everyone on that
segment receives the message. When a device wants to send out a
Layer 2 broadcast, the destination MAC address in the frame is
set to all ones. A MAC address of all ones is FF:FF:FF:FF:FF:FF
in hexadecimal. By setting the destination to this value, all
the devices will accept and process the broadcasted frame. The
broadcast domain at Layer 2 in referred to as the MAC broadcast
domain. The MAC broadcast domain consists of all devices on the
LAN that receive frame broadcasts by a host to all other
machines on the LAN. A switch is a Layer 2 device. When a
switch receives a broadcast, it forwards it to each port on the
switch except the incoming port. Each attached device must
process the broadcast frame. This leads to reduced network
efficiency, because available bandwidth is used for
broadcasting purposes. When two switches are connected, the
broadcast domain is increased. In this example a broadcast
frame is forwarded to all connected ports on Switch 1. Switch 1
is connected to Switch 2. The frame is propagated to all
devices connected to Switch 2. The overall result is a
reduction in available bandwidth. This happens because all
devices in the broadcast domain must receive and process the
broadcast frame. Routers are Layer 3 devices. Routers do not
propagate broadcasts. Routers are used to segment both
collision and broadcast domains.
Content
4.3 Switch Operation 4.3.10
Communication between switches and workstation When a
workstation connects to a LAN, it is unconcerned about the
other devices that are connected to the LAN media. The
workstation simply transmits data frames using a NIC to the
network medium. The workstation could be attached directly to
another workstation, using a crossover cable or attached to a
network device, such as a hub, switch, or router, using a
straight-through cable. Switches are Layer 2 devices that use
intelligence to learn the MAC addresses of the devices that are
attached to the ports of the switch. This data is entered into
a switching table. Once the table is complete, the switch can
read the destination MAC address of an incoming data frame on a
port and immediately forward it. Until a device transmits, the
switch does not know its MAC address. Switches provide
significant scalability on a network and may be directly
connected. Figure illustrates one scenario of frame
transmission utilizing a multi-switch network.
Content
Summary An understanding of the following key points
should have been achieved: - The history and function of
shared, half-duplex Ethernet
- Collisions in an Ethernet
network
- Microsegmentation
- CSMA/CD
- Elements affecting network performance
- The
function of repeaters
- Network latency
- Transmission time
- The basic function of Fast
Ethernet
- Network segmentation using routers, switches,
and bridges
- The basic operations of a switch
- Ethernet switch latency
- The differences between
Layer 2 and Layer 3 switching
- Symmetric and asymmetric
switching
- Memory buffering
- Store-and-forward
and cut-through switchings
- The differences between
hubs, bridges, and switches
- The main functions of
switches
- Major switch frame transmission modes
- The process by which switches learn addresses
- The
frame-filtering process
- LAN segmentation
- Microsegmentation using switching
- Forwarding
modes
- Collision and broadcast domains