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LAN Switching
Today's local-area networks (LANs) are becoming increasingly
congested and overburdened. In addition to an ever-growing population
of network users, several factors have combined to stress the
capabilities of traditional LANs:
• CPUs—In the mid-1980s, the most common desktop workstation was a PC.
At the time, most PCs could execute 1 million instructions per second
(MIPS). Today, workstations with 50 to 75 MIPS of processing power are
common, and I/O speeds have increased accordingly. Two modern
engineering workstations on the same LAN can easily saturate it.
• operating systems—Until recently, operating system design had
constrained network access. Of the three most common desktop operating
systems (DOS/Windows, the UNIX operating system, and the Mac OS), only
the UNIX operating system could multitask. Multitasking allows users to
initiate simultaneous network transactions. With the release of Windows
95, which reflected a redesign of DOS/Windows that included
multitasking, PC users could increase their demands for network
resources.
• applications—Use of client-server applications, such as Network File
System (NFS), LAN Manager, NetWare, and World Wide Web is increasing.
Client-server applications allow administrators to centralize
information, thus making it easy to maintain and protect. Client-server
applications free users from the burden of maintaining information and
the cost of providing enough hard disk space to store it. Given the
cost benefit of client-server applications, such applications are
likely to become even more widely used in the future.
Switching is a technology that alleviates congestion in Ethernet, Token
Ring, and Fiber Distributed Data Interface (FDDI) LANs by reducing
traffic and increasing bandwidth. Such switches, known as LAN switches,
are designed to work with existing cable infrastructures so that they
can be installed with minimal disruption of existing networks. Often,
they replace shared hubs. This case study describes how LAN switching
works, how virtual LANs work, and how to configure virtual LANs (VLANs)
in a topology that consists of Catalyst 5000 LAN switches.
Understanding Switching Basics
The term switching was originally used to describe packet-switch
technologies, such as Link Access Procedure, Balanced (LAPB), Frame
Relay, Switched Multimegabit Data Service (SMDS), and X.25. Today,
switching refers to a technology that is similar to a bridge in many
ways.
The term bridging refers to a technology in which a device (known as a
bridge) connects two or more LAN segments. A bridge transmits datagrams
from one segment to their destinations on other segments. When a bridge
is powered and begins to operate, it examines the Media Access Control
(MAC) address of the datagrams that flow through it to build a table of
known destinations. If the bridge knows that the destination of a
datagram is on the same segment as the source of the datagram, it drops
the datagram because there is no need to transmit it. If the bridge
knows that the destination is on another segment, it transmits the
datagram on that segment only. If the bridge does not know the
destination segment, the bridge transmits the datagram on all segments
except the source segment (a technique known as flooding). The primary
benefit of bridging is that it limits traffic to certain network
segments.
Like bridges, switches connect LAN segments, use a table of MAC
addresses to determine the segment on which a datagram needs to be
transmitted, and reduce traffic. Switches operate at much higher speeds
than bridges, and can support new functionality, such as virtual LANs.
Switching in the Ethernet Environment
The most common LAN media is traditional Ethernet, which has a
maximum bandwidth of 10 Mbps. Traditional Ethernet is a half-duplex
technology. Each Ethernet host checks the network to determine whether
data is being transmitted before it transmits and defers transmission
if the network is in use. In spite of transmission deferral, two or
more Ethernet hosts can transmit at the same time, which results in a
collision. When a collision occurs, the hosts enter a back-off phase
and retransmit later. As more hosts are added to the network, hosts
must wait more often before they can begin transmitting, and collisions
are more likely to occur because more hosts are trying to transmit.
Today, throughput on traditional Ethernet LANs suffers even more
because users are running network-intensive software, such as
client-server applications, which cause hosts to transmit more often
and for longer periods of time.
An Ethernet LAN switch improves bandwidth by separating collision
domains and selectively forwarding traffic to the appropriate segments.
Figure 10-1 () shows the topology of a typical Ethernet network in
which a LAN switch has been installed.
Figure 10-1 Ethernet switching.
In Figure 10-1 (), each Ethernet segment is connected to a port on the
LAN switch. If Server A on port 1 needs to transmit to Client B on port
2, the LAN switch forwards Ethernet frames from port 1 to port 2, thus
sparing port 3 and port 4 from frames destined for Client B. If Server
C needs to send data to Client D at the same time that Server A sends
data to Client B, it can do so because the LAN switch can forward
frames from port 3 to port 4 at the same time it is forwarding frames
from port 1 to port 2. If Server A needs to send data to Client E,
which also resides on port 1, the LAN switch does not need to forward
any frames.
Performance improves in LANs in which LAN switches are installed
because the LAN switch creates isolated collision domains. By spreading
users over several collision domains, collisions are avoided and
performance improves. Many LAN switch installations assign just one
user per port, which gives that user an effective bandwidth of 10 Mbps.
Understanding Virtual LANs
A virtual LAN (VLAN) is a group of hosts or network devices, such
as routers (running transparent bridging) and bridges, that forms a
single bridging domain. Layer 2 bridging protocols, such as IEEE 802.10
and Inter-Switch Link (ISL), allow a VLAN to exist across a variety of
equipment, including LAN switches.
VLANs are formed to group related users regardless of the physical
connections of their hosts to the network. The users can be spread
across a campus network or even across geographically dispersed
locations. A variety of strategies can be used to group users. For
example, the users might be grouped according to their department or
functional team. In general, the goal is to group users into VLANs so
that most of their traffic stays within the VLAN. When you configure
VLANs, the network can take advantage of the following benefits:
• control—Just as switches physically isolate collision domains for
attached hosts and only forward traffic out a particular port, VLANs
provide logical collision domains that confine broadcast and multicast
traffic to the bridging domain.
•—If you do not include a router in a VLAN, no users outside of that
VLAN can communicate with the users in the VLAN and vice versa. This
extreme level of security can be highly desirable for certain projects
and applications.
•—You can assign users that require high-performance networking to
their own VLANs. You might, for example, assign an engineer who is
testing a multicast application and the servers the engineer uses to a
single VLAN. The engineer experiences improved network performance by
being on a "dedicated LAN," and the rest of the engineering group
experiences improved network performance because the traffic generated
by the network-intensive application is isolated to another VLAN.
• management—Software on the switch allows you to assign users to VLANs
and, later, reassign them to another VLAN. Recabling to change
connectivity is no longer necessary in the switched LAN environment
because network management tools allow you to reconfigure the LAN
logically in seconds.
Figure 10-2 () shows an example of a switched LAN topology in which VLANs are configured.
Figure 10-2 Typical VLAN topology.
In Figure 10-2 (), a 10-Mbps Ethernet connects the hosts on each floor
to Catalyst 5000 LAN switches. 100-Mbps Fast Ethernet connects switches
A, B, C, and D to Switch E.
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