Allied Telesis CentreCOM AT-6870 User manual

Allied
Telesyn
International
CentreCOM MAC
Layer Bridge Models
AT-6870
AT-6875
User Manual
PN 613-10450-00

 Copyright 1996 Allied Telesyn International Corp.
All rights reserved. No part of this publication may be reproduced without prior written permission from
Allied Telesyn International.
Allied Telesyn reserves the right to make changes in specifications and other information contained in this
document without prior written notice. The information provided herein is subject to change without
notice. In no event shall Allied Telesyn be liable for any incidental, special, indirect, or consequential
damages whatsoever, including but not limited to lost profits, arising out of or related to this manual or the
information contained herein, even if Allied Telesyn has been advised of, known, or should have known,
the possibility of such damages.
Trademarks: CentreCOM is a trademark of Allied Telesyn International. Ethernet is a registered
trademark of Xerox Corporation.

iii
Statement
U.S. Federal
Communications
Warning: This equipment has been tested and found to comply with the
limits for a Class A digital device pursuant to Part 15 of FCC Rules.These
limits are designed to provide reasonable protection against harmful
interference when the equipment is operated in a commercial
environment.This equipment generates, uses, and can radiate radio
frequency energy and, if not installed and used in accordance with this
installation manual, may cause harmful interference to radio
communications. Operation of this equipment in a residential area is
likely to cause harmful interference in which case the user will be
required to correct the interference at his own expense.
Note: Modifications or changes not expressly approved of by the
manufacturer, can void your right to operate this equipment.
Danger :There are no "user-serviceable" parts inside the CentreCOM
Multiport Repeater. Due to the danger of exposed high voltage, DO NOT
operate or apply power to this product when the cover is removed.This
unit should be opened only by a trained and qualified technician.
Canadian
Department of
Communications
This digital apparatus does not exceed the Class A limits for radio noise
emissions from digital apparatus set out in the Radio Interference
Regulations of the Canadian Department of Communications.
Le présent appareil numérique n'émet pas de bruits radioélectriques
dépassant les limites appicables aux appareils numériques de le Classe A.
Prescrites dans le réglement sur le brouillage radioélectrique édicté par le
minestére des Communications du Canada.
Electrical Notice All Countries: Install in accordance with Local and National Electrical
Codes.
German Class B Bercheingung des herstellers/importeurs hiermit wird bescheingt,das der
AT-3004 oder AT-3004 Multiport Repeater in Uebereinstimmung mit den
Bestimmungen der AT-3004 funk-entstoert ist. Der Deutschen
Bundespost wurde das Inverkenhrbringen dieses Gerates angezeigt und

Statement
iv
die Berschtigung zur Uberprufung der Serie auf Einhaltung der
Bestimmungen eingeraumt.Von benutzer zusammengestellte Systeme,
die dieses Gerat beihalten, mussen den Bestimmungen von Vfg 243
entsprechen.
Notes The following are examples of special notices used throughout this
manual.They are Notes, Cautions, and Warnings.They are defined as
follows:
Note: This indicates a note of specific interest related to the immediate
text in which it is found.
Caution: This notice will be related to possible problem areas where
caution should be observed to avoid non-life threatening conditions and
minor damage to equipment, etc.
Warning: This notice will indicate a hazardous condition that may pose
the possibility of severe equipment damage, personal injury or death.

v
Table of Contents
Statement ...................................................................................................................... iii
U.S. Federal Communications............................................................................................................... iii
Canadian Department of Communications.......................................................................................... iii
Electrical Notice...................................................................................................................................... iii
German Class B........................................................................................................................................ iii
Notes.......................................................................................................................................................... iv
Table of Contents .......................................................................................................... v
Overview ..........................................................................................................................1
CentreCOM MAC Layer Bridge............................................................................................................. 1
Features...................................................................................................................................................... 2
Physical Description.................................................................................................................................. 2
Application Guidelines..................................................................................................................................... 4
Overcoming Physical Limitations........................................................................................................... 4
Increasing Network Performance........................................................................................................... 4
Mixing Media.............................................................................................................................................. 7
Fault Tolerance and Isolation.................................................................................................................. 8
Using Backup Bridges .............................................................................................................................. 9
Installation and Operation............................................................................................................................ 11
Unpacking................................................................................................................................................. 11
Site Selection............................................................................................................................................ 11
Factory Configuration ............................................................................................................................ 11
Operation................................................................................................................................................... 13
Specifications ................................................................................................................15
Software Features................................................................................................................................... 15
Hardware Features................................................................................................................................. 15
Performance ............................................................................................................................................. 15
Physical Characteristics ......................................................................................................................... 15
Power Characteristics............................................................................................................................. 15
Accessories ............................................................................................................................................... 16
IEEE 802.3 Quick Primer ............................................................................................17
IEEE 802.3 AUI Drop Cables............................................................................................................... 17

Table of Contents
vi
IEEE 802.3/10BASE2 (Thin Ethernet)............................................................................................... 17
IEEE 802.3/10BASE5 (Thick Ethernet)............................................................................................. 17
Propagation.............................................................................................................................................. 17
IEEE 802.3 Four Repeater Rule.......................................................................................................... 18
Network Frame Formats ............................................................................................19
Power Cords and Plugs ..............................................................................................21
Glossary ..........................................................................................................................23
Warranty and Service Information .........................................................................25

1
Chapter 1
Overview
CentreCOM MAC
Layer Bridge
❑ AT-6870 Learning Bridge
❑ AT-6875 SNMP Manageable Learning Bridge
IEEE 8023 Compliant Ethernet Compatible
The CentreCOM Bridges allow two IEEE 802.3 sub-networks to be
connected into a single network. Each sub-network may be IEEE 802.3
Ethernet or thin Ethernet.The CentreCOM Bridges transparently
forward packets between both networks. Since these bridges operate at
the MAC level, all higher level protocols, such as TCP/TP, DECnet™,and
Netware™, are passed unaffected.
Many common Local Area Network (LAN) problems are solved by the
CentreCOM Bridges.For example,the effective length of a network can be
increased by adding CentreCOM Bridges.Moreover, placing the bridges at
natural boundaries of the workgroups increases the network performance
in each segment.The reliability of a large network can be enhanced
through subnetting with CentreCOM Bridge, which allows heterogeneous
sub-networks to be interconnected.
The CentreCOM Bridges support high performance requirements
including packet filtering at 25,000 packets per second and sustained
forwarding at 12,500 packets per second.
The CentreCOM Bridges' address filter table supports 2048 station
addresses.With this high performance, the AT-6870 and the AT-6875 do
not lose data during peak network traffic periods.
Multiple CentreCOM Bridges can be used as backups in case of network
failure.The AT-6870 and AT-6875 support the IEEE 802.1 STP, which
provides redundancy link management and network loop checking. STP
allows backup bridges to provide fault tolerant networks by placing the
bridges that form loops into a standby state until an active bridge fails.
When failure occurs, the network is reconfigured and any loops are
eliminated.This network reconfiguration occurs without impacting the
network users or the applications.

Overview
2
The TCP/IP SNMP is supported by the AT-6875.The bridge can be
monitored and manipulated by many SNMP compatible management
stations.Several parameters of theAT 6875 can be managed under SNMP
including custom filtering, static forwarding entries, and STP.
Features
❑ Self-learning MAC layer bridges
❑ Filter at 25,000 PPS
❑ Forward at 12,500 PPS
❑ Two media options per network segment, thin (10BASE2) or AUI
(10BASE5)
❑ Supports up to 2,000 MAC addresses in filter table
❑ IEEE 802.1 revision D Spanning Tree Protocol (STP)
❑ Standalone or rack mount installation
❑ Simple Network Management Protocol (SNMP)
❑ Management Information Base II (MIB II)
❑ One year free software upgrades
❑ One year warranty
Physical Description The CentreCOM MAC Layer Bridge is packaged in a sturdy two piece
sheet metal enclosure with ventilation slots on the side and back.
All network attachments and power connectors are on the back panel;
status indicators are on the front.
The bridge configuration is controlled by the jumpers on the main board
inside the chassis. The top cover is attached to the main chassis by two
screws on each side.
Front Panel Six LEDs are on the bridge front panel (see Figure 1), arranged as two
sets.The first set contains two lights which display power and diagnostic
indications.The other set contains four network activity LEDs.
The names of each LEDand their functions are shown below in Table 1.
Figure 1: AT-6870 Front Panel
Light Function
PWR System Power
DIAG Diagnostic Testing Status
TX1 Channel 1 Transmitting
Table 1: AT-6870 Front Panel

AT-30xxTR Multiport Repeater
3
Back Panel
The back panel contains the following elements, shown in Figure 2:
❑ IEC-320/CEE-22 compatible power connector.
❑ Channel 1 connector for Ethernet or IEEE 802.3 10BASE5.
❑ Channel 1 thin Ethernet coaxial connector.
❑ Channel 2 connector for Ethernet or IEEE 802.3 10BASE5.
❑ Channel 2 thin Ethernet coaxial connector.
❑ Fan vent.
❑ Removable expansion slot cover.
The Ethernet/lEEE ffO2.3 10BASE5 network connector uses screw lock
fasteners to ensure secure connections.These ports use a DA-15P male
connector.
Product Code and
Serial Number
A label on the bottom of each CentreCOM MAC Layer Bridge indicates the
product code and serial number. If you need to call Allied Telesyn
Technical Support, have these numbers ready.
RX1 Channel 1 Receiving
TX2 Channel 2 Transmitting
RX2 Channel 2 Receiving
Light Function
Table 1: AT-6870 Front Panel
Figure 2: AT-6870 Back Panel

Overview
4
Application Guidelines
Overcoming Physical
Limitations
This section shows how bridges can be used to overcome common physical
limitations.The next section shows how to do this in such a way that
network performance is improved.
Maximum Network
Length
A bridge can be used to link several networks serially (see Figure 3) in the
same way as a repeater is used. The maximum network length only
applies to individual networks, so that the overall network length is not
bound by any limit. However, because each bridge introduces a small
delay in forwarded traffic, there are practical limits on how many bridges
can be used in this manner.
Maximum Number of
Stations
The maximum number of stations attached to a single network is 1024.As
more stations are added, the number of collisions increases and
performance degrades. Bridges can be used to solve this problem.
Although there is effectively no limit on the number of nodes that can be
attached to a bridged network, there are some practical considerations.
Once again, this is due to the delay introduced by each bridge through
which a frame traverses.
Increasing Network
Performance
This section discusses the clustering nature of network users and how
bridges can take advantage of this to provide dramatic performance
improvements.
Work Groups Most companies are divided into groups. These groups exist at many
levels.At one level there may be separate divisions, and within a division
there are usually teams of people associated with a project or department.
The important thing to realize about these groupings, which are often
referred to as Work Groups, is at the communication between members of
a group is of a much higher bandwidth than that between groups. This
communication includes interpersonal communication as well as transfer
of data between computers. These groups must also be able to
communicate with each other, though this will be of a lower bandwidth.
Figure 3: Extending Network
Length

AT-30xxTR Multiport Repeater
5
Placement of Bridges
in a Network The key to effective use of a bridge is to find the Work Groups within your organization and to place bridges between the networks in different
groups. Network traffic within each Work Group is isolated by a bridge
and has no effect on the overall network.When computers from different
groups need to communicate,bridges allow them to do so transparently,as
if there were just one big network.
Simple Bridged Network . In many cases, Work Group boundaries are
clearly defined, and it is obvious where to place bridges.This is especially
true for small organizations, which will only use one or two bridges, or
when previously disjointed networks are being interconnected. Figure 4
below shows this kind of configuration.
Analysis of Large Networks . When a large existing network must be
broken up into smaller pieces, it is useful to perform a thorough analysis
of the network traffic patterns.With e aid of a network monitor, you can
determine the amount of traffic between each node on the network.Work
Group boundaries exist at the points where the smallest amount of traffic
passes. Bridges should always be placed at the point of minimum traffic.
For example, consider the network shown in Figure 5.This network has
four nodes.The traffic patterns are shown in Table 2.
Figure 4: A Simple Bridged
Network
Figure 5: Analysis of Traffic
Patterns

Overview
6
In this case there are two clusters. The first contains nodes 1 and 2, and
the second contains nodes 3 and 4.Within each cluster, the traffic levels
are around 13000 frames per second, while the traffic between the groups
is around 4000 per second.A bridge could be placed between nodes 2 and
3. Note that placing a bridge between nodes and 2 in this case would
make the overall performance worse, because the traffic between these
two stations (130(0 frames/sec) is higher than the throughput of most
bridges.
As the network becomes larger it becomes increasingly difficult to find the
group boundaries without the assistance of network analysis application
programs.Some network installers can provide a network analysis service
to help you decide where to place bridges.
Network Topology A bridged network takes on one of two basic forms.The first is a cascaded
network, where a set of networks are connected together by a series of
bridges.An example of this configuration is given in Figure 6. A cascaded
network is useful when a small number of bridges is being used. If many
bridges are used, the delay associ ated with each bridge may accumulate
to unacceptable levels between networks at the extremities.
The second form used is a tree topology.The simplest form of a tree
topology is one level deep and is known as a backbone network. Figure 7
shows this type of network. ln a backbone network, each Work Group has
its own network, and there is one network used only for communicating
between the Work Groups.The advantages of a tree network far outway
the need for an additional network, and are listed below:
1. There is never more than two bridges between any two networks. The
delays introduced by bridges are therefore minimized.
From To Frames/Second (Average)
1 2 9000
1 2 1000
1 4 1000
2 3 1000
2 4 1000
3 4 9000
Table 2:Traffic Between Nodes
Figure 6: A Cascaded Bridged
Network

AT-30xxTR Multiport Repeater
7
2. As long as the backbone network is operating correctly, any fault within a
Work Group has no effect on other Work Groups.
3. Network faults are isolated to a single Work Group and are therefore
easier to find.
Mixing Media The AT-6870/ AT-6875 MAC layer bridge can also be used to interconnect
networks that use different media.
You may use Thin Ethernet on one channel, and Ethernet on the other
channel.This is not a good enough reason to use a bridge, however, since
inexpensive conversion devices are available. If you do need to install a
bridge and you need to interconnect Thick and Thin Ethernet, the
CentreCOM MAC Layer Bridge can save space and money because it has
built-in Thin Ethernet tranceivers. Figure 8 shows a bridged network
containing Ethernet subnetworks.
Figure 7: A Backbone Network
Figure 8: Mixing Media

Overview
8
Fault Tolerance and
Isolation A single large network has several problems relating to faults.Two are
listed below:
1. If the network fails all users lose connectivity.
2. Because of the network size, it may take a long time to find the fault.
Figure 9 shows the failure of a simple network.
When bridges are used,a network becomes much more fault tolerant.This
is especially true if a backbone network topology is used.
With a cascaded network,a fault will split the network into two disjointed
parts.This is shown in Figure 10.
In a backbone network two possibilities arise. If the backbone fails, the
network will be partitioned into as many partitions as there are networks
attached to the backbone. However, in the more common case of a failure
within one of the attached networks, only that network is affected, while
all other networks can still communicate with each other.This is shown in
Figure 11.
Figure 9: Failure on a Simple
Network
Figure 10: FailureofaCascaded
Bridged Network

AT-30xxTR Multiport Repeater
9
A bridged network is therefore inherently more fault tolerant, providing
service to many users even when network components fail.This has the
side effect of making it much easier to isolate the cause of the network
fault. Since only a small part of the bridged network fails, this provides a
much smaller area within which to search for a problem.
Using Backup Bridges The CentreCOM MAC Layer Bridge uses the IEEE 802.1 draft standard
STP to determine the network topology. If more than one bridge is
connected between the same two networks, one bridge will automatically
be designated as the master, and the others as slaves. Only the master
may forward traffic. If the master fails, however, one of the slave bridges
will become the new master and take over forwarding responsibility.
Figure 11: FailureofaBackbone
Network
Figure 12: Using a Backup
Bridge

Overview
10
The network depicted in Figure 12 would continue to operate if one of the
bridges was turned off accidentally,or if a cable between one of the bridges
and a network was unplugged.
By using backup (or redundant) bridges, it is possible to design a network
topology that is extremely resilient to failure. Unlike a normal bridged
network, which minimizes the loss of network availability, networks with
backup bridges may eliminate loss of availability even with multiple
network failures.Figure 13 shows a network configured in such a fashion.
Each subnetwork is connected to a single backbone network, and also to
each of its neighboring networks. Networks A and B can still communicate
after the failure of any other two networks.
Figure 13: Advanced Use of
Backup Bridges

AT-30xxTR Multiport Repeater
11
Installation and Operation
Unpacking Rough handling during shipping can damage electronic equipment.After
you unpack the CentreCOM MAC Layer Bridge, check carefully for
possible damage. If you notice any damage, contact the shipper.
Direct any additional questions about damage to the distributor from
whom the unit was purchased, or contact Allied Telesyn:
Allied Telesyn International
950 Kifer Road
Sunnyvale, CA 94086
Tel: [phone]
Fax: [phone]
Save the shipping carton and packing material in case you need to store or
ship the bridge back in the future.
Site Selection TheCentreCOM MAC Layer Bridge should be placed in a cool, well-
ventilated area. Air vents are located on each side of the bridge.The back
of the unit also has a fan vent. It is very important that none of the air
vents are obstructed by any material, wall, or other equipment.There
should be several inches of clearance between these vents and any other
object.
Failure to follow these ventilation guidelines may result in overheating
and eventual damage of the bridge.
Factory Configuration The CentreCOM MAC Layer Bridge is configured at the factory to use
Ethernet or IEEE 802.3 10BASE5 on both channels. If the bridge is used
in this environment, no additional configuration is required.
Changing The Factory
Configuration To use the bridge in other modes, some jumpers inside the unit must be relocated.To do this, follow the directions below:
1. Remove the power plug.
2. Remove the two screws on each side of the bridge that hold the top cover
in place.
3. Remove the top cover. The cover fits tightly, and it may be necessary to
use a screwdriver to initially dislodge the cover.
4. If necessary, carefully remove thin-Ethernet transformers with an IC
pulleror a flat screwdriver.Reposition and install thetransformers at the
specified location. Check for bent pins or a shifted transformer.
5. Replace the top cover and screws.
Caution: Make sure that the power cord is removed from the bridge before
opening the cover.

Overview
12
Settings
Thin Ethernet Transformer . Each channel has a transformer IC
component that is only for thin Ethernet operation.This component must
be moved to a spare socket if thin Ethernet is not used.The placement of
these transformers is given below in Table 3
Configuring Channel 1 Note: Channel 1 is set at the factory to operate with Ethernet or IEEE
803.2 10BASE5.
Setting Channel 1 for Thin Ethernet Operation:
1. Remove any transformer in position T4A.
2. Install the transformer into position T4.
Setting Channel 1 for Ethernet or IEEE 802.310BASE5 Operation:
(Channel 2 Factory Setting)
Figure 14: Location of Headers
and Transformers
Network Type Channel 1 Channel 2
Ethernet T4A (Thick) T2A (Thick)
IEEE 802.3 10BASE5 T4A (Thick) T2A (Thick)
Thin Ethernet T4 (Thin) T2 (Thin)
Table 3:Thin Ethernet Transformers

AT-30xxTR Multiport Repeater
13
1. Remove any transformer in position T4.
2. Install the transformer into position T4A.
Configuring Channel 2
Note: Channel 2 is set at the factory to operate with Ethernet or IEEE 803.2
10BASE5.
Setting Channel 2 for Thin Ethernet Operation:
1. Remove any transformer in position T2A.
2. Install the transformer into position T2.
Setting Channel 2 for Ethernet or IEEE 802.3 10BASE5 Operation:
(Channel 2 Factory Setting)
1. Remove any transformer in position T2.
2. Install the transformer in position T2A.
Operation
Power On When the power is first applied, the green PWR and red DIAG LEDs will
be lit. Each of the four green network activity LEDs will be OFF.
Diagnostics The CentreCOM MAC Layer Bridge executes a comprehensive set of
diagnostics when the power is first applied.The red DIAG LED will flash
ON for each test that is executed.After successfully executing the
diagnostic tests, the DIAG LED will be turned OFF. If any tests fail, the
DIAG LED will remain ON. In the latter case, the bridge should be
resumed for service.
Normal Operation Under normal operation, the four green activity LEDs will reflect the
reception and transmission of data packets on each channel.The activity
LEDs have the following meaning (Table 4):
The CentreCOM MAC Layer Bridge contains software that allows two or
more bridges to interconnect the same two networks. This feature allows
the use of backup bridges that take over when a bridge or its network
connection fails.The bridges communicate with each other to implement
this feature. Because of this, the activity LEDs will always blink about
once every few seconds, even if there is no other network activity.
Troubleshooting The next thing to check is that both network connections are firmly made
to the bridge and to the network.
Name Meaning When On
TX1 Channel 1 is forwarding a packet
RX1 Channel 1 is receiving a packet
TX2 Channel 2 is forwarding a packet
RX2 Channel 2 is receiving a packet
Table 4:Activity LEDs

Overview
14
The network itself may be faulty.This can usually be checked
independently of the bridge by establishing communication between two
computers that are directly attached to the network.
Finally, you should check that the bridge configuration matches the
network attachments being used.Turn off the power and remove the
power cord from the bridge. Remove the cover and inspect the intemal
configuration. Refer to the earlier section on bridge configuration for the
correct settings and make sure that:
1. All shunts are firmly installed in the correct locations.
2. The thin Ethernet transformers are inshiled in.the correct locations.
3. The thin Ethernet transformers do not have any bent pins.
4. The thin Ethernet transformers have all the pins inserted in the socket.
5. The thin Ethernet transformers are not shifted in the socket.