Download Bridge Algorithms

Unit 2
Bridging
Overview
Description
The focus of this unit centers on four different types of bridges:
•
Transparent learning bridge.
•
Source route bridge.
•
Translational bridge.
•
Source route Transparent bridge.
The algorithms associated with these bridges will be presented, including a
detailed discussion on the Spanning Tree algorithm.
Unit Table of Contents
This unit contains the following lesson:
Lesson
Pages
Length
Lesson 2-1: Bridge Algorithms
64-96
5 hours
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Unit 2: Bridging
Lesson 2-1: Bridge Algorithms
At a Glance
Bridges are data link layer connectivity devices that connect two or more
LANs. They are independent of the OSI upper layers. They may be used
to connect like networks, for example, Ethernet to Ethernet or token ring
to token ring, or they may be used to connect unlike networks, for example,
Ethernet to FDDI. Bridges use the MAC source and destination addresses
to relay frames between connected networks.
Bridges Operate at the Data Link Layer
Sending
Workstation
Receiving
Workstation
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Data
Bridges have three basic functions:
•
Forwarding a frame from one segment to another across the bridge.
•
Filtering a frame that does not need to cross the bridge to reach its
destination.
•
Flooding a frame to all ports when the location of the destination
address is unknown.
This lesson describes four types of bridges, transparent learning,
translational, source route, and SRT bridges, including the primary
algorithms employed by bridges to relay packets across a network.
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What You Will Learn
After completing this lesson, you will be able to do the following:
•
Identify the characteristics and operation of transparent learning,
translational, source route, and SRT bridges.
•
Diagram and explain the function of the Spanning Tree algorithm.
•
Identify performance issues in bridging.
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Tech Talk
66
•
Aging—Within the Spanning Tree algorithm, aging is the technique
that allows addresses maintained in the forwarding table to be removed
if they have not been accessed over a period of time.
•
Blocking—One of four Spanning Tree port states, blocking prevents a
port from sending and receiving data.
•
Bridge Protocol Data Unit—BPDUs are the configuration messages
used by bridges to calculate a Spanning Tree.
•
Flooding—When the forwarding table does not contain a Destination
address for a station on a network, the bridge delivers the frame to all
the interfaces on the network, except the interface (port) that received
the frame.
•
Forwarding—A bridge is able to forward or relay a frame onto an
interface by referring to its forwarding table of stored source addresses.
•
Interface—An interchangeable term for the port on an internetworking
device.
•
Learning—One of four Spanning Tree port states, learning is an
intermediary state when the bridge is building its forwarding tables.
•
Listening—During this Spanning Tree intermediary state, a port is
listening to BPDUs and determines which bridge is the root and
whether the port will go into the blocking or forwarding state.
•
Root bridge—Within the Spanning Tree algorithm, the root bridge has
the lowest priority and MAC address and is responsible for maintaining
a loop-free environment and for maintaining communication to other
bridges in the network.
•
Route Discovery—The route discovery process is used in source-route
bridging where the path from the source to the destination is
discovered by receiving a pre-determined path documented inside the
frame.
•
Routing Information Field—Used in source-route bridging, the RIF is a
field placed just before the information field of a frame. The RIF
contains a table of discovered paths including the ring and bridge
number.
•
Spanning Tree Algorithm—Bridges use the Spanning Tree Algorithm
to ensure a loop-free topology by enabling a single path through the
network.
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Lesson 2-1: Bridge Algorithms
Transparent Learning Bridge
Digital Equipment Corporation developed the transparent learning bridge
for transporting frames in an Ethernet network at the data link and
physical layers of the OSI. Both the source and the destination have the
same data link address format.
The transparent learning bridge does not take part in route discovery or
the route selection process. It does keep track of the location of each
workstation on the network by building a forwarding table of each MAC
address and the corresponding interfaces (ports) associated with each
workstation.
Transparent Learning Bridges Build Forwarding Tables
01-67-15-cb-63-37
25-a7-d8-16-87-10
Ethernet LAN
Port 1
Bridge
Address
Port
01-67-15-cb-63-37
25-a7-d8-16-87-10
11-e8-71-35-41-f5
33-56-91-62-31-71
1
1
2
2
Port 2
Ethernet LAN
11-e8-71-35-41-f5
33-56-91-62-31-71
The bridge operates in a promiscuous mode in that it receives and
examines every frame transmitted across the networks to which it is
attached. It learns the location of each workstation on the network by
reading the source address of every packet received and noting which
interface (port) the frame was received. The bridge enters this information
into a forwarding table, which the bridge updates constantly. The bridge
either forwards or drops frames based on the information in the forwarding
table.
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Unit 2: Bridging
The Basic Forwarding Process
When a bridge receives a frame, it compares the frame’s source and
destination address with the addresses in the forwarding table. Depending
on the results, the bridge performs the following actions:
•
If the source address is not present in the forwarding table, the bridge
adds the source address and corresponding interface to the table. It
then checks the destination address to determine if it is in the table.
•
If the destination address is listed in the table, it determines if the
destination address is on the same LAN as the source address. If it is,
then the bridge drops the frame since all the workstations have already
received the frame.
•
If the destination address is listed in the table but is on a different LAN
than the source address, then the frame is forwarded to that LAN.
•
If the destination address is not listed in the table, then the bridge
forwards the frame to all the LANs except the one that which originally
received the frame. This process is called flooding.
In some bridges, if the bridge has not accessed an address in the
forwarding table over a period of time, the address is removed to free up
memory space on the bridge. This process is referred to as aging.
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Spanning Tree Algorithm
The Spanning Tree algorithm is a protocol developed by the IEEE to
enable bridges to have multiple bridge connections between networks and
reduce problems that occur from redundant links or loops in LANs. A
maximum of eight bridges and seven active paths may exist between two
devices using the Spanning Tree algorithm. The algorithm ensures a loopfree topology in a multi-bridge network.
To fully understand how the Spanning Tree algorithm works, it is
necessary to understand how loops occur in transparent bridging.
Steps One Through Six: The Frame is Flooded Across the Network
Host 1
01-67-15-cb-63-37
LAN 1
Port 1
Address
Port
S 01-67-15-cb-63-37 1
D ?
Bridge A
LAN 2
Port 2
Port 1
Address
Port 1
Port
Address
Bridge B
S 01-67-15-cb-63-37 1
D ?
Port 2
Host 2
12-37-41-53-a7-d8
Port
S 01-67-15-cb-63-37 1
D ?
Bridge C
Port 2
LAN 3
= Flooding
S = Source
D = Destination
1. Host 1 sends a frame out over LAN 1. Bridge A receives the frame on
port #1.
2. Bridge A records the source address (01-67-15-cb-63-37) of the frame
and the port number on which the frame was received (port 1).
3. Bridge A then looks for the destination address of the frame. If the
destination address is not in the forwarding table prior to receiving the
frame, the bridge floods the frame onto LAN 2.
4. Both Bridge B and Bridge C receive the frame. Both bridges record the
source address and the port number on which the frame was received
(port 1).
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Unit 2: Bridging
5. Since neither bridge has the destination address in their forwarding
table they both flood the frame onto LAN 3. This results in two frames
being sent onto LAN 3.
6. Host 2 receives the frame.
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Steps Seven and Eight: Bridge B and C Exchange the Frame
Host 1
01-67-15-cb-63-37
LAN 1
Port 1
Bridge A
LAN 2
Port 2
Port 1
Address
Port 1
Address
Port
Bridge B
S 01-67-15-cb-63-37 1
D ?
Port 2
Host 2
12-37-41-53-a7-d8
Port
S 01-67-15-cb-63-37 2
D ?
Loop
Bridge C
Port 2
LAN 3
= Flooding
S = Source
D = Destination
7. The frame passed on by Bridge B is received not only by Host 2, but
also by Bridge C on its port 2, starting a loop.
When Bridge C receives the frame on port 2, it compares the source
address and port number to the Host 1 entry in its forwarding table. It
acknowledges the frame has the same source address. However, since the
frame was now received on port 2, the bridge updates its forwarding table
to associate the source address with port 2.
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Unit 2: Bridging
Steps Nine and Ten: A Loop is Created
Host 1
01-67-15-cb-63-37
LAN 1
Port 1
Bridge A
Loop
LAN 2
Port 2
Port 1
Port 1
Address
Bridge B
Port 2
Host 2
12-37-41-53-a7-d8
Port
S 01-67-15-cb-63-37 2
D ?
Bridge C
Port 2
LAN 3
= Flooding
S = Source
D = Destination
9. The destination address of Host 2 is still not in Bridge C’s forwarding
table, so the frame is flooded back onto LAN 2, creating a loop.
10. The same events in steps 7-9 occur for Bridge B when it receives the
frame flooded onto LAN 3 by Bridge C.
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Steps Eleven through Thirteen: The Frame is Discarded Due to the Loop
Host 1
01-67-15-cb-63-37
LAN 1
Port 1
Address
Port
S 01-67-15-cb-63-37 1
D ?
Bridge A
LAN 2
Port 2
Port 1
Address
Port 1
Port
S 01-67-15-cb-63-37 2
D 12-37-41-53-a7-d8 2
Address
Bridge B
Port 2
Host 2
12-37-41-53-a7-d8
Port
S 01-67-15-cb-63-37 2
D 12-37-41-53-a7-d8 2
Bridge C
Port 2
LAN 3
= Flooding
S = Source
D = Destination
11. When Host 2 replies to the received frame, both bridges record the Host
2 source address in their forwarding tables and associate the address
with their port 2 interface.
12. Then both bridges look at the destination address and find both Host 1
and Host 2 associated with port 2, falsely indicating that both hosts are
located on LAN 3.
13. Both bridges discard the frame, assuming that all workstations on the
same LAN have already received the frame and forwarding is not
necessary.
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Unit 2: Bridging
Check Your Understanding
♦ Diagram how a loop is created in a bridge not using the Spanning
Tree algorithm. Explain your diagram in terms of conflicting MAC
addresses and port assignments in the forwarding table.
♦ Without reading ahead, speculate on a method to create a loop-free
bridging environment. What would the bridge have to do to prevent
loops? Or how could the bridge detect that a loop exists and make
adjustments for it rather than dropping the frame altogether?
Creating a Loop-free Environment
The Spanning Tree algorithm ensures loop-free transmissions of frames by:
•
Assigning priorities to each bridge, thereby producing a logical tree
topology out of any physical arrangement of bridges.
•
Enabling a single path throughout a network.
•
Providing automatic reconfiguration of the Spanning Tree topology
around a failed bridge or data path.
In the Spanning Tree algorithm, a root bridge is established that serves as
the top of the logical topology through which all frames must travel.
Initially, all bridges are equal and claim the position of the root bridge by
transmitting a packet known as the Bridge Protocol Data Unit (BPDU).
The BPDU contains information on the bridge priority number, which is
assigned by the network manager, and its MAC address. The bridge with
the lowest priority, which will be explained on the next page, becomes the
root bridge. If the priorities are the same for all bridges, then the bridge
with the lowest MAC address becomes the root bridge.
Once the root bridge is established, all other bridges stop sending BPDUs.
Only the root bridge generates BPDUs and the other bridges update and
forward the packets they receive.
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Bridge Priority and MAC Addresses Determine the Root
Host 1
00-21-a7-11-56-68
LAN 1
Port 1
Root
Bridge
Bridge Priority = 1
Bridge A
MAC Address = 0000A2001427
Port 2
LAN 2
Port 1
Port 1
Bridge B
Port 2
Bridge Priority = 2
MAC Address =
0000A280213
Bridge C
Port 2
Bridge Priority = 3
MAC Address =
0000A2001531
LAN 3
Host 2
12-37-41-53-a7-d8
Once the root bridge has been elected, the other bridges determine the
least cost path to the root bridge, establish a root port, and a designated
bridge for each LAN.
1. The initial BPDU generated by the root bridge has an initial root cost of
zero.
2. As a bridge receives a BPDU from the root bridge, it adds its port path
cost to the BPDU. The cost number for each bridge’s port is determined
and assigned by the network manager. Frequently the cost
corresponds to the speed of the wire, for example, 100 Mbs wire would
have a lower cost than a 10 Mbs wire.
3. The BPDU is then sent to the other bridges across all ports.
4. As each bridge receives the BPDU, they add their path cost to the root
cost of the BPDU and send the BPDU on.
5. The total roots cost values of the BPDUs received on all bridge ports
are compared, and the port with the lowest cost path to the root bridge
is designated as the root port. Once a bridge establishes its root port,
the bridge blocks its redundant ports. Blocked ports will not forward or
accept frames.
6. Next a designated bridge for each LAN must be established. The
designated bridge for a LAN is the bridge with the lowest cost path to
the root bridge.
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Unit 2: Bridging
Bridge C is the Designated Bridge for LAN 1
LAN 1
10
10
Designated
Bridge for
LAN 1
Port 1
Bridge C
LAN 2
10
Port 1
Bridge A
Bridge B
Port 2
Port 2
10
Path
Cost
20
10
10
Port 2
Root Port
15
Port 1
Port 1
= Blocked Port
5
Bridge D
Port 2
10
Root Bridge
LAN 3
Port 1
Bridge E
Port 2
LAN 4
In the illustration above, the path cost from Bridge B to the root bridge,
Bridge E, using port 2 is 20. Using port 1 on Bridge B and transmitting
via Bridge C, the path cost is reduced to 15. The root port for Bridge B is
then designated as port 1 since the total path cost is lower. The same
holds true for Bridge A on LAN 1. Bridge C becomes the designated bridge
for LAN 1 since the path cost is lowest. Port 2 on both Bridge A and B is
blocked, based on path cost, to avoid a loop in the spanning tree.
As the Spanning Tree algorithm progresses, all the bridge ports are in one
of the following states:
•
Blocking—The port is prevented from sending or receiving frames.
•
Listening—The port is listening to BPDUs and determines which
bridge is the root and whether the port will go into the blocking or
forwarding state.
•
Learning—The bridge forwarding table is being built while the port is
listening.
•
Forwarding—The port is allowed to receive or transmit frames.
Once the Spanning Tree algorithm has completed, all the bridge ports are
in either the blocking or forwarding state. Loops are prevented since a
path has been established and ports not used in the path are blocked from
receiving or transmitting frames.
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Check Your Understanding
♦ In the diagram, label the following:
1. Root Bridge
2. Designated Bridge
3. Root port or ports
4. Port Costs
5. Total path cost
♦ After labeling the diagram, draw arrows indicating the path a
frame would take from one of the LANs to the root bridge.
LAN 1
Port 1
Bridge C
Port 1
Port 1
Bridge A
Bridge B
Port 2
Port 2
Path
Cost
LAN 2
Port 2
Port 1
Bridge D
Port 2
LAN 3
Port 1
Bridge E
Port 2
LAN 4
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Unit 2: Bridging
Translational Bridges
Translational bridges are a type of transparent bridge that connects LANs
that use different protocols at the data link and physical layers, for
example, FDDI and Ethernet.
Translational Bridge
Bridge
FDDI
Ring
Ethernet LAN
Source Route Bridges
Source route bridging is used in token ring networks. A source route
bridge links two or more rings together. There are fundamental
characteristics in how a source route bridge transmits a frame between
rings. A source route bridge does not create and maintain forwarding
tables. The decision to forward or drop a frame is based on information
provided in the frame.
The destination station is responsible for maintaining routing tables that
define a route to all workstations on the network. The source workstation
is responsible for determining the path of a frame to its destination. If no
route information is available, then the source station has the ability to
perform route discovery to learn the potential paths that can be taken.
Source Route Bridge
Bridge
Token
Ring
Token
Ring
Bridge
Token
Ring
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Route Discovery
A process called route discovery is used to determine the path of the frame
across a multiple token ring network. The process of route discovery is
based on the requirement that a pre-determined path between the source
and destination station must be established before a frame may be
transmitted. The path guides the frame from the source station to the
destination station.
A discovery frame, or explorer frame, is sent over the network by the
source station. A discovery frame is similar to other MAC frames, but
includes a routing information field (RIF). The discovery frame gathers
information on the path taken to the destination as it passes through the
network.
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All Routes Explorer Frames (ARE)
Host 6
Bridge D
Ring 2
Bridge E
Ring 3
ARE#3
ARE#2
Bridge C
Ring 3
ARE#1
Bridge A
Bridge B
Ring 1
ARE transmitted
Al l Rout es
Expl orer
Fr ames
Host 1
There are two types of explorer frames:
•
All Routes Explorer Frames—These are multiple explorer frames sent
across the network at the same time by the source station. Each frame
collects path information and the first frame to return to the source
becomes the path of choice to the destination.
•
Single Route Explorer Frame—A single frame is sent across the
network via a specific bridge, designated by the user during the NIC
set-up. Multiple SREs are generated, one for each bridge. The frame to
reach the destination first becomes the path of choice.
Single Route Explorer Frames (SRE)
Host 2
Bridge C
Ring 2
Ring 3
Bridge B
Bridge A
Ring 1
Host 1
80
SRE to 6
USE Bridge A
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Ro u t e
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Lesson 2-1: Bridge Algorithms
A typical discovery process might go as follows:
1. The source station determines that the destination for a frame is not on
the local ring.
2. The source station checks its routing tables for route information to the
destination. The routing table maintains information on the ring and
bridge pair identification.
3. If the destination station does not know the route, the source station
sends an explorer frame on to the network with the ring number the
station is attached to.
4. The bridge accepts the explorer frame and adds the assigned bridge
number, and the number of the ring to which the frame is then
forwarded.
5. The bridge then forwards the frame to all adjacent bridges except the
ring from which it received the frame.
6. Each bridge along the path adds its bridge number and the number of
the ring to which the frame is then forwarded.
Each Bridge Adds Information to the Discovery Frame
Source
Bridge 1
Token
Ring 1
Token
Ring 2
Frame
Information
Added
R1B1R2
Frame
Informa tion
Added
R1B1R2 B2R3
Bridge 2
Token
Ring 3
Destination
7. Once the frame arrives at the destination, the frame contains the route
it took to get there. The destination station returns the frame with a
bit set telling the frame to use the same route in reverse.
8. Each bridge that receives the frame uses the route information in the
frame to forward the frame to the next ring.
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9. When the source station receives the response to its explore frame, it
updates its routing tables and uses that route information for all
further transmissions with that destination.
Routing Information Field
The routing information field (RIF) is the part of the frame that contains
the routing information needed by the bridge to forward the frame. The
RIF is an optional section of the frame and only used if the frame must
leave the local ring.
Each time a frame is sent to a previously discovered destination on a
separate token ring, the path is copied to the routing information field of
the frame. The RIF is 2-18 bytes in length and is placed at the beginning of
the frame data field. The path includes the type of path (ARE/SRE) and
each ring and bridge used to reach the destination. The location of the
destination device is represented as a bridge with the identification of "0”.
Source Route Transparent Bridges (SRT)
Source route transparent bridges are transparent bridges that combine the
capabilities of the source route and the transparent bridge. When a SRT
bridge receives a frame with a RIF, the bridge handles the frame just as a
source route bridge would. If the bridge receives a frame without a RIF, it
handles the frame the same as a transparent bridge would. Source Route
Transparent bridges are commonly used in small token ring environments
where transparently bridging token ring frames is faster and requires less
processing, additionally as Layer 2 switches came into play the SRT
algorithm was used in high speed SRT switches to combine multiple token
rings into a virtual token ring by using transparent bridging between
combined rings and still offering SRB to rings that required route
discovery.
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Source Route Transparent Bridge (SRT)
SRT
Bridge
Token
Ring
SRT
Bridge
Token
Ring
Token
Ring
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Performance Issues in Bridging
This lesson is one in a series about routers. So the first question asked is,
“Why discuss bridging in the first place?” Placing bridges and routers in
perspective relative to each other is important to understand why routers
are replacing bridges in networks.
There are several constraints in bridging.
•
Transparent bridges use only a subset of the network topology,
facilitated by the Spanning Tree algorithm. They do not maintain a
routing table to reference the best path from the source to the
destination. Changes in the topology cause the slow process of
reconfiguration.
•
Destination workstations in source route bridging do maintain route
tables, but these tables are lost each time the system is shut down.
•
The number of interconnected bridges is limited, thus placing a limit on
the size of the network.
•
Bridges drop packets that are too large to forward. They are not able to
break packets into smaller sections or reassemble packets once
received.
•
Bridges do not assign priority to packets. All packets are treated the
same and forwarded as equals. This leads to network congestion.
•
Bridges do not provide error checking.
In the lessons to follow, the advantages of routers over bridges will become
apparent. Each of the constraints in bridging is resolved in routing.
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Lesson 2-1: Bridge Algorithms
Try It Out
Create a Bridging Game
Most of the information presented in the Routing course will be somewhat
intangible, in that much of the operation of routers and bridges cannot be
seen. Everything happens behind the scenes.
The challenge in this activity is to create a viable game that demonstrates
the operation of either a transparent learning bridge or a source route
bridge. The game must address the problems associated with successful
packet delivery and their solutions, for example, Spanning Tree Algorithm
or Discovery Route.
Materials Needed:
•
Windows 95 PC
•
Any Word Processor (e.g., MS Word)
•
Pen/Pencil and Paper
It is best to work in-groups for this activity, since teamwork allows the
group to tap into the strengths of many. Everyone has strengths and
weaknesses. Some people are great organizers, others can draw, others
can research, and others are fantastic writers. Teams of developers create
all the popular software computer games sold on today’s market.
Criteria for the game:
1. The focus of the game must be on either transparent learning or source
route bridging.
2. The design of the game must have an end result of increasing game
participants’ understanding of the type of bridging addressed in the
game.
3. Objectives must be stated for the expected outcomes of knowledge after
playing the game.
4. There must be at least one assessment component to the game to
document if participants understood more about bridging after using
the game (for example, a point system for successful progression
through the game).
5. The basic design of the game may be one of various designs (for
example, board game, role-playing, trivial pursuit).
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6. Creativity, clarity, attractive appearance, and engaging and fun
activities are a must.
7. Fellow students (not involved in the game design) must test the game.
Revisions are to be made to address problems discovered in the game.
Rubric: Suggested Evaluation Criteria and Weightings
86
Criteria
%
Clear objectives and assessments
10
Creativity, clarity, and attractive appearance
30
Quality of activities: engaging and fun
40
Cooperative group teamwork
20
TOTAL
100
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Routing
Lesson 2-1: Bridge Algorithms
Stretch Yourself
Spanning
Spanning Tree Poetry
Algorhyme
I think that I shall never see
A graph more lovely than a tree.
A tree whose crucial property
Is loop-free connectivity.
A tree that must be sure to span
So packets can reach every LAN.
First, the root must be selected.
By ID, it is elected.
Least cost paths from root are traced.
In the tree, these paths are placed.
A mesh is made by folks like me,
Then bridges find a spanning tree.
R. Perlman, INTERCONNECTIONS (poem entitled “Algorhyme”-page 54).
 1992 by Addison Wesley Publishing Company. Reprinted by permission
of Addision Wesley Longman.
Radia Perlman is the creator of the Spanning Tree Algorithm, and is a
well-known authority on the subject of bridges and routers. She wrote the
poem “Algorhyme” as a spoof of Joyce Kilmer’s (1886-1918) poem, “Trees.”
(Spurgeon, 1997)
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Materials Needed:
•
Windows 95 PC
•
Any Word Processor (e.g., MS Word)
•
Pen/Pencil and Paper
•
Color Pencils or Color Markers
1. Write an interpretation of Ms. Perlman’s poem referencing the actual
events that occur in the Spanning Tree Algorithm line by line.
Rubric: Suggested Evaluation Criteria and Weightings
Criteria
%
Overall analysis and synthesis
50
Correct referencing of Spanning Tree events with
each line in the poem.
50
TOTAL
100
Your Score
2. Illustrate an impressionistic Spanning Tree that can accompany Ms.
Perlman’s poem, in which the topology accurately represents the
algorithm. The tree must be accurate, but the illustration should be
creative, colorful, and with an impressionistic flare, actually look like a
tree.
Rubric: Suggested Evaluation Criteria and Weightings
88
Criteria
%
Use of creative impressionism
40
Accurate representation of the Spanning Tree
Algorithm
40
Quality illustration suitable for reproduction
20
TOTAL
100
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Routing
Lesson 2-1: Bridge Algorithms
Network Wizards
Comparing Source Route Bridges and Routers
Source route bridges operate very similarly to routers. Some references
even go as far as to say that source route bridges really are routers in
disguise, so to speak.
The confusion over networking terms is enough to frustrate even the most
experienced network managers. Often different names or terms are used
to represent the same thing. Joseph Bardwell said it quite well in his
poem, “Making the Connection”. (Optimized Engineering Corporation,
1999)
Making the Connection
Sometimes it amazes me
that routers work at Layer 3
when switches very well could do
the job at simply Layer 2.
But switches work at Layer 3.
Oh, how confusing this can be
when bridges work at Layer 2
and routers can be bridges too!
And when you hope there’d be no more
you find a switch at Layer 4.
So Layer 4, and 2, and 3
imply OSI conformity.
But these are simply building blocks
in what we’ll call an “Interconnect Box”.
Poem “Making the Connection” by Joseph Bardwell, copyright  1995-1999
by Optimizing Engineering Corporation, reprinted with permission.
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Materials Needed:
•
Windows 95 PC
•
Any Word Processor (e.g., MS Word)
•
Pen/Pencil and Paper
1. Jump ahead and research the main characteristics of a router.
2. Compare your router research to the information in this lesson on
source route bridges.
3. Write an explanation as to why some network experts say that source
route bridges should really be considered routers.
4. Document your resources.
Rubric: Suggested Evaluation Criteria and Weightings
Criteria
%
Analysis and synthesis of research
40
Well thought out comparison
40
Resources
20
TOTAL
100
Your Score
Summary
In this lesson, you learned the following:
90
•
The characteristics and operation of transparent learning,
translational, source route, and SRT bridges.
•
How to diagram and explain the function of the Spanning Tree
Algorithm.
•
The performance issues in bridging.
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Lesson 2-1: Bridge Algorithms
Review Questions
Name___________________
Lesson 2-1: Bridge Algorithms
Part A
1. A transparent learning bridge builds a forwarding table by:
a. reading the source MAC address of every received frame and noting
which port the frame was received on.
b. reading the destination MAC address of every received frame and
noting which port the frame was transmitted on.
c. reading the source logical network address of every received frame
and noting which port the frame was received on.
d. reading the destination logical network address of every received
frame and noting which interface the frame was transmitted on.
2. When a transparent learning bridge receives a frame, it compares the
frame’s destination address with addresses in the forwarding table.
What will happen if there is no match?
a. The frame will be dropped.
b. The frame will be flooded.
c. The frame will be broadcast out all ports.
d. The frame’s destination address will be added to the forwarding
table and then the frame will be flooded.
3. In the Spanning Tree algorithm, a designated bridge is:
a. the bridge with the highest path cost to the root bridge.
b. the bridge with the lowest path cost to the root bridge.
c. the bridge with the root port.
d. the bridge with the lowest priority and MAC address.
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Unit 2: Bridging
4. The type of bridge used only to connect two or more token rings is:
a. transparent bridge
b. learning bridge
c. source route bridge
d. translational bridge
e. SRT bridge
5. The type of bridge that provides a network connection between LANs
that use different protocols at the physical and data link layers is
called:
a. transparent bridge
b. learning bridge
c. source route bridge
d. translational bridge
e. SRT bridge
6. The bridge that links FDDI networks to networks using different
protocols is called:
a. transparent bridge
b. learning bridge
c. source route bridge
d. translational bridge
e. SRT bridge
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Lesson 2-1: Bridge Algorithms
7. Source-route bridges use what process to determine the path of a
frame?
a. Spanning Tree algorithm
b. Address Resolution Protocol
c. Route Discovery
d. Encapsulation
8. Bridges operate at what level of the OSI model?
a. Physical layer
b. Network layer
c. Data link layer
d. Application layer
e. Transport layer
9. As a discovery frame travels from ring to ring and crosses bridges, it
records the path in the:
a. Routing table
b. Routing information field
c. MAC frame
d. Logical network address
10. Which of the following is responsible for determining the path that a
SRB frame will use in order to reach a destination?
a. Source route bridge
b. destination station
c. source station
d. repeater
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Unit 2: Bridging
Part B
Diagram each of the following types of bridges, including descriptive labels.
1. Transparent Learning Bridge
2. Source Route Bridge
3. Translational Bridge
4. SRT Bridge
Part C
1. Diagram how the Spanning Tree algorithm functions to create a loopfree bridge topology. Include in your diagram descriptive labels.
2. Write an explanation of your Spanning Tree diagram.
Part D
Indicate each statement as either True (T) or False (F).
94
1.
All bridges use routing tables to select a path to a
workstation.
2.
Bridges do not assign priority to the packets they transmit.
3.
The number of interconnected bridges is limited.
4.
Bridges break large packets into smaller segments to allow
forwarding of the packet.
5.
Bridges provide error checking when there is a problem in
the network
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Lesson 2-1: Bridge Algorithms
the network.
6.
Transparent bridges use only a subset of the network
topology.
7.
All packets are examined and treated differently when
forwarded.
8.
Destination workstations maintain routing tables in source
route bridging.
9.
The routing tables in source route bridging must be rebuilt
each time the system is booted.
10.
The size of a network using bridges is unlimited.
Scoring
Rubric: Suggested Evaluation Criteria and Weightings
Criteria
%
Part A: Identify the characteristics and
operation of transparent learning,
translational, source route, and SRT bridges.
20
Part B: Identify the characteristics and
operation of transparent learning,
translational, source route, and SRT bridges.
20
Part C: Diagram and explain the operation of
the Spanning Tree Protocol.
40
Part D: Identify performance issues in
bridging.
20
TOTAL
100
Try It Out
100
Stretch Yourself
100
Network Wizards
100
FINAL TOTAL
400
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Your Score
95
Unit 2: Bridging
Resources
Intel Corporation. (1999). Intel Technology Glossary. Available On-line:
http://www.206.204.30.82/olc/glossary.cfm.
Keshav, S. (1997). An Engineering Approach to Computer Networking:
ATM Networks, the Internet, and the Telephone Network. Addison-Wesley
Publishing Company, Reading, Massachusetts.
Optimized Engineering Corporation. (1999). Network Interconnect Devices:
Repeaters, Bridges, Switches, Routers. Available On-line:
http://www.optimized.com/COMPENDI/L1-Inter.html#MakeConn.
Perlman, R. (1992). Interconnections: Bridges and Routers. . AddisonWesley Publishing Company, Reading, Massachusetts.
Spurgeon, Charles E. (1997). Practical Networking With Ethernet,
International Thomson Computer Press, Boston, Massachusetts.
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