A LAN
switch is a device that provides much higher port density at a lower
cost than traditional bridges. For this reason, LAN switches can
accommodate network designs featuring fewer users per segment, thereby
increasing the average available bandwidth per user. This chapter
provides a summary of general LAN switch operation and maps LAN
switching to the OSI reference model.
The trend toward fewer users per segment
is known as microsegmentation.
Microsegmentation allows the creation of private or dedicated segments,
that is, one user per segment. Each user receives instant access to the
full bandwidth and does not have to contend for available bandwidth with
other users. As a result, collisions (a normal phenomenon in
shared-medium networks employing hubs) do not occur. A LAN switch
forwards frames based on either the frame's Layer 2 address (Layer 2 LAN
switch), or in some cases, the frame's Layer 3 address (multi-layer LAN
switch). A LAN switch is also called a frame switch because it forwards
Layer 2 frames, whereas an ATM switch forwards cells. Although Ethernet
LAN switches are most common, Token Ring and FDDI LAN switches are
becoming more prevalent as network utilization increases.
Figure 22-1 illustrates a LAN switch
providing dedicated bandwidth to devices, and it illustrates the relationship
of Layer 2 LAN switching to the OSI data link layer:
Figure 22-1: A
LAN switch is a data link layer device.
The earliest LAN
switches were developed
in 1990. They were Layer 2 devices dedicated to solving bandwidth
issues. Recent LAN switches are evolving to multi-layer devices capable
of handling protocol issues involved in high-bandwidth applications that
historically have been solved by routers. Today, LAN switches are being
used to replace hubs in the wiring closet because user applications are
demanding greater bandwidth.
LAN switches
are similar to transparent bridges in functions such as learning the
topology, forwarding, and filtering. These switches also support several
new and unique features, such as dedicated communication between
devices, multiple simultaneous conversation, full-duplex communication,
and media-rate adaption.
Dedicated collision-free communication
between network devices increases file-transfer throughput. Multiple
simultaneous conversations can occur by forwarding, or switching,
several packets at the same time, thereby increasing network capacity by
the number of conversations supported. Full-duplex communication
effectively doubles the throughput, while with media-rate adaption, the
LAN switch can translate between 10 and 100 Mbps, allowing bandwidth to
be allocated as needed.
Deploying LAN switches requires no change
to existing hubs, network interface cards (NICs), or cabling.
LAN switches
can be characterized by the forwarding
method they support. In the store-and-forward switching method, error
checking is performed and erroneous frames are discarded. With the
cut-through switching method, latency is reduced by eliminating error
checking.
With the store-and-forward switching
method, the LAN switch copies the entire frame into its onboard buffers
and computes the cyclic
redundancy check (CRC). The frame is discarded if it contains a CRC
error or if it is a runt (less than 64 bytes
including the CRC) or a giant (more than 1518 bytes including
the CRC). If the frame
does not contain any errors, the LAN switch looks up the destination
address in its forwarding, or switching, table and determines the
outgoing interface. It then forwards the frame toward its destination.
With the cut-through
switching method, the LAN switch copies only the destination address
(the first 6 bytes following the preamble) into its onboard buffers. It
then looks up the destination address in its switching table, determines
the outgoing interface, and forwards the frame toward its destination. A
cut-through switch provides reduced latency because it begins to forward
the frame as soon as it reads the destination address and determines the
outgoing interface.
Some switches can be configured to
perform cut-through switching on a per-port basis until a user-defined
error threshold is reached, when they automatically will change to
store-and-forward mode. When the error rate falls below the threshold,
the port automatically
changes back to store-and-forward mode.
LAN switches
also can be characterized according to the proportion of bandwidth
allocated to each port. Symmetric switching provides evenly distributed
bandwidth to each port, while asymmetric switching provides unlike, or
unequal, bandwidth between some ports.
An
asymmetric LAN switch provides switched connections between ports of
unlike bandwidths, such as a combination of 10BaseT
and 100BaseT. This type of switching is also called 10/100
switching. Asymmetric
switching is optimized for client-server traffic flows where multiple
clients simultaneously communicate with a server, requiring more
bandwidth dedicated to the server port to prevent a bottleneck at that
port.
A symmetric
switch provides switched connections between ports with the same
bandwidth, such as all 10BaseT or all 100BaseT. Symmetric switching is
optimized for a reasonably distributed traffic load, such as in a
peer-to-peer desktop environment.
A network manager must evaluate the
needed amount of bandwidth for connections between devices to
accommodate the data flow of network-based applications when deciding to
select an asymmetric
or symmetric switch.
LAN switches
can be categorized according to the OSI
layer at which they filter and forward, or switch, frames. These
categories are: Layer 2, Layer 2 with Layer 3 features, or multi-layer.
A Layer 2 LAN switch is operationally
similar to a multiport bridge but has a much higher capacity and
supports many new features, such as full-duplex operation. A Layer 2 LAN
switch performs switching and filtering based on the OSI data link layer
(Layer 2) MAC address. As with bridges, it is completely transparent to
network protocols and user applications.
A Layer 2 LAN switch with Layer 3
features can make switching decisions based on more information than
just the Layer 2 MAC address. Such a switch might incorporate some Layer
3 traffic-control features, such as broadcast and multicast traffic
management, security through access lists, and IP fragmentation.
A multi-layer switch makes switching and
filtering decisions on the basis of OSI data
link layer (Layer 2) and OSI network-layer
(Layer 3) addresses. This type of switch dynamically decides whether to
switch (Layer 2) or route (Layer 3) incoming traffic. A multi-layer LAN
switch switches within a workgroup and routes between different
workgroups.
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