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Spanning Tree Protocol

Spanning Tree Protocol
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1

Overview

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1

7.1

Redundant
Topologies

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2

7.1.1


Redundancy
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2

7.1.2


Redundant topologies
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3

7
.1.3


Redundant switched topologies

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5

7.1.4

Broadcast storms

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6

7.1.5

Multiple frame transmissions
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7

7.1.6

Media access control database instability

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7.2

Spanning
-
Tree Protocol

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8

7.2.1

Redunda
nt topology and spanning tree

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8

7.2.2


Spanning
-
tree protocol

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7.2.3


Spanning
-
tree operation

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11

7.2.4

Selecting the root bridge

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7.2.5


Stages of spanning
-
tree port states
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7.2.6

Spanni
ng
-
tree recalculation
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14

7.2.7

Rapid spanning
-
tree protocol

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15

Summary

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16


Overview


Redundancy
in a network is critical. It allows networks to be fault tolerant. Redundant
topologies protect against network downtime, or nonavailability. Downtime can be
caused by the failure of a single link, port, or network device. Network engineers are
often requi
red to balance the cost of redundancy with the need for network
availability.


Redundant topologies based on switches and bridges are susceptible to broadcast
storms, multiple frame transmissions, and MAC address database instability. These
problems can ma
ke a network unusable. Therefore, redundancy should be carefully
planned and monitored.


Switched networks provide the benefits of smaller collision domains,
microsegmentation, and full duplex operation. Switched networks provide better
performance.


Redun
dancy in a network is required to protect against loss of connectivity due to the
failure of an individual component. However, this provision can result in physical
topologies with loops. Physical layer loops can cause serious problems in switched
networks
.


The Spanning
-
Tree Protocol is used in switched networks to create a loop free logical
topology from a physical topology that has loops. Links, ports, and switches that are
not part of the active loop free topology do not forward data frames. The Spannin
g
-
Tree Protocol is a powerful tool that gives network administrators the security of a
redundant topology without the risk of problems caused by switching loops.



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Students who complete this module should be able to perform the following tasks:




Define red
undancy and its importance in networking



Describe the key elements of a redundant network topology



Define broadcast storms and describe their impact on switched networks



Define multiple frame transmissions and describe their impact on switched
networks



Ide
ntify causes and results of MAC address database instability



Identify the benefits and risks of a redundant topology



Describe the role of spanning
-
tree in a redundant
-
path switched network



Identify the key elements of spanning
-
tree operation



Describe the p
rocess for root bridge election



List the spanning
-
tree states in order



Compare Spanning
-
Tree Protocol and Rapid Spanning
-
Tree Protocol


7.1

Redundant Topologies

7.1.1


Redundancy


This page will explain how redundancy can improve network reliability and
pe
rformance.


Many companies and organizations increasingly rely on computer networks for their
operations. Access to file servers, databases, the Internet, intranets, and extranets is
critical for successful businesses. If the network is down, productivity
and customer
satisfaction decline.


Increasingly, companies require continuous network availability, or uptime. 100
percent uptime is perhaps impossible, but many organizations try to achieve 99.999
percent, or five nines, uptime. Extremely reliable networ
ks are required to achieve
this goal. This is interpreted to mean one hour of downtime, on average, for every
4,000 days, or approximately 5.25 minutes of downtime per year. To achieve such a
goal requires extremely reliable networks.


Network reliability
is achieved through reliable equipment and network designs that
are tolerant to failures and faults. Networks should be designed to reconverge rapidly
so that the fault is bypassed.


The figure overleaf illustrates redundancy. Assume that a car must be use
d to get to
work. If the car has a fault that makes it unusable, it is impossible to use the car to go
to work until it is repaired.


On average, if the car is unusable due to failure one day out of ten, the car has ninety
percent usage. Therefore, reliabi
lity is also 90 percent.



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A second car will improve matters. There is no need for two cars just to get to work.
However, it does provide redundancy, or backup, in case the primary vehicle fails.
The ability to get to work is no longer dependent on a single

car.



Both cars may become unusable simultaneously, one day in every 100. The second
car raises reliability to 99 percent.


7.1.2


Redundant topologies


This page will explain the concept and benefits of a redundant topology.


A goal o
f redundant topologies is to eliminate network outages caused by a single
point of failure. All networks need redundancy for enhanced reliability.


A network of roads is a global example of a redundant topology. If one road is closed
for repair, there is l
ikely an alternate route to the destination.




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Consider a community separated by a river from the town centre. If there is only one
bridge across the river, there is only one way into town. The topology has no
redunda
ncy.


If the bridge is flooded or damaged by an accident, travel to the town centre across the
bridge is impossible.


A second bridge across the river creates a redundant topology. The suburb is not cut
off from the to
wn centre if one bridge is impassable.


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7.1.3


Redundant switched topologies


This page will explain how switches operate in a redundant topology.


Networks with redundant paths and devices allow for more network uptime.
Redundant topo
logies eliminate single points of failure. If a path or device fails, the
redundant path or device can take over the tasks of the failed path or device.



If Switch A fails, traffic can still flow from Segment 2 to Segment 1 and to the r
outer
through Switch B.


Switches learn the MAC addresses of devices on their ports so that data can be
properly forwarded to the destination. Switches flood frames for unknown
destinations until they learn the MAC addresses of the devices. Broadcasts and
multicasts are also flooded.


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A redundant switched topology may cause broadcast storms, multiple frame copies,
and MAC address table instability problems.


7.1.4

Broadcast storms


This page will explain the effects of
broadcasts and multicasts in a switched network.


Broadcasts and multicasts can cause problems in a switched network.


Multicasts are treated as broadcasts by the switches. Broadcast and multicast frames
are flooded out all ports, except the one on which t
he frame was received.


If Host X sends a broadcast, like an ARP request for the Layer 2 address of the router,
then Switch A will forward the broadcast out all ports. Switch B is on the same
segment and also forwards all broadcasts. Switch B receives all
the broadcasts that
Switch A forwarded and Switch A receives all the broadcasts that Switch B
forwarded. Switch A forwards the broadcasts received from Switch B. Switch B
forwards the broadcasts received from Switch A.


The switches continue to propagate b
roadcast traffic over and over. This is called a
broadcast storm. This broadcast storm will continue until one of the switches is
disconnected. Since broadcasts require time and network resources to process, they
reduce the flow of user traffic. The networ
k will appear to be down or extremely
slow.


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7.1.5

Multiple frame transmissions


This page will explain multiple frame transmissions in a redundant switched network.


In a redundant switched network it is possible for an end device to re
ceive multiple
frames.



Assume that the MAC address of Router Y has been timed out by both switches. Also
assume that Host X still has the MAC address of Router Y in its ARP cache and sends
a unicast frame to Router Y. The router receiv
es the frame because it is on the same
segment as Host X.


Switch A does not have the MAC address of Router Y and will therefore flood the
frame out its ports. Switch B also does not know which port Router Y is on. Switch B
then floods the frame it receive
d. This causes Router Y to receive multiple copies of
the same frame. This results in unnecessary utilization of network resources.


7.1.6

Media access control database instability


This page will explain how incorrect information can be forwarded in a red
undant
switched network.



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In a redundant switched network it is possible for switches to learn the wrong
information. A switch can incorrectly learn that a MAC address is on one port, when
it is actually on a different port. In this example the MAC address

of Router Y is not
in the MAC address table of either switch.



Host X sends a frame directed to Router Y. Switches A and B learn the MAC address
of Host X on port 0.


The frame to Router Y is flooded on port 1 of both switches. Switche
s A and B
receive this information on port 1 and incorrectly learn the MAC address of Host X
on port 1. When Router Y sends a frame to Host X, Switch A and Switch B also
receive the frame and will send it out port 1. This is unnecessary, but the switches
h
ave incorrectly learned that Host X is on port 1.


In this example the unicast frame from Router Y to Host X will be caught in a loop.


7.2

Spanning
-
Tree Protocol

7.2.1

Redundant topology and spanning tree


This page will teach students how to create a loo
p free logical topology.


Redundant network topologies are designed to ensure that networks continue to
function in the presence of single points of failure. Work is interrupted less often for
users because the network continues to function. Any interrupti
ons that are caused by
a failure should be as short as possible.


Reliability is increased by redundancy. A network that is based on switches or bridges
will introduce redundant links between those switches or bridges to overcome the
failure of a single li
nk. These connections introduce physical loops into the network.
These bridging loops are created so if one link fails another can take over the function
of forwarding traffic.


When the destination of the traffic is unknown to a switch, it floods traffic
out all
ports except the port that received the traffic. Broadcasts and multicasts are also
forwarded out every port except the port that received the traffic. This traffic can be
caught in a loop.


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In the Layer 2 header, there is no Ti
me To Live (TTL) value. If a frame is sent into a
Layer 2 looped topology of switches, it can loop forever. This wastes bandwidth and
makes the network unusable.


At Layer 3, the TTL is decremented and the packet is discarded when the TTL
reaches 0. This c
reates a dilemma. A physical topology that contains switching or
bridging loops is necessary for reliability, yet a switched network cannot have loops.



The solution is to allow physical loops, but create a loop free logical topology. F
or
this logical topology, traffic destined for the server farm attached to Cat
-
5 from any
user workstation attached to Cat
-
4 will travel through Cat
-
1 and Cat
-
2. This will
happen even though there is a direct physical connection between Cat
-
5 and Cat
-
4.


T
he loop free logical topology created is called a tree. This topology is a star or
extended star logical topology. This topology is the spanning
-
tree of the network. It is
a spanning
-
tree because all devices in the network are reachable or spanned.


The al
gorithm used to create this loop free logical topology is the spanning
-
tree
algorithm. This algorithm can take a relatively long time to converge. A new
algorithm called the rapid spanning
-
tree algorithm was developed to reduce the time
for a network to co
mpute a loop free logical topology.



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7.2.2


Spanning
-
tree protocol


This page will explain how STP can be used to create a loop free network.


Ethernet bridges and switches can implement the IEEE 802.1d Spanning
-
Tree
Protocol and use the spanning
-
tree algo
rithm to construct a loop free shortest path
network.


Shortest path is based on cumulative link costs. Link costs are based on the speed of
the link.



The Spanning
-
Tree Protocol establishes a root node called the roo
t bridge. The
Spanning
-
Tree Protocol constructs a topology that has one path for every node on the
network. This tree originates from the root bridge. Redundant links that are not part of
the shortest path tree are blocked.


It is because certain paths are

blocked that a loop free topology is possible. Data
frames received on blocked links are dropped.


The Spanning
-
Tree Protocol requires network devices to exchange messages to detect
bridging loops. Links that will cause a loop are put into a blocking stat
e.


Switches send messages called the bridge protocol data units (BPDUs) to allow the
formation of a loop free logical topology. BPDUs continue to be received on blocked
ports. This ensures that if an active path or device fails, a new spanning
-
tree can be

calculated.


BPDUs contain information that allow switches to perform specific actions:




Select a single switch that will act as the root of the spanning
-
tree.



Calculate the shortest path from itself to the root switch.


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Designate one of the switches as th
e closest one to the root, for each LAN
segment. This switch is called the designated switch. The designated switch
handles all communication from that LAN segment towards the root bridge.



Choose one of its ports as its root port, for each non
-
root switch.

This is the
interface that gives the best path to the root switch.



Select ports that are part of the spanning
-
tree. These ports are called designated
ports. Non
-
designated ports are blocked.



7.2.3


Spanning
-
tree operation


This page w
ill teach students about the ports and devices that are found in an STP
switched network.


When the network has stabilized, it has converged and there is one spanning
-
tree per
network.


As a result, for every switched network the following elements exist:




One root bridge per network



One root port per non
-
root bridge



One designated port per segment



Unused, or non
-
designated ports



Root ports and designated ports are used for forwarding (F) data traffic.


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Non
-
designated ports discard data

traffic. These ports are called blocking (B) or
discarding ports.


7.2.4

Selecting the root bridge


This page will explain how a root bridge is selected in an STP network.


The first decision that all switches in the network make is to identify the root b
ridge.
The position of the root bridge in a network affects the traffic flow.


When a switch is turned on, the spanning
-
tree algorithm is used to identify the root
bridge. BPDUs are sent out with the bridge ID (BID). The BID consists of a bridge
priority t
hat defaults to 32768 and the switch MAC address. By default BPDUs are
sent every two seconds.



When a switch first starts up, it assumes it is the root switch and sends BPDUs that
contain the switch MAC address in both the root and sen
der BID.



These BPDUs are considered inferior because they are generated from the designated
switch that has lost its link to the root bridge. The designated switch transmits the
BPDUs with the information that it is the root bridge as

well as the designated bridge.

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These BPDUs contain the switch MAC address in both the root and sender BID. The
BIDs are received by all switches. Each switch replaces higher root BIDs with lower
root BIDs in the BPDUs that are sent out. All switches recei
ve the BPDUs and
determine that the switch with the lowest root BID value will be the root bridge.



Network administrators can set the switch priority to a smaller value than the default,
which makes the BID smaller. This should only be

implemented when the traffic flow
on the network is well understood.


7.2.5


Stages of spanning
-
tree port states


This page will explain the five port states of a switch that uses STP.


Time is required for protocol information to propa
gate throughout a switched
network. Topology changes in one part of a network are not instantly known in other
parts of the network due to propagation delay. Data loops can occur when a switch
changes the state of a port too quickly.


Each port on a switch

that uses the Spanning
-
Tree Protocol has one of five states, as
shown in the figure overleaf.


In the blocking state, ports can only receive BPDUs. Data frames are discarded and no
addresses can be learned. It may take up to 20 seconds to change from this

state.


Ports transition from the blocking state to the listening state. In this state, switches
determine if there are any other paths to the root bridge. The path that is not the least
cost path to the root bridge returns to the blocking state. The list
ening period is called

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the forward delay and lasts for 15 seconds. In the listening state, data is not forwarded
and MAC addresses are not learned. BPDUs are still processed.



Ports transition from the listening state to the learning st
ate. In this state, data is not
forwarded, but MAC addresses are learned from traffic that is received. The learning
state lasts for 15 seconds and is also called the forward delay. BPDUs are still
processed.


Ports transitions from the learning state to t
he forwarding state. In this state user data
is forwarded and MAC addresses continue to be learned. BPDUs are still processed.


A port can be in a disabled state. This disabled state can occur when an administrator
shuts down the port or the port fails.


T
he time values given for each state are the default values. These values have been
calculated on an assumption that there will be a maximum of seven switches in any
branch of the spanning
-
tree from the root bridge.


7.2.6

Spanning
-
tree recalculation


This
page will describe the convergence of a spanning
-
tree network.


A switched internetwork has converged when all the switch and bridge ports are in
either the forwarding or blocking state. Forwarding ports send and receive data traffic
and BPDUs. Blocking po
rts only receive BPDUs.


When the network topology changes, switches and bridges recompute the spanning
-
tree and cause a disruption in network traffic.


Convergence on a new spanning
-
tree topology that uses the IEEE 802.1d standard can
take up to 50 seco
nds. This convergence is made up of the max
-
age of 20 seconds,
plus the listening forward delay of 15 seconds, and the learning forward delay of 15
seconds.


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7.2.7

Rapid spanning
-
tree protocol


This page will describ
e the Rapid Spanning
-
Tree Protocol.


The Rapid Spanning
-
Tree Protocol is defined in the IEEE 802.1w LAN standard. The
standard and protocol introduce new features:




Clarification of port states and roles



Definition of a set of link types that can go to for
warding state rapidly



Concept of allowing switches in a converged network to generate BPDUs rather
than relaying root bridge BPDUs


The blocking state of a port is renamed as the discarding state. The role of a
discarding port is that of an alternate port.

The discarding port can become the
designated port if the designated port of the segment fails.


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Link types have been defined as point
-
to
-
point, edge
-
type, and shared. These changes
allow rapid discovery of link failure in switched net
works.



Point
-
to
-
point links and edge
-
type links can go to the forwarding state immediately.


Network convergence should take no longer than 15 seconds with these changes.


The Rapid Spanning
-
Tree Protocol, IEEE 802.1w, will eventually
replace the
Spanning
-
Tree Protocol, IEEE 802.1d.


Summary


Redundancy is defined as a duplication of components that allows continued
functionality despite the failure of an individual component. In a network, redundancy
means to have a backup method to co
nnect all devices. Redundant topologies increase
network reliability and decrease downtime caused by a single point of failure.


A redundant switched topology may cause broadcast storms, multiple frame
transmissions, and MAC address table instability probl
ems. A broadcast storm is

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caused by multiple hosts that send and receive multiple broadcast messages. The
result is that they continue to propagate broadcast traffic over and over until one of
the switches is disconnected. During a broadcast storm, the net
work appears to be
down or extremely slow. Multiple frame transmissions occur when a router receives
multiple copies of a frame from multiple switches due to an unknown MAC address.
These excessive transmissions cause the router to time out. When a switch
incorrectly
learns a MAC address of a port, it can cause a loop situation and instability for the
MAC address table.


Since switches operate at Layer 2 of the OSI model, all forwarding decisions are
made at this level. Layer 2 does not provide a TTL value,

which is the set amount of
time a packet is provided to reach a destination. The problem is that physical
topologies contain switching or bridging loops necessary for reliability, yet a switched
network cannot have loops. The solution is to allow physical

loops, but create a loop
free logical topology.


The loop free logical topology created is called a tree. The topology is a star or
extended star that spans the tree of the network. All devices are reachable or spanned.
The algorithm used to create this l
oop free logical topology is the spanning
-
tree
algorithm.


The Spanning
-
Tree Protocol establishes a root node, called the root bridge. The
Spanning
-
Tree Protocol constructs a topology that has one path for every node on the
network. This results in a tree
that originates from the root bridge. Redundant links
that are not part of the shortest path tree are blocked. It is because certain paths are
blocked that a loop free topology is possible. Data frames received on blocked links
are dropped.


Switches send
messages called the bridge protocol data units (BPDUs) to allow the
formation of a loop free logical topology. BPDUs continue to be received on blocked
ports. BPDUs contain information that allow switches to perform specific actions:




Select a single switc
h that will act as the root of the spanning
-
tree.



Calculate the shortest path from itself to the root switch.



Designate one of the switches as the designated switch.



Choose one of its ports as its root port, for each non
-
root switch.



Select ports that are
part of the spanning
-
tree. These ports are called
designated ports.


IEEE 802.1w LAN standard defines the Rapid Spanning
-
Tree Protocol. It serves to
clarify port states and roles, define a set of link types, and allow switches in a
converged network to gen
erate BPDUs rather than use the root bridge BPDUs. The
blocking state of a port is renamed as the discarding state. The role of a discarding
port is that of an alternate port. The discarding port can become the designated port if
the designated port of the

segment fails.