Allied Telesis CentreCOM AT-3675 Bridge User manual

Allied
Telesyn
CentreCOM 
AT-3675 Bridge
SNMP Manageable
User Manual

 Copyright 1994 Allied Telesyn International, Corp.
All rights reserved. No part of this publication may be reproduced without prior written permission from
Allied Telesyn International, Corp.
Allied Telesyn International, Corp. 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 International, Corp. 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
International, Corp. has been advised of, known, or should have known, the possibility of such damages.
Trademarks: CentreCOM is a registered trademark of Allied Telesyn International, Corp. Ethernet is a
registered trademark of Xerox Corporation. UNIX is a registered trademark of UNIX System
Laboratories. Novell and NetWare are registered trademarks of Novell, Inc. Microsoft and MS-DOS are
registered trademarks and LAN Manager and Windows for Workgroups are trademarks of Microsoft
Corporation. 3Com is a registered trademark of 3Com. PC-NFS is a trademark of Sun Microsystems, Inc.
PC/TCP is a registered trademark of FTP Software, Inc. DECnet is a registered trademark of Digital
Equipment Corporation.

xi
Table Of Contents
Electrical Safety and Installation Requirements .....................................................i
Table Of Contents ..........................................................................................................xi
Chapter 1
Overview ........................................................................................................................... 1
CentreCOM AT-3675 SNMP-Manageable Bridge...................................................................................... 1
Performance ............................................................................................................................................... 1
Management............................................................................................................................................... 2
Physical Description........................................................................................................................................ 2
Features Summary.......................................................................................................................................... 3
Chapter 2
Installation ....................................................................................................................... 5
Site Selection..................................................................................................................................................... 5
Factory Configuration..................................................................................................................................... 5
Changing Configuration with Transceivers................................................................................................. 6
Switching Configuration with Jumpers........................................................................................................ 6
Upgrading Bridge Firmware......................................................................................................................... 8
Chapter 3
Operation .......................................................................................................................... 9
Power On........................................................................................................................................................... 9
Diagnostics........................................................................................................................................................ 9
Normal Operation............................................................................................................................................. 9
Backup Operation........................................................................................................................................... 10
Application Guidelines................................................................................................................................... 10
Fault Tolerance and Isolation....................................................................................................................... 14
Chapter 4
Troubleshooting ............................................................................................................ 17
Product Code and Serial Number................................................................................................................ 17
Appendix A
Data Cabling Techniques ............................................................................................ 19

xii
10BASE-T....................................................................................................................................................... 19
UTP Wiring
Specifications.................................................................................................................................. 19
10BASE-T Point-to-Point Cabling....................................................................................................... 20
10BASE-FL/FOIRL Ethernet ................................................................................................................... 23
10BASE2 (Thin) Ethernet........................................................................................................................... 23
10BASE5 (Thick) Ethernet ......................................................................................................................... 23
AUI Drop Cables.................................................................................................................................... 24
Appendix B
Glossary ............................................................................................................................25
Appendix C
Technical Support Fax Order .....................................................................................31
Incident Summary......................................................................................................................................... 31
Appendix D
CentreCOM AT-3675 Manual Feedback ....................................................................33
Index .................................................................................................................................35

1
Chapter 1
Overview
CentreCOM AT-3675 SNMP-Manageable Bridge
The CentreCOM AT-3675 SNMP-Manageable Learning Bridge allows you
tointerconnect two Ethernet sub-networks.Eachsub-networkmaybeIEEE
802.3 10BASE2 (thin) Ethernet or 10BASE5 (thick) Ethernet or pre-802.3
Ethernet. Unshielded Twisted Pair (UTP) can also be connected to the bridge
through a transceiver. The AT-3675 bridge transparently forwards packets.
Since the bridge operates at the Media ACcess (MAC) level, all higher level
protocols, such as Transmission Control Protocol (TCP), DECnet™ and
NetWare®, are passed unaffected.
Performance The four repeater rule does not apply across the bridge. The effective length
of a Local Area Network (LAN) can be increased by adding a CentreCOM
AT-3675 SNMP-Manageable Learning Bridge. Placing bridges at natural
boundaries of the workgroups increases the network performance at each
segment by diminishing network traffic. The reliability of a large network can
be enhanced by a bridge and allows sub-networks of different media types to
be interconnected.
The CentreCOM AT-3675 is self-learning, adjusting to reconfiguration of sub-
nets to provide continuous isolation of local traffic. The AT-3675 provides
packet filtering at 25,000 Packets Per Second (PPS) and sustained forwarding
at 12,500 PPS. Its filter table supports 2,048 MAC-level station addresses.
Multiple AT-3675 bridges can be used as backups in case of network failure.
The AT-3675 supports the IEEE 802.1 Spanning Tree Protocol (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
Management Simple Network Management Protocol (SNMP) is supported by the AT-3675.
The bridge can be monitored and manipulated by many SNMP-compatible
management stations. Since it supports the second version of the
Management Information Base (MIB-II) and Bridge MIB, parameters of the
AT-3675 can be managed under SNMP including custom filtering, static
forwarding entries and STP.
Physical Description
The AT-3675 bridge is housed in the slim-line chassis. All network
attachments, the power connector and status indicators are located on the
front panel. See Figure 1.
Media selection for the bridge’s A and B channels are controlled by jumper
blocks on the main board inside the chassis.
There are three pairs of indicators on the AT-3675 front panel. Two pairs
indicate network activity forChannels A and B. The third pair of indicators is
System Power and Diagnostics. The names of the indicators and their
functions are described in Table 1.
Table 1: AT-3675 Front Panel Indicators
The two 10BASE2 network connectors are standard Bayonet Nut Couples
(BNCs). The two 10BASE5 network connectors are standard Attachment
Unit Interfaces (AUIs) and accept standard 10BASE2, fiber optic
(10BASE-FL, FOIRL), 10BASE-T transceivers and 10BASE5 drop cables.
Since the AT-3675 bridge has the same dimensions as the AT-3600 Series
Repeaters, it may be mounted in an ATI rackmount or desktop chassis.
Indicator Function
Transmit A Channel A Transmitting
Receive A Channel A Receiving
Transmit B Channel B Transmitting
Receive B Channel B Receiving
Diagnostics Diagnostic Test Status
System Power Power is ON
Figure 1: AT-3675 Front Panel
POWERSYSTEM ACTIVITY
Illumination indicates
line voltage available.
Indicator continually illuminated
during self test, then off.
Indicator will flash if error detected.
CHANNEL BCHANNEL B
TRANSMIT
RECEIVE
TRANSMIT
RECEIVE
10BASE2 10BASE5 DIAGNOSTICS
3675 CentreCOMTM
IEEE 802.3/ETHERNETBRIDGE
with SNMPManagement
CHANNEL A
10BASE2 10BASE5
CHANNEL A
SYSTEM POWER

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3
Not all chassis slots must be populated and populated slots need not be
contiguous. Table 2 lists chassis models available.
Table 2: Department Concentrators Chassis Available
Features Summary
❑Self -learning SNMP-Manageable Learning Bridge
❑Filter at 25,000 PPS
❑Forward at 12,500 PPS
❑Thinnet (for 10BASE2) and thicknet (for 10BASE5 or pre-802.3
Ethernet) media options on each network segment
❑Fiber optic (10BASE-FL, FOIRL) or UTP with the addition of an
optional transceiver(s)
❑Supports 2,048 MAC addresses in filter table
❑IEEE 802.1 revision D STP
❑Standalone, desktop, or rackmount, chassis installation
❑SNMP management
❑MIB-II and Bridge MIB (RFC 1493)
❑One year free software upgrades
❑One year warranty
Number of Modules Desktop Model Rackmount Model
1 AT-36E1 AT-36C1
2 AT-36E2 AT-36C2
3 AT-36E3 AT-36C3
4 AT-36E4 AT-36C4
8 AT-36E8 AT-36C8

5
Chapter 2
Installation
Before installing the CentreCOM AT-3675 SNMP-Manageable Learning
Bridge, check to see that the package contents are complete. If any of the
following items are missing or damaged, contact your sales
representative.
❑CentreCOM AT-3675 SNMP-Manageable Learning Bridge
❑User Manual
❑Warranty statement and registration cards
❑Power cord (U.S.A. version only)
Save the shipping carton and packing material in case you need to ship or
store the bridge in the future.
Site Selection
Before installing the AT-3675 bridge, read the safety installation
requirements starting on page i.
The AT-3675 bridge should be placed in a cool, well-ventilated area. Air vents
are located on each side of the bridge. It is very important that none of the
vents is obstructed. Several inches of clearance should be allowed between
these vents and any other object, e.g., a wall or other equipment.
Failure to follow these ventilation guidelines may result in overheating and
eventual damage of the bridge.
Factory Configuration
The AT-3675 bridge is configured at the factory to use the AUI ports on both
channels. The AUI connectors (labeled 10BASE5) are active. If you use the
bridge in anIEEE 802.3 10BASE5 (or a pre-802.3Ethernet) environment, no
additional configuration is required.
!
Attention
!
Caution

Installation
6
Changing Configuration with Transceivers
You can attach cable of different media to one or both channels via a
transceiver. Table 3 list the transceivers avaiable.
Table 3: ATI Micro Transceiver Choices
Switching Configuration with Jumpers
Each channel can be switched between the BNC connector (labeled
10BASE2) and the AUI connector (labeled 10BASE5). To switch connectors,
you move a jumper block inside the bridge chassis.
Channels A and B are set at the factory to operate with pre-802.3Ethernet or
IEEE 802.3 10BASE5.
To determine which port is connected
If you are not sure which port is active for a given channel, you can check as
follows:
1. Connect one of the channel’s two ports to a cable that is transferring data.
2. If the channel’s Receive indicator flashes, you have selected the active
port, if not, the other port is active.
To change the jumper block
If you need to switch port connections for Channel A and/or Channel B, find
the locations of the jumper blocks as shown in Figure 2. Look up the proper
jumper block position(s) as listed in Table 4.
1. Remove the power cord and any cables from the AT-3675 front panel.
Openingthebridge without unplugging the powercordmayexposepersonnel
to high voltage, risking the possibility of injury or death. See safety
statements starting on page i.
2. Remove the three screws along the front edge of the top.
3. Turn the bridge upside-down and loosen the top cover by removing the
three screws along each side and the three screws along the back.
4. Remove the top cover by sliding it towards the back. If the cover fits
tightly, use a screwdriver to dislodge it.
Model Description
AT-MX25F Fiber Optic (FOIRL, 10BASE-FL), SMA Connector
AT-MX26F Fiber Optic (FOIRL, 10BASE-FL), ST Connector
AT-210T/AT-210TS/AT-MX20T UTP (10BASE-T)
AT-MX10/AT-MX10S Thin Ethernet (10BASE2)
Note
Warning

CentreCOM AT-3675 SNMP-Manageable Learning Bridge
7
The person handling the jumper block(s) must be grounded to prevent
electrostatic discharge.
5. Carefully remove jumper block(s) with an IC puller or a flat screwdriver.
6. Carefullyrepositionand reinstall the jumperblock(s).Checkforbentpins
or misalignment between the jumper block and socket.
7. Reinstall the top cover and screws.
Table 4: Jumper Block Options for Media Selection
Figure 2: Location of Jumper
Blocks/Sockets
Network Type Channel A Channel B
Pre-802.3 Ethernet T4A (Thick) T2A (Thick)
IEEE 802.3 10BASE5 T4A (Thick) T2A (Thick)
IEEE 802.3 10BASE2 T4 (Thin) T2 (Thin)
!
Caution
T4A
THICK THIN
T4 T2A
THICK THIN
T2
FRONT
REAR
CHANNEL A CHANNEL B

Installation
8
Upgrading Bridge Firmware
The bridge firmware resides in PROMs on the motherboard. See Figure 3.
To swap out the firmware PROMs
1. Remove the power cord and any cables from the AT-3675 front panel.
Openingthebridge without unplugging the powercordmayexposepersonnel
to high voltage, risking the possibility of injury or death. See safety
statements starting on page i.
2. Remove the three screws along the front edge of the top.
3. Turn the bridge upside-down and loosen the top cover by removing the
three screws along each side and the three screws along the back.
4. Remove the top cover by sliding it towards the back. If the cover fits
tightly, use a screwdriver to dislodge it.
The person handling the PROM(s) must be grounded to prevent electrostatic
discharge.
5. Carefully remove PROM(s) with an IC puller or a flat screwdriver.
6. Carefully reposition and reinstall the PROM(s). Check for bent pins or
misalignment between the PROM and socket.
7. Reinstall the top cover and screws.
Figure 3: Location of the
PROMs on the AT-3675 Logic
Board
Warning
!
Caution
Front Panel of Bridge
Channel A LEDs
ODD EVEN B6 B7 B8
B8B11
Transmit Receive

9
Chapter 3
Operation
This chapter describes the features of the CentreCOM AT-3675 SNMP-
Manageable Learning Bridge and shows the basic operating procedures. It
covers how the bridge can be used to overcome common physical limitations
and how to employ the bridge in such a way that network performance is
improved.
Power On
When the power is first applied, the green System Power and red Diagnostics
indicators are illuminated. The green network Activity indicators are OFF.
Diagnostics
The AT-3675 bridge executes a comprehensive set of diagnostics when the
power is first applied. The red diagnostic LED stays ON and the port’s
Receive and Transmit indicators flash as the tests are executed. The
Diagnostic indicator turns OFF after successful completion of the tests. If
any tests fail, the Diagnostic indicator remains ON. In the latter case, the
bridge should be returned for service. Refer to the inside back cover for the
ATI location near you.
Normal Operation
Under normal operation, the green Activity indicators reflect the reception
and transmission of data packets, as summarized in Table 5.
Table 5: Activity LEDs
Indicator Meaning When ON
Transmit Channel is Forwarding a Packet
Receive Channel is Receiving a Packet

Operation
10
Backup Operation
The AT-3675 bridge software allows you to establish redundant
interconnections between the same two networks. This feature provides
backup should the bridge or its network connection fail. The primary bridge
and its backup communicate with each other in accordance with the SNMP-
standard STP protocol. Because of this signal, the Activity indicators will
blink every few seconds, even if there is no other network activity. The STP
algorithm can be disabled using a network management package such as
SunNet™ Manager or SNMPc.
Application Guidelines
Inplanning the placementofbridges, network topology, network length,node
count and natural cluster boundaries are all factors you must take into
consideration for the best performance.
Network Topology A network with bridged sub-networks takes on one of two basic forms. The
first is a cascaded network, where a set of sub-networks are connected
together by a series of bridges. An example of this configuration is given in
Figure 4. A cascaded-bridged network is useful when the number of bridges
is small. If many bridges are used, the delay associated with each bridge may
accumulate to unacceptable levels between networks at the extremities.
The second form used is a tree topology. The simplest configuration of a tree
topology is one level deep and is known as a backbone network. Figure 5
showsthis type oftreetopology. In abackboneconfiguration,each workgroup
has its own network, and there is one network used only for communicating
between the workgroups. The advantages of a tree network topology are
listed below:
❑There are never more than two bridges between any two
sub-networks. The delays introduced by bridges are therefore
minimized.
❑As long as the backbone network is operating correctly, any fault
within a sub-network has no effect on other sub-networks.
❑Faultsareisolatedto a single sub-network and are thereforeeasierto
find.
Figure 4: A Cascaded Bridged
Network
Sub-network A Sub-network B Sub-network C Sub-network D
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Mixing Media The AT-3675 bridge can also be used to interconnect network segments that
use different media.
You may use any medium on either channel. This alone is not a good reason to
use a bridge since inexpensive conversion devices are available. However, if
you need to install a bridge and need to interconnect different media, the
AT-3675 can apply in this case. Figure 6 shows a network that mixes media
using a bridge.
Figure 5: A Backbone Network
Sub-network A
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Sub-network B Sub-network C
Backbone Network
Figure 6: Example of Mixing
Media
Thick
Ethernet
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Thin
Ethernet

Operation
12
Increasing Maximum
Network Length A bridge may be used to link sub-networks serially in the same way a
repeateris used. Figure 7 showsthisuseofthe cascaded-bridge topology. The
maximum network length only applies to individual sub-networks, so that the
overall network length is not bound by any limit. Since there are practical
limits on how many bridges can be used in this manner, however, each bridge
introduces a small delay in forwarded traffic.
Exceeding Maximum
Number of Stations The maximum number of stations attached to a single network is 1,024. 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 passes.
Improving Network
Performance In one way or another companies are divided into groups. Network users can
be considered in terms of natural clusters or workgroups. At one level there
may be separate divisions and within a division there are usually teams of
people associated with a project or department. When placing new bridges,
you can take advantage of natural clustering to provide dramatic network
performance improvements.
The important thing to realize about natural clusters is that the
communication among members of a group produces a much higher volume of
network activity than communication between groups. Communication
within groups includes inter-personal communication as well as transfer of
data between computers. Although groups must be able to communicate with
each other, inter-group communication produces less traffic.
Placement of Bridges. The key to effective use of bridges is to find the
boundaries between workgroups within your organization and to split the
network by adding bridges at natural boundaries between different groups.
Network traffic within each workgroup 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 as if they were one network.
Figure 7: Extending Network
Length
Sub-network A Sub-network B
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Simple Bridged Network. In many cases, workgroup boundaries are
clearly defined and it is obvious where to place the bridges. This is especially
true for small organizations, which will only use one or two bridges or when
previously separate networks are being interconnected. Figure 8 below
shows this kind of configuration.
Analysis of Large Networks. When a large existing network is broken up
into smaller networks, it is useful to perform a thorough analysis of the
network traffic patterns. With the aid of a network monitor, you can
determine the amount of traffic between nodes throughout the network.
Workgroup boundaries (or natural cluster boundaries) exist at the points
where the smallest amount of traffic passes. Bridges should always be placed
at these points of minimal traffic.
For example, consider the network shown in Figure 9. This network of four
nodes exhibits traffic patterns listed in Table 6.
Figure 8: A Simple Bridged
Network
Marketing Engineering
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Figure 9: Analysis of Traffic
Patterns
Group 1 Group 2
Node 2
Node 1 Node 4
Node 3

Operation
14
Table 6: Traffic Patterns
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 13,000 frames per second, while the traffic between the groups is
around 4,000 per second. A bridge could be placed between nodes 2 and 3.
Placing a bridge between nodes 1 and 2 in this case would make the overall
performance worse, because the traffic between these two stations (13,000
frames/second) 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. In these cases, you may wish to consult network installers who can
provide a network analysis service to help you decide where to place bridges.
Fault Tolerance and Isolation
A single large network has several problems relating to faults. Two are listed
below:
❑If the network fails and coax (thin and thick) is used, all users lose
connectivity
❑Dependingonthenetworksize,itmaytakealongtimetofindthefault
Figure 10 shows the failure of a simple 10BASE2 or 10BASE5 network.
When bridges are used, a network becomes much more fault tolerant. This is
especially true if a backbone network topology is used.
From Node To Node Frames/Second (Average)
1 2 9000
1 3 1000
1 4 1000
2 3 1000
2 4 1000
3 4 9000
Note
Figure 10: Failure of a Simple
Coax Network Without a
Bridge
Failure here affects
all nodes.

CentreCOMAT-3675 SNMP-Manageable Learning Bridge
15
With a cascaded network, a fault will split the network into two disjointed
parts. This is shown in Figure 11.
In a backbone network, two possibilities arise. If the backbone fails, the
network will be partitioned into as many partitions as there are sub-networks
attached to the backbone. However, in the more common case of a failure
within one of the attached sub-networks, only that sub-network is affected,
while all other sub-networks can still communicate with each other. This is
shown in Figure 12.
A bridged network is therefore inherently more fault tolerant, providing
service to many users even when network components fail. This has the
advantage 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 to search for a problem.
Figure 11: Failure of a Cascad-
ed- Bridged Network
Sub-network A Sub-network B Sub-network C Sub-network D
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Failure here partitions
the network: 
1) Sub-nets A and B are up.
2) Sub-net D is isolated.
3) Sub-net C is down.
Figure 12: Failure of a Back-
bone Network
Sub-network A
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Sub-network B Sub-network C
Backbone Network
Failure here only affects Sub-network B.
Sub-networks Aand C are still connected.