Ethernet Switches: Faster Than a Speeding Hub
Laura Chappell
The first time I saw a Kalpana switch in 1991, I had no idea how a switch worked or how
it could improve network performance. I certainly had no idea that by 1996 the
networking industry would be having a love affair with switch technology. This article
briefly explains switch technology and describes some solutions for managing and
troubleshooting networks that are connected by switches.
WHAT IS A SWITCH?
On a 10Base-T network, a hub connects devices, or stations, to form a network, as shown
in Figure 1. When a station sends a request or reply to another station on the network,
that communication, which is transmitted as an Ethernet packet, first travels through the
hub. Like a multiport repeater, the hub then broadcasts the packet to each of its ports,
regardless of the actual destination of the packet. For example, when Station A transmits
a packet for the RESUME.DOC file to the SERV1 server, the packet passes through the
hub, which then repeats the packet to all of the ports on the hub.
Unfortunately, hubs have a shared media segment, which is essentially a single Ethernet
bus on which all stations' communications must travel. (See Figure 1.) All stations
connected to the hub must take turns sending packets on this "bus," sharing its 10 Mbit/s
of bandwidth. For example, on a 48-port hub, a station competes with 47 other stations to
send packets across 10 Mbit/s of bandwidth. If one station is sending packets across the
segment, all other stations must wait until the segment is free.
Switches employ a different architecture to reduce bandwidth contention. Unlike a hub, a
switch is an intelligent device that provides logical connections between stations. The
switch maintains a table that lists all the stations connected to the switch by their
hardware address and the port that corresponds to each station. When a packet passes
through the switch, the switch reads the packet's destination address, refers to the table to
determine the appropriate station's address and corresponding port, and then forwards the
packet only to that port.
For example, when Station A transmits a packet addressed to SERV1, the switch reads
the address on the packet, finds SERV1's address in the table, determines which port is
connected to SERV1, and forwards the packet only to SERV1's port, as shown in Figure
2. When Station A transmits a broadcast packet, the switch transmits the broadcast packet
to all active ports.
Because a switch is intelligent and provides each station with almost dedicated
bandwidth, networks connected by switches are fast and efficient. To increase speed even
more, switch manufacturers are producing switches with both 10 Mbit/s ports and a 100
Mbit/s port. You connect your most active network device, such as a server or router, to
the 100 Mbit/s port, and connect all other stations to the 10 Mbit/s ports.
Switches do have one disadvantage: They cost significantly more than hubs do. For
example, 3Com Corporation's SuperStack LinkBuilder FMS II Stackable Hub with 12 10
Mbit/s ports has a suggested retail price of U.S. $499, and the 24-port version has a
suggested retail price of U.S. $899. On the other hand, 3Com Corporation's SuperStack
LinkSwitch 1000 Switch with 12 10 Mbit/s ports and one 100 Mbit/s port has a suggested
retail price of U.S. $2,699, and the 24-port version has a suggested retail price of U.S.
$3,699.
TYPES OF SWITCHES
Ethernet switches typically use one of two types of forwarding technologies: cut-through
or store-and-forward.
Cut-Through Switches
If a switch uses cut-through technology, it begins forwarding a packet to the appropriate
destination port after it receives the first six bytes of the packet. (The first six bytes
contain the packet's destination address field.) The delay introduced by the switch, or the
latency period, is minimal because the switch holds the packet for only a six-byte
timeframe (approximately 4.8 microseconds) before forwarding it.
Although a cut-through switch does improve performance, it has one major drawback: It
begins forwarding a packet before it can determine the packet's validity. In Ethernet, the
cyclical redundancy check (CRC) field, which determines whether the packet is valid or
corrupted, appears at the end of the packet.
If a station is transmitting corrupted packets, a cut-through switch forwards these
corrupted packets. If a cut-through switch interprets a corrupted packet as a broadcast
packet, the switch will forward the corrupted packet to all stations connected to the
switch. Transmitting corrupted packets can cause serious congestion and time-outs on the
network.
Store-and-Forward Switches
If a switch uses store-and-forward technology, it stores the entire packet before
forwarding it. As a result, the switch can check the packet's CRC field to determine if the
packet is valid or corrupted. Unlike a cut-through switch, a store-and-forward switch does
not forward corrupted packets.
Unfortunately, because the store-and-forward switch must hold the entire packet in
memory before forwarding it, latency time increases. The latency time depends on the
size of the packet since the entire packet must be stored before the switch can forward it.
ANALYZING SWITCHED TRAFFIC
Although switches can improve performance and decrease traffic, they do affect how you
troubleshoot Ethernet networks. For example, if a network has a 10Base-T hub, you can
connect Novell's LANalyzer for Windows to the hub and take advantage of the way the
hub forwards packets to every port. You can eavesdrop on all communications between
stations and use these communications to troubleshoot performance problems.
If the network has a switch, however, a LANalyzer for Windows station can hear only
broadcast traffic and its own communications. To monitor traffic on a network connected
by a switch, you must purchase a switch management solution. To choose the best
solution for your network, you should consider the size of your network and your budget.
Distributed Analysis Solution
A distributed analysis solution, such as Novell's ManageWise, allows you to manage all
your network devices from a central console. Information about network traffic and
performance travels from the ManageWise LANalyzer Agent NetWare loadable module
(NLM) across the network to the central console on which you can view, manage, and
configure your local and remote network devices.
Most NetWare communications are client-server communications: A station sends
request packets to a server, and the file server transmits replies. By installing a
ManageWise LANalyzer Agent NLM on a NetWare file server, you can track all
communications to and from that server, regardless of whether a switch forwarded the
packets. (See Figure 3.)
Monitor Port Solution
Some switches include a monitor port that may support a company's proprietary network
analyzer or a third-party network analyzer, such as Network General's Sniffer.
A monitor port allows you to analyze network communications in one of three ways: port
tapping, circuit tapping, or switch tapping. Port tapping enables the network analyzer to
monitor all communications to and from a specific port. Circuit tapping enables the
analyzer to look at a specific conversation between two switch ports. Switch tapping
enables the analyzer to look at all of the communications flowing through the switch.
The following switches offer a monitor port solution:
? Bay Networks, Inc.'s Model 28000 Series switches and Model 3000 with 3328
host module switch
? Cisco Systems' Catalyst 5000 (with software version 2.1 or greater), Catalyst
1200 (with software version 3.0 or greater), Catalyst 1700, and EtherSwitch EPS-
2115M
? SMC's EliteSwitch ES/1 and EliteSwitch ES/1 ATX
? 3Com Corporation's SuperStack LinkSwitch 2200 Switch and LANplex 2016,
2500, and 6000
Standalone Network Analyzer
On a network that has only two or three switches, you can add an external tap to the
server's port. An external tap is a hub with at least three ports. You simply plug your
server, a switch, and a network analyzer such as Novell's LANalyzer for Windows into
these ports, and the network analyzer allows you to listen to all communications between
the server and its clients. (See Figure 4.)
An external tap works with any switch and offers the same functionality as ManageWise
and the LANalyzer Agent NLMs do. You can purchase a four- or five-port hub from
SMC or Farallon Communications, Inc. for approximately U.S. $90.
CONCLUSION
If your hub is a bottleneck on your network or if you need high-bandwidth support, using
a switch will improve network performance and allow you to use your existing cabling,
network interface boards, and drivers. If you want to investigate switch technology, you
may want to start with the products mentioned in this article. (For more information, see
"Switch Companies.")
Laura Chappell researches, writes, and lectures on NetWare protocol performance,
troubleshooting, and optimization. She is the coauthor of several books published by
Novell Press. You can reach Laura at lchappell@imagitech.com. |
发表于 2006-5-25 14:17:57
发表于 2006-5-25 15:48:48