Multi-segment token-ring self study

Madge.connect
Madge Training
TRN-02A
Multi-segment Token Ring
Self-Study Guide

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Madge Training TRN-03 - Multisegment Token Ring
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Copy of Sg_trn02A © Madge Networks Page 1 of 93
Table of Contents
1. COURSE PREREQUISITES AND OBJECTIVES .................................................3
1.2 COURSE OVERVIEW ................................................................................................ 3
1.3 COURSE PREREQUISITES ......................................................................................... 3
1.4 TOPICS YOU WILL COVER ......................................................................................... 3
1.5 COURSE OBJECTIVES.............................................................................................. 4
2. COURSE MAP ................................................................................................5
3. HOW TO USE THIS GUIDE..............................................................................6
3.1 WHAT IS COVERED IN THIS GUIDE ............................................................................. 6
3.2 HOW TO USE THIS GUIDE......................................................................................... 6
4. REVIEW TOKEN RING BASICS .......................................................................6
4.1 ANSWERS TO REVIEW.............................................................................................. 8
5. BRIDGING FUNDAMENTALS.........................................................................11
5.2 SERIAL BRIDGES .................................................................................................. 17
5.3 PARALLEL BRIDGES .............................................................................................. 18
5.4 REMOTE BRIDGES ................................................................................................ 18
5.5 BACKBONE TOPOLOGY .......................................................................................... 19
5.6 DUAL BACKBONES................................................................................................ 21
5.7 BRIDGING METHODS ............................................................................................ 22
5.8 TRANSPARENT BRIDGE OPERATION .......................................................................... 23
5.9 SOURCE ROUTE BRIDGE OPERATION....................................................................... 28
5.10 SOURCE ROUTE TRANSPARENT BRIDGES................................................................ 30
5.11 WHAT MAKES A ROUTER DIFFERENT FROM A BRIDGE?............................................... 30
5.12 SELF STUDY: COMBINING BRIDGING AND ROUTING .................................................. 33
5.13 TEST YOUR UNDERSTANDING: BRIDGING FUNDAMENTALS.......................................... 34
6. THEORY OF SOURCE ROUTE BRIDGING......................................................36
6.1 SELF STUDY : - RING NUMBERS AND BRIDGE NUMBERS.............................................. 37
6.2 RING NUMBERS AND BRIDGE NUMBERS - 3 RULES TO REMEMBER ................................. 38
6.3 SELF STUDY - DRAW A FRAME WITH THE MAXIMUM NUMBER OF HOPS............................ 40
6.4 TEST YOUR UNDERSTANDING: THEORY OF SOURCE ROUTING....................................... 43
7. THEORY OF SOURCE ROUTE BRIDGING: EXPLORER FRAMES ....................44
7.1 SELF STUDY: ALL ROUTES EXPLORERS .................................................................... 45
7.2 WHY USE ALL ROUTES EXPLORERS? ....................................................................... 46
7.3 DISADVANTAGES OF ALL ROUTES EXPLORERS........................................................... 46
7.4 WHY USE SPANNING TREE EXPLORERS?................................................................... 48
7.5 EXPLORATION STRATEGIES .................................................................................... 50
7.6 PROS AND CONS OF ARE OUT SR RETURN................................................................ 51
7.7 PROS AND CONS OF STE OUT ARE BACK ................................................................. 52
7.8 PROS AND CONS OF STE OUT SR BACK ................................................................... 52
7.9 CONTROL HEADER FIELDS..................................................................................... 52
7.10 BRIDGE DECISION PROCESS ................................................................................ 55
7.11 REVIEW OF EXPLORATION STRATEGIES: A REAL WORLD ISSUE.................................... 55
7.12 TEST YOUR UNDERSTANDING: EXPLORER FRAMES................................................... 57
8. SPANNING TREE..........................................................................................57
8.1 ADDRESSING FORMATS FOR SPANNING TREE FRAMES ................................................. 60
8.2 CANONICAL AND NON-CANONICAL ADDRESSING FORMATS ............................................ 60
8.3 ELECTION OF ROOT BRIDGE - THE BRIDGE ID........................................................... 62
8.4 DETERMINING THE DESIGNATED BRIDGES - ROOT PATH COST ..................................... 65

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8.5 SELF-STUDY: PREDICTING THE SPANNING TREE......................................................... 66
8.6 REVIEW SPANNING TREE ....................................................................................... 67
8.7 TEST YOUR KNOWLEDGE: SPANNING TREE................................................................ 68
9. RING AND BRIDGE NUMBER CONFIGURATION WORKSHOP .........................69
9.1 PRE-REQUISITES FOR THIS WORKSHOP..................................................................... 69
9.2 PRACTICAL OBJECTIVE.......................................................................................... 69
9.3 POINTS TO REMEMBER .......................................................................................... 70
9.4 INSTRUCTIONS TO FOLLOW..................................................................................... 70
9.5 YOU WILL HAVE COMPLETED THIS EXERCISE WHEN..................................................... 70
10. SPANNING TREE WORKSHOP ....................................................................72
10.1 PRE-REQUISITES FOR THIS WORKSHOP ................................................................... 72
10.2 PRACTICAL OBJECTIVE........................................................................................ 72
10.3 POINTS TO REMEMBER ........................................................................................ 72
10.4 INSTRUCTIONS TO FOLLOW ................................................................................... 73
10.5 YOU WILL HAVE COMPLETED THIS EXERCISE WHEN................................................... 74
10.6 SPANNING TREE WORKSHOP: ANSWERS ................................................................. 75
11. EXPLORER FRAMES WORKSHOP...............................................................76
11.1 PRE-REQUISITES FOR THIS WORKSHOP ................................................................... 76
11.2 PRACTICAL OBJECTIVE........................................................................................ 76
11.3 POINTS TO REMEMBER ........................................................................................ 76
11.4 INSTRUCTIONS TO FOLLOW ................................................................................... 77
11.5 ANSWERS TO EXPLORER WORKSHOP ..................................................................... 80
11.6 EXPLORER WORKSHOP CONCLUSIONS.................................................................... 81
12. APPENDIX- TEST YOUR UNDERSTANDING: ANSWERS................................83
12.1 BRIDGING FUNDAMENTALS .................................................................................. 83
12.2 SOURCE ROUTING THEORY .................................................................................. 85
12.3 EXPLORER FRAMES............................................................................................. 85
12.4 SPANNING TREE................................................................................................. 86
13. NOVELL® SOURCE ROUTE SERVER END STATION SOFTWARE: UPDATE ...86
14. EXPLORER FRAMELOGS FOR WINDOWS95 ...............................................87
15. APPENDIX- GLOSSARY OF TERMS .............................................................89

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1. Course prerequisites and objectives
1.1.1 Course code TRN-03
TRN-03 Multi-segment Token Ring 6 hours
1.1.2 Teaching method
Self-paced learning
1.2 Course overview
TRN-03 provides an introduction to principles of building multi-segment Token Ring
networks followed by a detailed analysis of source route bridging.
We look at the possibilities of optimising the strategy of clients when setting up a
connection to a server by analysing the use of broadcast frames.
The control of broadcast frames is a key strength of the Madge Ringswitch, hence a
clear understanding of these principles is indispensable prior to learning how to
configure the Ringswitch
1.3 Course prerequisites
Students are expected to have completed the following or equivalent courses:
TRN-01 Single segment Token Ring
TRN-02 Madge hubs
1.4 Topics you will cover
• Key bridging techniques • Madge Smart Ringbridge
• Transparent bridging
• Source Route bridging
• Bridge Configuration
• Spanning Tree
• The Routing Information Field • Exploration strategies
• Hop count control • Multi-segment Token Ring design
essentials

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1.5 Course objectives
By the end of this course you will be able to:
• Identify poor multisegment network design
• List benefits of key Token Ring bridge topologies
− Source Route bridging
− Transparent bridging
− Routing
• Describe how Transparent bridges filter and forward frames
• Describe how Source Route bridges filter and forward frames
• State the 2 purposes of Spanning Tree for Token Ring networks
• Draw and notate a simple correctly resolved Spanning Tree
• Analyse Token Ring traffic flow using RIFs to determine network paths
• Explain how use of RIF can provide load balancing over duplicate routes from
client to server/mainframe
• Use captured traffic to determine client exploration strategy for server connections
− AREs - All Routes Explorers
− STEs - Spanning Tree Explorers
• Explain benefits of AREs, STEs and SR (specifically Routed) frames
• Explain use of AREs and STEs by different providers
• Assess broadcast strategies used in different client/server environments and
implement measures to reduce ARE traffic
• Configure ring numbers and bridge numbers on the Smart Ringbridge
• Fault find basic spanning tree configurations using Trueview and the LCD display

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2. Course Map
Here is an outline of the learning activities for this course which can be taken in one
day. This includes approximate timings. Use this information to pace yourself
through the training.
9.00 Review of Token Ring Basics
Basic Concepts of Bridging and Routing
10.30 Using Source Routing for establishing links between Token Ring stations
12.30 Lunch
1.30 Bridge Configuration Workshop
2.00 Spanning Tree Workshop
3.00 Explorer Frames Workshop
4.30 Review Workshops
5.00 Finish
Course Map

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Student Notes
3. How to use this guide
3.1 What is covered in this guide
The Multi-segment Token Ring Module is a self-paced learning package aimed at
engineers who understand Token Ring basics and wish to learn about Madge Token
Ring switching. To do this you need to grasp the principles of source route bridging
and transparent bridging with Token Ring.
3.2 How to use this guide
This manual includes a variety of activities which allow you to cover the topics in a
way which will suit your own style of learning.
Look out for the icons on the left. Try to cover all the suggested practical activities
and check your knowledge using the workbook reviews.
If you are taking Madge Certification tests make sure you cover the topics and
objectives listed in the first chapter as well as the more detailed objectives for each
module or chapter.
4. Review Token Ring basics
ICON KEY
) Key
Information
Workbook
Review
" Self-Study
Lab Exercise

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Student Notes
Before we start work on Bridging Concepts make sure you have grasped the basic
concepts of Token Ring itself. This is covered in the first Token Ring course known as
TRN-01.
1. Under token ring a “special” node is designated to be in charge of the ring and
generating and controlling the token. This node is called the:
a) Ring Error Monitor
b) Active Monitor
c) Configuration Monitor
d) Active Report Server
2. The IEEE Token Ring standard occupies which two layers in the ISO model?
a) Layer 1 2
b) Layer 1 3
c) Layer 2 3
d) Only resides in layer 2
3. When a data frame is sent around a Token Ring:
a) It is copied into every station on the ring and then passed on
b) It is copied into every active station on the ring and then passed on
c) It is copied into the destination and then passed on
d) It is removed by the destination and an acknowledgement is passed on
4. The following diagram describes certain so-called functional addresses
available on certain token ring stations
Active Monitor (C000 0000 0001)
Ring Parameter Server (C000 0000 0002)
Ring Error Monitor (C000 0000 0008)
Configuration Report Server (C000 0000 0010)
NetBIOS (C000 0000 0080)
Bridge (C000 0000 0100)
Functional Address Bits
The functional address bits are used to allow:
a) a node to adopt a single specific function e.g. Active Monitor
b) a node to adopt multiple functions simultaneously e.g. bridge and
network management (REM)
c) network management applications to monitor and gather statistics
5. What is the principle function of MAC frames ?
) Only one of the possible answers is correct. Read all the answers carefully

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Student Notes
a) to maintain the operation of the ring
b) fault diagnosis
c) to carry data
d) to purge the ring
6. Look at the diagram of a Token Ring frame below. The CRC check is done to
ensure that
a) Any part of a frame received by any station has not been corrupted
b) The data portion carried by the frame has not been corrupted
c) The frame does not have the CRC bit set
d) Frame type, address and data in the frame have not been corrupted
SDEL
Access
Control
EDEL FSFC Destination
Address
Source
Address
DATA
FCS
(CRC)
Priority
Token or Frame ?
Frame type:
- MAC frame
- Data frame
Active monitor ?
Frame Check
Sequence
Address Recognised ?
Frame Copied ?
1 byte 1 byte 1 byte 6 bytes 6 bytes 1 byte 1 byte4 bytes
Last Bit (EDI)
Error Detected
Optional
RIF
CRC checked
4.1 Answers to review
1. Under token ring a “special” node is designated to be in charge of the ring and
generating and controlling the token. This node is called the:

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Student Notes
a) Ring Error Monitor
b) Active Monitor
c) Configuration Monitor
d) Active Report Server
The first active adapter or node on the ring will become the Active Monitor and
will manage tokens and frames and generally maintain the ring.
2. The IEEE Token Ring standard occupies which two layers in the ISO model?
a) Layer 1 2
b) Layer 1 3
c) Layer 2 3
d) Only resides in layer 2
The Token Ring standard specifies cabling types and signalling at layer 1-the
physical layer - and a token ring passing protocol for media access
control (MAC) at layer 2 - the datalink layer. A high sublayer (LLC) is
provided as 802.2 which provides access to layer 3 protocols like IP and
IPX. This is not strictly part of the Token Ring protocol, but in practice
MAC and LLC are inseparable.
3. When a data frame is sent around a Token Ring:
a) It is copied into every station on the ring and then passed on
b) It is copied into every active station on the ring and then passed on
c) It is copied into the destination and then passed on
d) It is removed by the destination and an acknowledgement is passed on
4. The following diagram describes certain so-called functional addresses
available on certain token ring stations
Active Monitor (C000 0000 0001)
Ring Parameter Server (C000 0000 0002)
Ring Error Monitor (C000 0000 0008)
Configuration Report Server (C000 0000 0010)
NetBIOS (C000 0000 0080)
Bridge (C000 0000 0100)
Functional Address Bits
The functional address bits are used to allow:
a) any node to adopt a single specific function e.g. Active Monitor
b) a specific node to adopt a single specific function e.g. Active Monitor
c) certain nodes to adopt functional roles e.g. bridge or network
management
d) network managers only to monitor and gather statistics

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Student Notes
5. What is the principle function of MAC frames ?
a) to maintain the operation of the ring
b) fault diagnosis and recovery
c) to carry data
d) to purge the ring
6. Look at the diagram of a Token Ring frame below. The CRC is check done to
ensure that.
a) Any part of a frame received by any station has not been corrupted
b) The data portion carried by the frame has not been corrupted
c) The frame does not have the CRC bit set
d) Frame type, address and data in the frame have not been
corrupted

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Student Notes
5. Bridging Fundamentals
By the end of the session
you will be able to...
„ State the purpose of a bridge
„ List bridge topologies and types
– eg serial, loop, backbone, remote
„ List the merits of the backbone topology
„ Compare and contrast bridging methods
– source route
– transparent
„ Explain the difference between a bridge and a
router

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Student Notes
„ Connects two physical rings
„ Single logical network
„ Forwards or Filters Frames
„ Keeps local traffic local
Ring A Ring B
2 4
3 5
1
6
Figure 1. Purpose of a bridge
5.1.1 What is the purpose of a bridge?
The purpose of a bridge is to connect two (or possibly more) rings into one logical
network. This means that when station 2 sends frames to station 3, stations on ring
B will not see the frames.
In other words local traffic does not cross the bridge, it remains local.
However when station 1 needs to talk to station 6, the bridge will allow this. So a
bridge will forward or filter a frame as required. In general the end stations are
unaware that the bridge is performing this task on their behalf.
There are other specific benefits to be gained from bridging between Token Rings.
Use your knowledge of Token Ring to list these benefits. Use the hint words in the
table to get you thinking.
What is the purpose of a Bridge?

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Student Notes
Turn the page when you are ready to review your answers.
• Flexibility
• Performance
• Reliability
• Compatibility
• Security
• Overcoming cabling
distances

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Student Notes
Benefits of bridging with Token Ring
„ Flexibility
– More rings so increased
number of nodes
– 255 on STP, 144 on UTP
ring
„ Performance
– Increase bandwidth
– Each ring has its own token
„ Reliability
– MAC Processes are per ring
– If one ring beacons, no effect
on the other
„ Comaptibility
– Different network
speeds/types
– 16Mbps to 4Mbps
– Token Ring to FDDI
„ Security
– Filters by address or
protocol
„ Overcome cabling
distances
– Remote Link greater than
2KM
5.1.2 Benefits of bridging with Token Ring
There are a great many benefits to be gained from linking Token Ring segments using
bridges.
Segmenting a ring with a bridge provides you with 2 separate rings. This means you
can increase the number of nodes on the network, with up to 255 nodes on an STP
ring and 144 on a UTP ring.
The MAC processes run independently on each Token Ring and each ring has its own
Token. This means two things: firstly performance is improved as each station gets
increased access to the token, thus increasing that station’s share of the 16 Mbps
bandwidth.
Secondly, any failure, whether it be soft errors or beaconing, is confined to the ring
where the problem originated. This means, for example, that a troubleshooting ring
could be created where potentially faulty devices could be isolated without affecting
connected rings. This means the reliability of all rings can be maintained.
Bridges generally work most effectively when connecting LAN segments of the same
speed and network type. A bridge can be used to link a 16Mbps ring with a 4Mbps

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Student Notes
ring and of course this is a very useful feature on networks where there are
applications which must run on 4Mbps rings, but the majority want to take
advantage of 16Mbps.
The down side of this is that in some cases frames will have to be truncated to 4K
from a larger frame size. This means additional processing as the frame must be
stored and then divided up with new token ring headers and trailers being added.
Security is a key feature that a bridge is able to provide. The bridge software can
analyse the MAC and reject a frame based on its MAC address, thus blocking traffic
from a specific workstation. It can also look within the LLC header which contains the
SAP (Service Access Point). This indicates whether a frame is destined for an IPX, IP
or other stack. This allows the bridge to confine certain protocols to one side of the
bridge.
Remote links can be used between sites to overcome the cabling distance limitss of
Token Ring. CAUs for example cannot be further than 2Kms apart and require fibre
connections to achieve this.

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Student Notes
Bridge Topologies
R e m o te b r id g e s
S e r ia l b r id g e s
P a r a lle l b r id g e s
Much of network design is down to common sense and experience.
Apply your existing knowledge and assess serial, parallel and remote topologies
according to the criteria in the leftmost column:
Serial Parallel Remote
Reliability and
Availability
Performance (Hop
Counts, Filtering,
Placing of Servers
Maintainability
(Effect of adding new
rings)

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Student Notes
Bridge Topologies
Remote bridges
Serial bridges
Parallel bridges
„ Frames must cross rings
„ Hop count limitations
„ If one bridge breaks, lose communication
„ Alternative routes
„ Load balancing (in Source Route bridging)
„ Redundancy
„ Overcome physical distance limitations
„ Counted as one hop
5.2 Serial bridges
Small token ring networks often grow by adding new rings in series with existing
rings. This is a simple installation task and involves only the cost of a bridge and two
cables. But there are several disadvantages: a frame may need to make several hops
to reach its destination, each bridge adding delay and thus decreasing the
performance of the application; any bridge or ring in the path between the
workstation and the server can be a single point of failure; if you continue to install
bridges in series, frames will soon encounter the hop count limit (7 hops with IBM
bridges) - this means that a workstation cannot communicate with a server if the
frames need to cross more than 7 bridges to get there, as the frame will be discarded
after 7 hops.
Incidentally, you will hear the term latency used to describe the delay introduced by
a bridge as a frame crosses it. A traditional token ring bridge is a Store and Forward
device. The bridge checks a flag in the MAC header. If the frame is to be bridged the
frame is then stored in memory; the integrity of the frame is checked by recalculating
the CRC for the frame and comparing it with the existing CRC - if there is no match
the frame is discarded; the frame is forwarded when the token becomes available on
the destination ring. .

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Student Notes
Clearly all of these actions will add delay to each frame as it crosses each bridge.
Additionally since each frame., large or small, must be stored and checked the overall
latency will vary slightly dependent on frame size.
Each additional bridge hop will add to additional latency and diminish the
performance of applications running between clients and servers which use that path
to maintain sessions.
5.3 Parallel Bridges
A solution using parallel bridges is more expensive than one using bridges in series,
simply due to the amount of hardware required. Configuration is also more complex.
However, this is outweighed by the benefits of redundancy and load balancing, in
which frames have alternative routes to their destination.
Redundancy means that a standby bridge is waiting to take over in case the active
bridge fails. This works with both transparent and source route bridges.
Load balancing means that traffic between two rings can be shared by both bridges.
This decreases the burden on each bridge and allows each workstation to find the
fastest path to its destination. Load balancing works only with source route bridges.
5.4 Remote bridges
Remote bridges are deployed to overcome large physical distances between rings. In
the case of Madge CAUs 2 kilometres is the maximum distances between CAUs on a
ring using fibre cables.
Remote bridges operate in pairs or “bridge halves”. Each bridge half has two physical
interfaces. A token ring interface allows the device to insert into the local ring. It has a
device driver which operates normal bridging software, source route or transparent.
The bridge also has a remote interface which passes the frames to the second bridge
half. This link could be any type of remote link such as ISDN, Megastream or T1 and
is simply a pipe for the data frames to be bridged.
Between the two interfaces the bridge provided software to translate between the
different frame types.
When a frame crosses a remote bridge this counts a single bridge hop.

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Student Notes
Backbone Topology
„ Ease of growth
„ Reduced number of hops
„ Centralised positioning of servers
„ Well suited to large networks
5.5 Backbone Topology
First of all, take some time to analyse the benefits of the backbone network.
Reliability and
Availability
Performance (Hop
Counts, Filtering,
Placing of Servers)
Maintainability
rings)
Turn the page to see some of the possible responses to this exercise.
Reliability and
Availability
• Users can easily be placed on another ring in case of
failure of local ring.

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Student Notes
• One central ring is used as a common path for all
traffic between rings. Backbone is single point of
failure, but a dual backbone overcomes this problem.
Performance (Hop
Counts, Filtering,
Placing of Servers)
• The number of hops is small and predictable: 1 hop
onto the backbone (where servers may be located) or
2 hops onto any other ring (also where servers or
gateways may be located).
• It is possible to add further rings to the sub rings, but
this creates hierarchies; the further from the
backbone you are the more hops your frames have to
make to reach any services, so performance will be
diminished.
• A backbone is well suited to large networks where
servers can be placed on the backbone or on specific
sub-rings. In either case the total latency (due to
bridge hops) can be easily predicted.
Maintainability
rings)
• Easy to grow - each bridge allows one new ring to be
added.

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Student Notes
5.6 Dual backbones
001
002
003
004
AA
BB1
AA
AA
AA
BB
BB
BB
BB
BB2
Dual backbones are probably the most popular types of network to be found on large
Token Ring sites. This is because of the extra resilience provided by the second
backbone, which also provides an extra path between all rings.
An example of this would a user on 001 wishing to reach a server on 004. Let’s
assume the normal path might be via bridge A and BB1. If ring BB1 or bridge A
becomes unavailable the path via bridge B and BB2 can be used instead.

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Student Notes
Bridging Method
OSI Layers
Repeaters
7
6
5
4
3
2
1
Routers
Transparent Bridges
Source Route Bridges
Source Route Transparent Bridges
Application
Presentation
Session
Transport
Network
Data Link
Physical
5.7 Bridging Methods
To clearly understand the role of bridges we will first compare bridges with repeaters
and routers.
Repeaters function at the physical level (level 1) and simply regenerate the electrical
signals at bit level. This is a means of extending the physical network and does not
segment the network in any way. Whether applied to Token Ring or Ethernet there
are specified limits to the number of repeaters that can be deployed.
Bridges use information in the MAC (or LLC) headers to determine whether to forward
a frame. This means they work in a way which is hidden from network layer protocols
like IP and IPX; they need have no awareness that this is how traffic is being passed
round the network. Bridges, whether transparent, source route or hybrid occupy the
Data Link Layer, layer 2, of the OSI model.
Routers are devices which can only handle frames by examining the network header.
A node does not need the services of a router if the target node is on its own subnet. If
the node cannot match the target subnet address with its own it will send a routing
request to a router on its subnet. We will discuss routing in a little more detail later.
Transparent Bridge Operation

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Student Notes
Addr Port
A
B
C
D
E
F
1
1
1
2
2
2
„ Each bridge has a table containing
– Destination addresses it knows
– Port on which that address can be found
Ring A
B
C
A
Ring C
E
F
11
1 2 21
22
D
Ring B
Addr Port
A
B
C
D
E
F
1
1
1
1
2
2
5.8 Transparent bridge operation
A transparent bridge maintains a table which stores each known address with the
port on which the node with that address can be located. The process of forwarding
the frame is thus very simple.

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Student Notes
Transparent Bridge Operation
NO: Forward frame on known port
DATALLCHDR DA SA DATALLCHDR DA SA Trailer
YES:
Are SA and DA
on same port?
YES: Discard frame
NO: Forward frame to all other ports.
Is DA in Address
Table?
Record SA in
Address Table
Once the bridge enters forwarding mode it has to build a picture of all local and
remote nodes and put the information in the Address Table. The flow chart above
describes this. Once a frame has arrived at a bridge port the bridge records the
address of the frame in the Address Table. Let’s use the diagram on the previous page
as an example. When stations A, B and C insert into ring A, Ring Poll frames will be
generated which will be seen by the bridge. The bridge thus discovers that these
stations are to be found on port 1 and builds the bridge table accordingly.
That allows us to understand how the addresses of stations on directly attached rings
are discovered by the bridge. What about other stations?
In fact, the bridge is not concerned whether frames arrive from directly attached
stations or remote stations. Addresses are learned by inspecting the source addresses
of all frames on the segment, even if these frames were not generated by stations
directly attached to the local ring.
Consider the situation where bridge 1 has started up and acquired the addresses of
the directly attached stations on ring A due to Ring Poll. The next event to occur will
be that requests are sent by the stations to contact servers, which may be several
hops away.

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Student Notes
When station A sends a request to talk to server F (for example) bridge 1 will forward
the request on all ports (port 2 in this case). This is because F’s address is not in the
table. Bridge 2 will receive the request frame and likewise it will forward it on all
ports.
Server F will then start returning frames to station A via bridge 1. Bridge 1 will then
record the source address in its Address Table, with port 2 as the port on which the
frames were seen.
Once the session between A and F has been established, bridge 1 will have discovered
that station F can be reached on port 2 and this will be placed in the table.
5.8.1 Source and destination address on same port?
The final part of the transparent bridging logic is a check to see if the destination
address for the frame is on the same bridge port as the source address. If this is the
case then the bridge must not forward the frame as the frame originated from that
segment.
5.8.2 Transparent bridging support with the Madge Ringswitch
You may like to apply this information to a real life switch or multi-port bridge.
The Madge Smart Ringswitch is not part of this training module but it is interesting to
note that this device can store up to 10,000 MAC addresses in its table.
This number is so large simply because the switch must keep track of all active
stations, including those not directly attached to the switch. The switch must know
through which port it can forward each packet to ensure delivery.
5.8.3 Self-study exercise
Look back at the last two slides and examine the bridge tables for bridges 1 and 2.
Account for each entry in each table. For the purpose of illustrating your point you
can treat any of the nodes as workstations or servers.
Hint: “A” could be a Netware server.
1. Why do addresses A,B,C and D appear in the table for bridge 1?
_________________________________________________________

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Student Notes
2. What caused addresses E and F to appear in bridge 2’s table?
_________________________________________________________
3. Describe how address A might have come to appear in bridge 2’s table.
_________________________________________________________

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Student Notes
Answers
1. Why do addresses A,B,C and D appear in the table for bridge 1?
Addresses A,B and C appear as available on port 1 and D appears as available on port
2. The addresses have been learned by the bridge when it receives its first frames
from these nodes. In the case of token ring, Ring Poll frames (AMPs and SMPs) would
have told the bridge that these stations are local on port 1.
2. What caused addresses E and F to appear in bridge 2’s table?
Same as 1. These are stations local to the bridge on port 2.
3. Describe how address A might have come to appear in bridge 2’s table.
“A” could be the address of a server which station E on ring 3 is logged into. Under
IPX a SAP request is sent out by station E with “A” as the preferred server. Initially
address “A” is not present in bridge 2’s table. Bridge 2 forwards the frame. Bridge 1
receives it and forwards it on the ring where server “A” is inserted .
Server A responds and the response frames appear on port 1 of bridge 2. Bridge 2 will
keep on seeing frames from A on port 1 and the entry in bridge 2’s table will be kept
until traffic from A ceases to appear, or shortly thereafter.
5.8.4 Address ageing
In fact the entry will not disappear immediately from the table. A so-called “ageing
timer” of between 20 and 30 seconds applies after which the address is finally
removed. During this period a new frame from the same source can arrive which will
reset the ageing timer to 0.

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Student Notes
Source Route Bridge Operation
„ RIF built up as explorer frame is broadcast
across the rings
„ Frame reaches server with complete RIF
„ Server uses RIF to get back to station
Ring/bridge pairsHDR DA SA Trailer
Ring A
B
C
A
Ring C
E
11
1 2 21
22
D
Ring B
LLC DATA
RIF
Station
Server
F
5.9 Source Route Bridge Operation
As we mentioned earlier, routing and bridging are different, the key distinction being
that routers use information in the level 3 network header, and bridges use
information in the MAC or LLC.
So why are we suddenly talking about Routing again? Source Route Bridging is not
similar to Routing in any sense. Source Route Bridging uses a number of techniques
initiated by the workstation which wishes to start a session with a server. The
initiating station is the “source” of this route determination activity. “Routing” refers
to the technique of recording the route taken by the frame as it passes over each
bridge, and using this information later for finding a path.
This means that unlike transparent bridges where information is held in the bridge
tables, a source routing bridge stores information in the Data frame.
Initially, a station wishing to perform source routing sets a flag in the Token Ring
header to indicate that the frame must be processed by Source Route bridges. This
flag actually indicates the presence of a RIF or Routing Information Field. We will
discuss the precise location of this flag later.

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Student Notes
If there is no RIF at this point the two stations wishing to start a session must
establish a route and fill in a RIF. This process is called route discovery and will be
covered in full later. Once fully formed, the RIF will contain a list of each ring and
bridge the frame must cross to reach its destination, normally a server of some kind.
Let’s use the diagram above as an example of this. Imagine a frame passing from A on
ring A to server F on ring C. What ring and bridge information needs to be stored in
the Routing Information Field to let the frame pass?
RIF Indicator
set?
Ring
No.
Bridge
No.
Ring
No.
Bridge
No.
Ring
No.
Bridge
No.
0
As you can see from the answers at the end of the section, the only possibly confusing
part is that the final bridge number is actually 0. This is because a frame is always
destined for an end station on a ring. Therefore, when traversing a ring a RIF must be
terminated with a 0.
You should realise that you have almost certainly sat in front of a station which
performs Source Routing to reach its server, you may even have loaded or configured
the drivers to do this. For example, have you ever loaded the following drivers?
Novell Madge IBM
LSL
TOKEN
IPXODI
ROUTE
NETX
SMART IPX SR=Y DEVICE=DXMA0MOD.SYS
DEVICE=DXMCMOD.SYS
DEVICE=DXMTMOD.SYS
In each case you have loaded source routing, in the last case it is built into the low
level driver. This is because IBM workstations need to talk to IBM mainframes which
expect to be installed on a source route bridged token ring network.
Answers to quiz
RIF Indicator
set?
Ring No. Bridge No. Ring No. Bridge No. Ring No. Bridge
No.
Yes 00A 1 00B 2 00C 0
Source Route Transparent Bridging

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Student Notes
Frame arrives
at bridge port
YES: bridge the frame
using Source Routing
NO: bridge the frame
using Transparent
Marked for
Source Routing?
5.10 Source Route Transparent Bridges
Many bridges, including the Madge Smart Ringswitch, can perform either
Transparent or Source Route Bridging. On a bridge such as this, if the port is enabled
for both Transparent and Source Route bridging the frame follows the flow diagram
above.
The bridge checks the RIF indicator flag. A frame with it set will be source routed,
otherwise the frame is transparently bridged.
5.11 What makes a router different from a bridge?
Before we conclude our introduction to the principles of bridging with Token Ring let’s
revisit our definitions of routing and bridging from earlier.
Bridges use information in the MAC or LLC headers to determine whether to forward
a frame. This means they work in a way which is hidden from network layer protocols
like IP and IPX.
Routers are devices which can only handle frames by examining the network header.
A node does not need the services of a router if the target node is on its own subnet. If
the node cannot match the target subnet address with its own it will send a routing
request to routers on its subnet.
The key differences between a bridge and a router can be summarised as follows:

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Student Notes
Routers tend to connect networks of differing topologies e.g. Token Ring and
Ethernet. They join networks with different network numbers or subnet addresses.
Bridges tend to connect segments of the same media, and with the same subnet
address.
Routers support specific network protocols (IPX, IP, AppleTalk) and allow
internetworking to occur between subnets which support one of these layer 3 network
types.
Routers do not perform a hidden role. A station must decide that a frame is not on his
subnet, and then send a specific frame to routers on the subnet. A router only
handles packets addressed to itself. Only when a frame has been sent to the router
itself can the router forward the frame to the next hop. The final router will pass the
frame to the specific network where the end station resides. All routers need some
method of discovering the next hop to a particular subnet. An example of this is RIP,
Routing Information Protocol which provides Router Updates on a routine basis and
whenever the topology of the network changes.
Bridges are different from routers in all these points as they operate in a way which is
hidden to all layer 3 protocols which are specifically supported by routers.
The following slide summarises these points on routers versus bridges:
Routers versus Bridges

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Student Notes
„ Handle frames based on
Network Header
„ Tend to connect networks
of differing LAN types (or
LAN to WAN)
„ Support specific protocols:
IP/IPX/Appletalk
„ Protocol required for
updating routing
information
„ Handle frames based on
LLC/MAC Header
„ Tend to connect networks
of same LAN type
„ Support any LLC based
protocol - network layer is
unaware
„ Protocol required to
maintain Spanning Tree
– “loop avoidance” protocol

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Student Notes
Routing /Bridging Combination
Source Routing
Bridge
Ring A Ring B
Net Ware
Server/
Router
IPX Segment 000002
IPX Segment
00000A
Net Ware
Client
Load ROUTE. COM
Load ROUTE. NLM
5.12 Self study: Combining Bridging and Routing
In order to fully understand the concepts of bridging and routing in a Novell
environment we can look closer at a simple but typical Novell network.
Look at the diagram and try answering the following questions:
1. How many IPX segments (networks or subnets) can you see on the diagram?
________________________________
2. The router on ring B receives frames from segment 00000A. It needs to pass these
frames to a station on ring A. What end station software is needed at both ends?
________________________________
3. What does ROUTE.NLM do?
________________________________
4. What is the difference here between the role played by the router and that played
by the bridge?
________________________________
5.12.1 Answers to Self Study
1. How many IPX segments (networks or subnets) can you see on the diagram?
2 segments, 000002 and 00000A routed by the Novell server

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Student Notes
2. The router on ring B receives frames from segment 00000A. It needs to pass these
frames to a station on ring A. What end station software is needed at both ends?
ROUTE.NLM is needed on the server end and ROUTE.COM on the workstation
end.
3. What does ROUTE.NLM do?
ROUTE.NLM is the same as ROUTE.COM on the workstation i.e. it sets the RIF indicator
flag so that the bridge knows to use a RIF. ROUTE.NLM has the job of exploring possible
routes to reach ring A. These drivers have nothing do with routing they enable source
routing in the frames which allows the bridge to pass the frame and fill in the RIF.
4. What is the difference here between the role played by the router and that played
by the bridge?
________________________________
Routing occurs at level 3 - IPX frames are routed between IPX segment 00000A and
000002.The same frames are then source route bridged in order to reach ring A.
IPX segment 00000A could be any MAC type (Token Ring, FDDI, Ethernet).
5.13 Test your understanding: Bridging Fundamentals
Complete the following self-test by answering the following questions. Check your
answers are by using the answer key located in Appendix A.
1. What type of device is hidden from layer 3 protocols like IPX?
a) A repeater
b) A transparent bridge
c) A source route bridge
d) All of the above
2. A router works at which layer of the OSI model
a) Physical
b) Datalink
c) Network
d) Transport

In the multiple choice tests only one of the possible answers is correct. Read all the
answers carefully)

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Student Notes
3. Which of the following is a benefit of a traditional bridge between network
segments?
a) a cabling fault on one ring can never be passed to the second ring
b) a bridge can easily link Token Ring and Ethernet
c) bridges add an insignificant delay (latency) as frames pass through
them
d) none of the above
4. A device which copies frames based on the source and destination MAC
address is called
a) a repeater
b) a router
c) a transparent bridge
d) a source routing bridge
5. Why is the following RIF invalid?
Ring No. Bridge No. Ring No. Bridge No. Ring No.
FFF 1 00B 3 0
6. A correctly formed single backbone topology provides
a) alternative routes from client rings to the server
b) single hop between any two rings
c) single hop to the backbone (server) ring
d) all of the above
7. In a Token Ring frame the RIF is
a) present if requested by the bridge
b) present if requested by end station software
c) optional if the RIF flag is set
d) mandatory
8 A transparent bridge acquires addresses and assigns each address to one of
its ports.
It does this by
a) Examining the source and destination address
b) Examining the source address only
c) Examining the destination address only
d) None of the above

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Student Notes
6. Theory of Source Route Bridging
„ Define valid Ring and Bridge numbers
„ Explain how the Routing Information Field
(RIF) is formed
„ State how clients and servers use the RIF
„ Describe the benefits of each Route
Discovery Method
„ Log Route Discovery session and compare
theory with practice
During this session you will learn the basic rules that end-stations and bridges have
to follow to perform source routing. You will look at the different Route Discovery
Methods and see how they can provide benefits such as load balancing without
having a severe negative impact on performance.

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Student Notes
Theory of Source Route Bridging
001 002 00F
003
101
1
1
A
A
1
2
3
4
Remote Bridge
Parallel
Bridges
1
6.1 Self Study : - Ring numbers and Bridge numbers
At this moment you are not expected to understand any of the details of how source
routing works. However with the knowledge you have picked up, spend a few
moments studying the above diagram.
Ask yourself if the network is valid and would actually function.
1. Are the bridge numbers valid?
2. Are the ring numbers within specification?
3. Why are there 2 bridges called “A”?
If you found this difficult, do not worry, the topic is fully covered over the next 3
pages.
Turn the page for answers to these questions.

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Student Notes
Answers to quiz
1. So that unambiguous paths are recorded in each RIF; bridge numbers between the
same 2 rings must be unique. Otherwise bridge numbers need not be unique.
2. Ring numbers must always be unique
3. Bridge A is made of two bridge halves. In fact, from the point of view of the token
ring interface on each bridge half, this is a single bridge. The medium for
transmitting frames from one ring to the other happens to be a phone link (T1 or
similar) rather than the internals of a local bridge.
6.2 Ring numbers and bridge numbers - 3 rules to remember
Source route bridging is achieved by close co-operation between end-stations and
bridges.
Source routing has to be requested by the end-station. In the case of Novell this
means loading ROUTE.COM, ROUTE.NLM or something similar. It is the bridges
which do all the hard work after that. Since each bridge adds to the RIF of an explorer
frame as it crosses the rings, it is the bridges which must be configured with the ring
and bridges numbers.
Let’s list the basic rules of ring and bridge numbers.
1. Every ring must have a unique ring number consisting of 3 hex digits. Valid ring
numbers range from 000 to FFF.
2. Every bridge must have a number consisting of 1 hex digit. Valid ring numbers
range from 1 to F. 0 is invalid as this is used to mark the end of a RIF. Bridge
numbers only need to be unique when a bridge is joining 2 rings in parallel with
another bridge.
3. So that RIFs are always unambiguous the combination of Ring Number + Bridge
Number + Ring Number must unique on the bridged network.
This means that the diagram on the previous page is indeed correct with its
duplicated bridge numbers. Don’t forget there are only 15 valid bridge numbers, so a
larger network is bound to have duplicated bridge numbers.
Now we have laid down the basic rules we can examine in detail the process of route
discovery and the roles played by the stations and the bridges.
The slide below shows in some detail how a station indicates that it wants its frames
source route bridged. Please don’t worry if you don’t understand it straight away, we
are going to discuss this in detail shortly.

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Student Notes
Flagging the frame
for Source Routing
Example:
0000F6123456 No RIF
8000F6123456 RIF present
Individual/Group
bit in the
Source Address is
always Individual ...
I U
LG
Manufacturer ID Serial #
I
U
LF
Manufacturer ID Serial #
R
Token Ring
Frame
DATALLCRIFHDR DA SA Trailer
So use it to indicate
presence of a RIF
The real point behind all this complexity is that the station needs to locate an unused
bit somewhere in the MAC frame to use as an indicator that the frame needs to be
source routed. The source routing specification says that a certain bit should be used.
In fact we use the high order bit of the source address. This can in theory be used to
indicate whether the address is a group or an individual address. We know that a
source address is always an individual address, as a group address can only be used
for destinations.
That’s why, if we analyse a source route bridged source address, we see 8000F6 etc
(in the case of a Madge address). Fortunately, we don’t normally see this - the detail is
handled by the network analysis software, in our case Madge Framelogger.
Routing Information Field (RIF)
So remember, the end station indicates source routing by setting the high order bit of
the source address. There is no special bit reserved solely for this purpose.)

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Student Notes
Routing Information
Field
Ring
Bridge
Ring
Bridge
Ring
Bridge
Control B 0
2bytes Maximum of 18 bytes
001
A
002 003
DATALLCRIFHDR DA SA Trailer
„ 2 bytes minimum
– Control information only
„ 18 bytes maximum: Ring + Bridge pairs
– ie maximum hop count (7 bridges ) reached
„ Last bridge number always 0
– destination node is on a ring
In fact when the station sets the RIF indicator flag it also sets up a 2 byte control
field. This field contains, amongst other things, the exploration strategy to be used
by the station when looking for the server. The control field is something we will
discuss in much more detail later in the document.
You can see from its position in the MAC header of the data frame that the Routing
Information Field contains information which allows LLC to carry data (possibly other
protocols) over bridges. This makes source routing a layer 2 protocol.
Source route bridging actually follows one of two protocols: IBM or IEEE. The key
difference between the two is the maximum hop count, 7 in the case of IBM, 13 in the
case of IEEE. This means that on an IBM network the frame will be discarded if a
bridge notices that a frame has already crossed 7 bridges.
6.3 Self study - Draw a frame with the maximum number of hops
To understand how RIFs are built up it is useful to consider the maximum size of RIF
that is possible on an IBM network.
First, there are 2 bytes of control information. A ring number requires 3 hex digits (1
1/2 bytes) and a bridge number requires 1 hex digit (1/2 byte) so a ring/bridge pair
require 2 bytes. This means the simplest possible RIF would be:
Control
header
Ring Bridge
bytes
..

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Student Notes
2
2 bytes 001 A 002 0
Don’t forget the final bridge number, 0, which is only there to terminate the RIF. The
search for a server must end on a ring.
Right, its time for you to try drawing a RIF with the maximum of 7 hops. Imagine a
network with 7 bridges and the appropriate number of rings.
Once you have drawn this count up the total number of bytes for this RIF.
Then you can turn the page to see if you agree with the “official answer”.

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Student Notes
6.3.1 Answers
Here is an example RIF which has reached maximum length.
2 bytes 001 A 00
2
B 003 C 004 D 005 E 006 F 007 1 008 0
This RIF is 18 bytes in length and consists of 8 ring/bridge pairs allowing a total of 7
bridge hops. The last bridge number is null as the frame must land finally on a ring.
Constantly Circulating Frames
Unique Ring Numbers
„ Ring number not allowed to
be in RIF more than once
„ Stops constantly circulating
frames
Control 002 1 004 2 002
Ring
002
Ring
004
321
0
One final point about how RIFs are constructed.
We have already made it clear that ring numbers must be unique. A bridge always
expects to see a particular ring number only once in any RIF. If it recognises that the
ring number of its output ring (002 in the example above) is in the RIF it will not write
the Ring Number into the RIF again; it will discard the frame. The purpose of all this
is to prevent frames constantly circulating the network. This is vital on switched
networks where loops are deliberately introduced to provide alternative paths and
thereby load balancing.
Control
Header
2 bytes
Ring/Bridg
e

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Student Notes
6.4 Test your understanding: Theory of Source Routing
1. In a token ring frame the Routing Information Field (RIF) is positioned
a) before the MAC address
b) after the destination MAC address and before the source MAC address
c) after the source MAC address and before the data
d) after the data
2. In source routing, each ring number is identified by
a) a 2-digit hexadecimal number (00-FF)
b) a 2 digit decimal number (00-99)
c) a 3 digit hexadecimal number (000-FFF)
d) a 3 digit decimal number (000-999)
3. The control header in the RIF is:
a) always exactly 2 bytes long
b) at least 2 bytes long
c) optional
4. Two source routing bridges with the same bridge number must not
a) exist on the same network
b) attach to the same ring
c) join the same two rings

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Student Notes
7. Theory of Source Route Bridging: Explorer Frames
Source Route Bridging
All Routes Explorer (ARE) Frames
001 002 003
1031
2
3
B
101 1021
2
3
A
AREARE
We can now apply the basic knowledge we learned in the previous chapter. In the
diagram you can see a perfectly valid resilient network with multiple paths from the
source end station to the destination end station.
The destination end station could be a server (as pictured) but it could equally be a
mainframe or another workstation.
Ring numbers are unique, bridge numbers are unique where necessary, that is when
joining the same two rings e.g. 001 and 002.
The end station wishes to start a session with the destination and sends an explorer
frame to find it. It sets the RIF indicator and also sets up the first 2 bytes of the
Control header. In this header it indicates the exploration strategy.
This is the strategy requested by the source route bridging end station to explore the
network and find the end station. This is fixed in the first 3 bits of the Control header
at the start of the RIF.
Over the next few pages we will be looking at the different frame types needed for
source routing. The first examples will be general. Later, we look at how these frames
are used in specific manufacturer environments for example Novell.
)

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Student Notes
First we will investigate a common strategy which is to explore all routes round the
network to reach the server. This uses an All Routes Explorer frame or ARE.
All Routes Explorer (ARE) Frames
2
3001 002 003
103101 102
3
A
1
2
1
B
This is the result once the ARE has crossed onto rings 101 and 002. The frame will be
copied by bridge 2 ,bridge 3 and bridge A.
7.1 Self study: All Routes Explorers
1. Given that the frame will go in both directions (via 101 and 002) can you determine
how many copies will end up on the destination ring?
Answer:________________________
2. How many copies of the frame will be seen on ring 101 (not necessarily at the same
time)?
Answer:_____________________________________
All Routes Explorers

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Student Notes
ARE: 001-3-002-3-003-0ARE: 001-3-002-3-003-0
001 002
1031
2
3
B
101 1021
2
3
A
ARE: 001-A-101-1-102-0ARE: 001-A-101-1-102-0
ARE: 001-2-002-2-003-0ARE: 001-2-002-2-003-0
ARE: 001-2-002-3-003-0ARE: 001-2-002-3-003-0
ARE: 001-3-002-2-003-0ARE: 001-3-002-2-003-0 001 002 003
1
2
3
B
101 1021
2
3
A
ARE: 001-A-101-1-102-1-103-0ARE: 001-A-101-1-102-1-103-0
ARE: 001-2-002-2-003-B-103-0ARE: 001-2-002-2-003-B-103-0
ARE: 001-3-002-3-003-B-103-0ARE: 001-3-002-3-003-B-103-0
ARE: 001-3-002-2-003-B-103-0ARE: 001-3-002-2-003-B-103-0
ARE: 001-2-002-3-003-B-103-0ARE: 001-2-002-3-003-B-103-0
The answer to both questions is 5. The explorer frames will reach the destination ring
from both directions, 4 frames via the top route, 1 via the bottom route.
The 4 frames will carry on round from ring 003 and reach 101. Ring 101 will already
have seen the same explorer frame coming via bridge A, so the total for 101 is also 5.
7.2 Why use All Routes Explorers?
With AREs it is possible (as in our case) for multiple copies of the ARE to arrive at the
destination. The destination will respond to each of these with a specifically routed
frame which will follow the route given in the ARE’s original RIF. This means the
specifically routed frame will go back the way the ARE came.
Eventually a number of specifically routed frames will get back to the source, and the
source will store in the cache the first route it receives.
7.3 Disadvantages of All Routes Explorers
With All Routes Explorers numerous copies of the request can arrive at the
destination and this is not always desirable.
For this reason an alternative strategy is to use a single route explorer to reach the
destination.
This whole process allows the source station to determine the best route by taking the
path with the shortest round trip delay. This is a key principle in the theory of source
route bridging.
)

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Student Notes
Clearly there needs to be a way to provide a single route to the destination in first
place. The end station needs plenty of help from the bridges to do this. A technique
called Spanning Tree is used to provide this. The term Spanning Tree is used to refer
to a network that has only one path to get between any pair or rings.

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____________________________________________________________________________________
Student Notes
Example Spanning Tree
001 002 003
1031
2
3
B
101 1021
2
3
A
Only one route from any ring to any other ring
= Standby Bridge
From the diagram it is clear that something has determined that certain bridges must
be “turned off” these are both bridges numbered 3 and also bridge 102-1-103. The
bridges remaining active are called designated bridges. This means that these are the
only bridges that will pass Spanning Tree Explorer frames. Simply expressed, this
means that the Spanning Tree Explorer frames will traverse the designated bridges to
explore the route to the destination station.
7.4 Why use Spanning Tree Explorers?
The whole point of this is that there is only one route to the server, one route from
one ring to any other ring. This means only one copy of an STE frame appears on
each ring but the destination station is still found.
)
Note: Do not get confused with transparent bridges. A designated bridge under
transparent bridging will carry data frames. The spanning tree is there to provide a
loop free path for data itself. There is no concept of explorer frames. This means that
a standby bridge is basically inactive. It only becomes active when it has to take over
from the active bridge in case of failure.

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Student Notes
Specifically Routed Frame
001 002 003
1031
2
3
B
101 1021
2
3
A
SR: 001-2-002-2-003-0

001 002 003
1031
2
3
B
101 1021
2
3
A
SR: 001-2-002-2-003-0

001 002 003
1031
2
3
B
101 1021
2
3
A
SR: 001-2-002-2-003-0

Let’s just review the basic process again:
A Specifically Routed Frame is sent in response to each ARE or STE received by the
destination
Note, the RIF remains unaltered, but a single bit, the direction bit, is altered. This
tells the bridges en route to read the RIF backwards.
In the case of AREs one or more specifically routed frames will be received by the
source station. It will then use the RIF in the first specifically routed frame it receives
for its new session with the destination. If it sent an STE out the return frame will
also follow the spanning tree and that is what the source station will use for its
session with the destination.
)

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Student Notes
Exploration Strategies
Broadcast
Indicator
Length of
Routing Info Direction
bit
Maximum
frame size
Unused
3 bits 5 bits 1 bit 3 bits
Header DA SA RI
LLC
Header
Data Trailer
Control 0-8 Ring Bridge Number fields
(in bytes)
000 Specifically Routed Frame
100 All Routes Explorer (SR return)
110 Spanning Tree Explorer (ARE return)
111 Spanning Tree Explorer (SR return)
000: 516 bytes000: 516 bytes 100: 8144 bytes100: 8144 bytes
011: 4472 bytes011: 4472 bytes 111: Initial value111: Initial value
7.5 Exploration Strategies
Note: all reference books call the first 3 bits of the control field the “Broadcast
Indicator”. We will always refer to them as providing the “Exploration Strategy”.
The options which can be set by the source station driver software determine the
strategy which the frames will follow when crossing the bridges to the destination
station.
This then gives the complete picture of how end station interact when trying to find a
route for a new session.
3 bit
code
Indicator Exploration Strategy
000 Specifically Routed
Frame
Exploration is complete, use the specified
route
100 All Routes Explorer (SR
return):
Explore All Routes from the source, the
destination replies, the source caches the
RIF in the first SR frame it gets back
)

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Student Notes
3 bit
code
Indicator Exploration Strategy
110 - Spanning Tree Out, All
Routes Explorer Return
Possibly the ideal strategy, find the
destination with an STE, then discover the
best route on the way back. This allows load
balancing.
111 Spanning tree out, use
SR (the same route)
back
This option uses no AREs, but means only
one path is used. This means no load
balancing, so it takes away one advantage of
spanning tree.
7.6 Pros and cons of ARE out SR return
The main benefit of this strategy is that once the source station has received its first
response to its ARE out it knows it has the fastest route. Subsequent explorations
may provide a different station with a different route. This is the benefit of the round
trip calculation the strategy provides.
In this case the destination (normally a server of some kind) receives multiple copies
and must process them all. Each bridge and ring can become overloaded if there are
many alternate paths.
7.6.1 Broadcasts using All Routes Explorers
An All Routes Explorer can be sent as a unicast if the network address (MAC address)
of the server or end station is known. But what if the MAC address is not known?
Then we must send a broadcast.
This would be the case if the source station is a Novell workstation wishing to start a
session with a Novell server. The workstation would send an IPX SAP request
otherwise known as a Get Nearest Server request.
Every bridge will copy every ARE it sees. The AREs sent by the station are broadcast
frames and will be processed by every station on every ring on which that ARE is
seen. This includes all the servers which should not be spending valuable CPU time
processing broadcast frames not intended for them.

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Student Notes
7.7 Pros and cons of STE out ARE back
The benefit compared to the first strategy (ARE out) is that the end station (normally a
server) only receives one frame and is itself responsible for sending out AREs. This
means load balancing can be combined with a lower processing overhead on the
server , bridges and other stations. However the STE is often sent as a MAC broadcast
frame.
7.7.1 Broadcasts using Spanning Tree Explorers
An STE broadcast must still be processed by all stations on the ring, but the
overhead is reduced since only one copy appears on each ring.
7.8 Pros and cons of STE out SR back
The advantage of this is that the load on bridges, rings, destinations and other
stations is reduced.
But no load balancing is provided.
7.9 Control Header Fields
After the 3 bit broadcast indicator you see the following fields:
Length of routing information - very useful if you are a bridge wishing to skip forward
to the start of LLC data which would allow you to perform filtering
Direction bit - tells the bridge in which direction to read the RIF. Once the server has
decided which RIF to use it will start to use Specifically Routed frames. To do this it
must change the direction bit to tell the bridge not to read the RIF left to right (from
the station), but right to left (from the server).
Maximum frame size - rarely used by drivers.
Exploration Example
Spanning Tree Out with All Routes Explorer Return

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Student Notes
001 002 003
1031
2
3
B
101 1021
2
3
A
STE: 001-2-002-0
STE: 001-A-101-0
001 002 003
1
2
3
B
101 1021
2
3
A
ARE: 003-B-103-1-102-1-101-1-001-0
ARE: 003-2-002-2-001-0
ARE: 003-3-002-3-001-0
ARE: 003-3-002-2-001-0
ARE: 003-2-002-3-001-0
103
Station chooses best RIF, then uses SR frames
Let’s look at what might be considered the ideal exploration strategy: STE out, ARE
return.
Let’s do this by going through the procedure followed by the end station which wants
to find a server using source routing. This time let’s consider a Novell workstation:
1. Driver software is loaded on the end station to ensure the RIF indicator flag (bit 1
of the source address) will be set. This is ROUTE.COM for a workstation, but
equally ROUTE.NLM carries out the same job on a Novell server. ROUTE.COM
must also set the broadcast indicator, in this case to 110: STE out, ARE return.
2. Once the IPX stack has been loaded, requester software called NETX.COM (or
something similar in the case of VLMs or Client32) is loaded. This sends out a SAP
request for a server. If the Preferred Server option was included then a response
from a named server will be expected.
The STE will take two routes in order to reach all rings. Only one explorer will arrive
at the server.
The server can then use a unicast All Routes Explorer to find the best path back to
the client. Note that this is a unicast as the MAC address of the client is known.
)
3. The exploration strategy will now be followed - with one large proviso: the server is
at liberty to ignore the strategy requested by the end station. For example, certain
older versions of ROUTE.NLM ignore requests to perform AREs back to the
workstation.

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Student Notes
7.9.1 The ideal exploration strategy?
The full description of the exploration strategy used here is Spanning Tree Explorer
Out, All Routes Explorer Return then Specifically Routed.
The station finds its server with an STE; the server explores the route back with a
unicast ARE; the end station chooses the “best” RIF from those it receives and uses
the specified route to return to the server. This is the route it uses for the duration of
the session. In the case of a Novell workstation, this is until the user unloads NETX.
When NETX is reloaded a new STE will be transmitted (if this is the chosen
exploration strategy). The detail of all this is going to become clear when you try this
in the practical session which is coming up after the next chapter.

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Student Notes
Bridge Decision Process: 3 Logical Bridges
Specifically Routed Frame
Forward if RIF ring bridge
numbers match
unless
• Bridge Rules broken
Source
Routed
frame?
Yes
No
Ignore frame
All Routes Explorer
Always Forward,
unless:
• AREBridge Rules broken
Spanning Tree Explorer
Forward if bridge configured for STEs,
unless:
• STE Bridge Rules broken
Check RIF
ARE Bridge Rules
Never forward if
• already been on next ring
• hop count exceeded
• hop count can be less than
max of 7 ie configurable
• filtered (MAC address etc)
STE Bridge Rules
Never forward if
• already been on next ring
• max hop count of 7 exceeded
• not configurable to less
•filtered (MAC address etc)
7.10 Bridge Decision Process
The flow chart above summarises the possible roles played by each source routing
bridge. You can even think of this process in terms of three logical bridges within the
physical bridge. Each bridge deals with a different type of source routed frame: all
routes explorer; spanning tree explorer; specifically routed.
7.11 Review of Exploration Strategies: a real world issue
The picture overleaf summarises the key points you need to “take home” on Source
Route Bridging Exploration Strategies.
The key point to remember is that each strategy has strengths and weaknesses.
Factors to consider include: need for load balancing, level of AREs; difficulty of
applying uniform strategy with differing platforms and driver versions.

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Student Notes
Exploration Strategies
„ Each strategy has strengths and weaknesses
„ Full route discovery means using AREs
– otherwise no load balancing
„ After route discovery, ALL subsequent frames
are Specifically Routed
„ Log transactions to identify ACTUAL behaviour
– server might not follow request of client
OUT BACK
ARE SR
STE ARE
STE SR
If load balancing is a pre-requisite then AREs must be used at some point.
The last, and perhaps the most important point to remember about designing source
routed networks is as follows.
Clients, servers, mainframes and gateways might not obey the rules described above.
The real world is more complex. This means that as an engineer you must become fully
acquainted with ACTUAL behaviour by logging. That’s precisely why in the practical
session we will start this learning process using Madge’s own Framelogger.

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Student Notes
7.12 Test Your Understanding: Explorer Frames
1. If source route bridging is implemented, bridges joined in parallel can provide:
a) alternative routes for traffic
b) load balancing
c) contingency in case of failure
d) all of the above
2. If transparent bridging is implemented, bridges joined in parallel can provide:
a) alternative routes for traffic
b) load balancing
c) contingency in case of failure
d) all of the above
3. Specifically routed frames are copied by every source routing bridge.
a) True
b) False
4. Spanning Tree Explorer frames are copied by every source routing bridge.
a) True
b) False
5. All Routes Explorer frames are copied by every source routing bridge.
a) True
b) False
6. When applied to source routing bridges the spanning tree protocol provides a
path for
a) single route explorer frames only
b) single route and all route explorer frames only
c) non-explorer frames only
7. The Spanning Tree Protocol is recognised
a) by bridges only
b) end stations only
c) bridges and end stations
8. Spanning Tree

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Student Notes
„ Identify the steps taken by the Spanning Tree
algorithm to nominate:
– the Root bridge
– Designated bridges
– Standby bridges
„ Explain the parameters which can be
configured to influence the Spanning Tree
„ Describe the key fields of the Spanning Tree
frame
„ Explain how Spanning Tree eliminates loops
– providing a single route between any two rings
The purpose of Spanning Tree when used with Token Ring can be described in very
simple terms. The Spanning Tree on its own can be applied to any network topology
and is guaranteed to provide a network without loops, where there is only ever one
route from one station to any other. In terms of Token Ring this means there is a way
for a frame to be sent out so that it reaches its destination using only one route. The
result of that is that only one frame arrives at the destination.
To achieve this state each bridge has to establish whether it should forward frames or
not, i.e. should it be forwarding or blocking. Its mode while it is establishing this is
called learning mode; it is learning the topology of the network to identify its role.
In fact, the bridges must co-operate using some sort of protocol to ensure that certain
bridges are switched off to eliminate loops. With Token Ring we can use the analogy
suggested earlier. A source routing bridge contains three logical bridges for:
1. All Routes Explorers
2. Spanning Tree Explorers
3. Specifically Routed Frames
Spanning Tree can effectively “switch off” the second of these two logical bridges. In
other words the Spanning Tree Algorithm cannot affect the passing of AREs and SR
frames. These will always be passed unless the bridge rules (hop count etc.) will be
broken in doing so.
Example Spanning Tree

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Student Notes
1
3
Ring 101 Ring 102
Ring 104Ring 103
4
2
Root
(Designated)
Designated
Standby
„ Path from 103 to 104, discovered by
an STE frame, is via 101 and 102
Designated
Here is an example Spanning Tree consisting of bridges 1, 2 and 3. An STE frame
sent by a station wishing to start a session with a server on 104 must follow the pre-
determined Spanning Tree. Bridge 4 is in standby but will forward AREs and SR
frames.
A single Root bridge is the centre of the spanning tree. Other bridges will measure
the cost of sending frames to the root bridge. Only bridges on the cheapest route will
be enabled by the protocol for forwarding STEs.
For rings 103 and 104 the cheapest bridges to use are bridges 3 and 4. These become
the designated bridges. The root bridge is always designated.
But how does the Root Bridge get chosen in the first place?
Spanning Tree Formation
Election of the Root Bridge

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Student Notes
„ Each bridge has a Bridge Label
– 4 hex digits (8000h or C000h)
„ Each bridge has a number of MAC Addresses
– 12 hex digits (0000F6123456)
– read in non-canonical (MSB first)
„ Bridge ID is Bridge Label + Mac Address
– (8000)(00006F482C6A)
– MAC Address read in Canonical (LSB first)
„ Lowest Bridge ID becomes Root Bridge
8.1 Addressing formats for Spanning Tree frames
Before the discussion on Root bridge election we need to clarify the issue of address
formats.
Because Spanning Tree is a standard originated from the world of Ethernet, it uses
Ethernet-style addressing. This means that addresses in frames are held in canonical
format.
If you are not aware of canonical and non-canonical addressing then you should read
the next section and complete the exercise.
If you do understand canonical and non-canonical addressing then skip to the
section Election of Root Bridge - the Bridge Id
8.2 Canonical and non-canonical addressing formats
A change of network type means an even greater overhead for the bridge as address
translation may be required. Ethernet and FDDI use canonical addressing. This
means the bits in an address byte are read from right to left, or from the least
significant to the most significant. Token Ring uses non-canonical addressing, i.e.
addresses are read most significant bit (MSB) first. So when a token ring station
needs to talk to an FDDI station across a bridge the bridge must perform the
translation.
Remember, the contents of the address field are not changed by this process. It is
merely a matter of reading a byte from the left most bit (high order or MSB) which is

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Student Notes
the non-canonical way, or from the right most bit (low order or LSB) which is the
canonical way.
Incidentally 00 00 F6 as the first 3 bytes of a MAC address specifies a Madge Token
Ring Adapter. This can be easily identified in canonical format as you just swap the
hex characters round.
Expressed as a byte:
Non-canonical
Expressed in bits
Read MSB to LSB
Same bits
Read LSB to MSB
i.e. Canonical
Canonical
F6 MSB
11110110LSB LSB
01101111MSB
6F
E6 MSB
11100110LSB LSB
01100111MSB
67
Test your powers of bit manipulation now by filling the gap in the following table.
Turn to the following page for the answers.
Non-canonical
Expressed in bits
Read MSB to LSB
Same bits
Read LSB to MSB
i.e. Canonical
Expressed as a
byte:
Canonical
F7 11110111
10000110 61
Analysis of a single Token Ring address byte, binary and hex
Analysis of a single Token Ring address byte, binary and hex

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Student Notes
8.2.1 Answers to questions on canonical and non-canonical addressing
Non-canonical Non-canonical Canonical Canonical
F7 11110111 11101111 EF
86 10000110 01100001 61
8.3 Election of Root Bridge - the Bridge Id
Each bridge has a 4hex digit value assigned to it called the Bridge Label. This
appears in hex when you log the frame using Framelogger, and in decimal when you
configure the value using Madge Trueview Bridge manager.
Another difference is that under Trueview Bridge manager the Bridge Label is called
the Bridge Priority.
Manufacturers assign default values to the Bridge Label, 8000h and C000h being two
possible defaults.
Each bridge port has a MAC address assigned to it. A multi-port bridge or switch
might have 4 or more MAC addresses, normally assigned sequentially. These are read
as normal MAC addresses i.e. non-canonically or Most Significant Bit first (LSB)
Spanning tree takes the Bridge Label and appends the lowest MAC address to it to
form the Bridge Id. The Spanning Tree Protocol will run allowing all bridges to
exchange Bridge Ids. There is guaranteed to be a single Bridge Id which is the lowest
and this will become the root bridge. Bridge Ids are read canonically or Least
Significant Bit first (LSB).

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Student Notes
HELLO Bridge Protocol Data Unit (BPDU)
SDEL AC FC DA SA DATA FCS EDEL FS
LLC header HELLO BPDU
42h 42h
DSAP SSAP Control
UI
Root ID Bridge ID
Root
Path
Cost
Protocol Identifier
Protocol Version ID
BPDU Type
Flags
Message Age
Max Age
Hello Time
Forward Delay
Port Identifier
RI
3 bytes
35 bytes
The exchange of Bridge Ids happens during a so-called Election process during
which each bridge assumes it is the only bridge on the network and sends out a
frame announcing itself as the root.
This frame is called the Bridge Protocol Data Unit or HELLO frame.
These frames are sent to a specific functional address which means they are not
broadcast frames. This is important, as it means workstations will not be burdened
with the large quantity of these frames sent out during the root election.
We are not concerned with all the detail of the frame. We will look at Root Id, Root
Path Cost and Bridge ID. During the Root Election each bridge will send out BPDU
frames with its own Bridge Id in both the Bridge Id and the Root Id fields. It assumes
initially that it should be the Root Bridge. However it will also receive BPDU frames
from other bridges. When it sees a BPDU with a Root ID that is lower than its own it
Once the election is over we will see that the bridge with the lowest Bridge ID actually
becomes the root.

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Student Notes
stops sending out BPDUs on that specific ring. It then prepares its own BPDU with the
Root Id it has just discovered and sends these BPDUs away from the (potential) root
bridge. In turn this Root Id will be compared to those sent by neighbouring bridges
until all l Root Ids have been compared. Eventually, all bridges will be sending BPDUs
with the same Root Id. In other words they agree which bridge should be the root
bridge: the bridge with the lowest Bridge Id.
Election of Root Bridge
3
1Ring 101 Ring 102
Ring 104Ring 103 4
2
Hello BPDU
Hello BPDU
100008005aabcdef
800000006f654321
Hello BPDU
800000006f123456
Hello BPDU
100000006fabcdef
The diagram shows this process clearly. All bridges will start sending BPDUs on all
ports. Each bridge will examine the Bridge Ids in the BPDUs it receives. Eventually
only one bridge per ring send BPDUs, all with the same Root Id.
What is the Root Id of the root bridge?
____________________________ (Answer is over the page).
Warning: the Bridge Id is nothing to do with the Bridge Number. It’s the bridge
number that gets written into the RIF as the RIF is being built up. The Bridge Id is
used solely for determining the Spanning Tree.
)

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