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Training objective

Understand the background of the creation ofthe STP protocol

Understand STP working principle

Understand RSTP workingprinciple

Use Huawei STP protocol commandsskillfully

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Training content

Applications of transparent network bridges;

Why introduce spanning tree protocols?

Spanning tree protocol working principle;

Performance of spanning tree protocol;

Why introduce fast spanning tree protocolFast spanning tree protocol working principle;

What are the differences between spanning trees and fast spanning trees?Configuration and common commands of spanning trees;

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Expand LAN abilityAddress information of free dynamic studying web site

Question:Ordinary transparent network bridges will not make any sign for forwarded messages. As aresult, if there is a loop in the network, messages might be forwarded circularly and continuously, leading

to network congestion

Applications of Transparent Network Bridges

A DCB

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Occurrence of Path Loops

A'

LAN 1

LAN 2

A

B1 B2 B3

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Why introduce spanning tree protocol?

Remove path loops that might exist in the bridging network by disconnecting redundant links

Activate redundant backup links to restore network connection when the current active path fails

LAN A

LAN D

LAN C

LAN B

ROOTLAN E

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Basic principle of spanning tree protocol

Basic idea: transmit special information among network bridges (configurationinformation) and contain the following information to do the following:

Select one from all network bridges in the network to be used as the root bridge;

Calculate the shortest path fromthe local bridge to the root bridge;

For eachLan first select a bridge nearest to that network bridge as a designated bridge,to handle the data forward on its Lan;

For the network bridge, select a root port, and the path given from this port will be theoptimal path from this bridge to the root bridge;

Select ports (designated ports) contained on the spanning tree except the root port.

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Configuration message is also called bridge protocol data unit(BPDU)Main content includes:

Identifier of root bridge (Root ID)

The smallest path cost from a designated network bridge to a root bridge(RootPathCost)

Identifierof a designated network bridge

Identifierof a designated port of a designated network bridge

I.e., (RootID, RootPathCost,DesignatedBridgeID, DesignatedPortID)

Contents of short messages

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on gura on messageformat

DMA SMA L/T LLC Header payload

0x0180c2000000 0x01000010 BPDU data

Domain Seizure byte

Protocol ID 2

Protocol version 1

BPDU type 1

Flag bit 1

Root bridge ID 8

Root path cost 4

Designatedbridge ID

8

Designatedport ID

2

Message Age 2

MaxAge 2

HelloTime 2

ForwardDelay 2

Destination MACaddress

The destinationaddress of aconfigurationmessage is afixed groupbroadcasting

address

Source MACaddress

The

source MACaddress is theMAC address ofthe bridge

sending this

configurationmessage

Fixed link headof configuration

message

Framelength

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Compare the configuration messages received from every port with its own configuration

message to obtain the configuration message of the highest priority so as to replace its ownconfiguration message. This mainly includes:

Select RootID of the root bridge : the RootID of the configuration message with thehighest priorityCalculate the overhead of the shortest path to the root bridge RootPathCost: if it is the

root bridge itself, then the shortest path overhead is 0; otherwise, that overhead is the sumbetween the RootPathCost of optimal configuration message it receives and the

overhead of the port that receives this configuration message

Select Root Port: if it is the root bridge itself, then the root port is 0; otherwise, the rootport is the one that receives the optimal configuration message

Select designated port: including other ports in the Forwarding status on the spanning tree

Send new configuration message from a designated port

rocess ng con gura onmessages

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Assume there are two configuration messagesC1 and C2, then:

If RootID of C1 is less than RootID of C2, then C1 is prior to C2

If RootID of C1 and C2 is the same, but if the RootPathCost of C1 is less than C2, then C1 isprior to C2

If RootID and RootPathCost of C1 and C2 are the same, but if the TransmitID of C1 is lessthan C2, then C1 is prior to C2

If RootID, RootPathCost, and TransimitId of C1 and C2 are the same, but if the PortID of C1

is less than C2, then C1 is prior to C2

optimal configuration

message?Configuration message prioritycomparison rules:

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B92

Port1

Port4Port3

Port2

Port5

81,0,81 41,19,12541,12,315

41,12,111

41,13,90

Blocking

Blocking

Select Port4 as the root port according to the priority of the configuration messagereceived, select Port1 and Port2 as designated ports, and meanwhile block Port3 andPort5.

Send new configuration messages from Port1 and Port2: (41, 13, 92), among of them:

RootId = 41RootPathCost = 12+1 = 13

RootPort = Port4

An instance of receiving and processingconfiguration messages

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What to do if links fail?

Hello TimeThe network bridge cyclically sends configuration messages from a designated port at the

period ofHello Time

Message Age and Max AgeConfiguration messages stored at a port have aMessage Agefield, which growsproportionally with time. Each time one configuration message with a shorter message age isreceived, its own configuration message is updated. Whenthe message age reaches the

Max Ageand when no configuration message whatever is received in a period oftime, the network bridge will regard it as failure on the links connected to this port, and will startfailure processing

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Processing I of link fault

When the message age of the configurationmessage of Port4 is timeout, thisconfiguration message is abandoned, and a

new spanning tree are calculated, so as toselect Port3 as the new root port, while theconfiguration message of network bridge 92remains unchanged

B92

Port1

Port4Port3

Port2

Port5

41,13,9241,12,315

41,12,11141,13,90

41,13,92

41,13,92

Blocking

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B92

Port1

Port4Port3

Port2

Port5

41,12,31541,12,111

41,13,90

41,14,92

41,14,9241,14,92

41,14,92

Processing II of link fault

When the message age of the configurationmessage of Port3 is timeout, thisconfiguration message is abandoned, and a

new spanning tree are calculated, so as toselect Port5 as the new root port, while theconfiguration message of network bridge 92changes to 41 14 92

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Processing III of link fault

When the message age of the configurationmessage of Port5 is timeout, thisconfiguration message is abandoned, and it

is taken itself as the root bridge to send theconfiguration message (92 0 92) till aconfiguration message of a higher priority isreceived from any port

B92

Port1

Port4Port3

Port2

Port5

41,12,31541,12,111

41,13,90

92,0,92

92,0,9292,0,92

92,0,92 92,0,92

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Problems of temporary loops

When the topology structure changes, newconfiguration messages are broadcast to thewhole network only after a certain delay.

Before all bridges get the message of thischange,

if ports at the Forwarding status in the originaltopology structure have not realized that they

should stop forwarding in the new topology,temporary loops might exist;

if ports blocked in the original topologystructure have not realized that they should

ow o avo emporary

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ow o avo emporaryloops?

Ports will go through a certain delay to change from

the blocked status to the Forwarding status. Thistime should at least be twice the maximum time forconfiguration messages to be broadcast to thewhole network

Forward Delay Maximum delay for configurationmessage to be broadcast to the whole network

Design interim status: ports at the interim statusmerely learns address info of sites, but does notforward any data

Ports enter the interim status from the blocked staafter the Forward Delay;

Then enter the Forwarding status after the ForwarDelay

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Port status Port ability

Disabled Not receive/send any message

Blocking Not receive/forward data, receive but nottransfer BPDUs, and not learn addresses

Listening Not receive/forward data, receive andtransfer BPDUs, but not learn addresses

Learning Not receive/forward data, receive andtransfer BPDUs, and start to learn addresses

Forwarding Receive and forward data, receive andtransfer BPDUs, and learn addresses

Statuses of ports

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Port status shift

Disabled

Forwarding

Learning

Listening

Blocking

(1,2)

(2)(1)

(3) (5)

(5)

(4)

(4)

(4)

(2)

(1)

(1,2)

(1,2)

1) port enabled

2) port disabled

3) port is selected as root port ordesignated port

4) port is selected as backup port( blocked)

5) Forward Delay

ge o a ress

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ge o a ressinformation

Topology structure change can enable shifts of the

relative position of sites in the spanning tree, so ththe MAC address info the network bridge originallylearnt might be incorrect. So, the learnt MACaddress info should also have an age, and if theaddress is not proven to be correct within this age,the address info is then discarded.

There are two ages in the spanning tree protocol:

A longer survival age is used when the topology istable

A shorter age is used when the topology changesWhen the topology changes, not all network bridgewill discover this change. So the topology changeinfo should be broadcast to the whole network

e re a ve pos on o s es

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e re a ve pos on o s eschange

LAN A

LAN D

LAN C

LAN B

ROOT

LAN E1

2

3

B2

B1

B3

B4

roa cas o opo ogy

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Topo logy change no t im es s age

T opo l ogy c hange r es pm es s age

T opo l ogy c hange m es

ROOT

B

B

B

B

B

roa cas o opo ogychange message

na equacy o spann ng ree

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na equacy o spann ng reeprotocol

It takes a port twice the Forward Delay tochange from the blocked status to theForwarding status, so after change of

network topology structure, it takes at leasttwice the Forward Delay to restoreconnectivity

Using a single spanning tree instance in the

whole switching network might lead to alonger convergence time and more frequenttopology changes. In switching networks that

Note:network diameter means that themaximum value of quantity of switch passcommunication between random two terminaldevices

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Fast spanning tree protocol

The fast spanning tree protocol evolved fromthe spanning tree protocol, and shares thesame basic concepts;

A fast spanning tree has all functions of aspanning tree;

The purpose of improving a fast spanningtree is to try to restore network connectivityas soon as possible when the networktopology structure changes

mprovemen o as

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mprovemen o asspanning trees

In the new topology structure, a new root port canimmediately enter the Forwarding status, if the original rootport has entered the blocked status, and the designated portof the opposite side switch connected to the new root port is

in the Forwarding status

LAN

A

LAN

A

LAN

A

TO ROOT

Rootport

LANA

LANA

LANA

T O R O O T

RootportF

F

Designatedport

Designatedport

Designatedport

Designatedport

F FFF

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The designated port can make a handshakewith the connected bridge so as to enter theForwarding status quickly

Handshake request

Handshakeresponse

LANA

LAN B

TO ROOT

Rootroot

Designatedport

F

Att ti !

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Attention!

Two points for attention:Handshakes must occur on point to point links.

After the first handshake, the non-edge designated port of theresponding bridge will change into the discarding status, andwill continue to initiate handshakes to adjacent bridges.

L A N A

Non-point topoint link

LAN C

LAN A

LAN B

Designatedport

F

Designatedport

Designatedport

D D

Handshakediffusion

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Ports at the edge of a network, i.e., thosedirectly connected to terminals but not toother bridges can directly enter the

forwarding status, without any delay.LAN

A

LAN

ALANA

TO ROOT

RootportLANA

Edge port

F

er ormance o as

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er ormance o asspanning tree

Effect of the first improvement: it takes onlyseveral microsends from finding topology changeto restoring connectivity, and without transfer ofany configuration message.

Effect of the second improvement: networkconnectivity can be restored within the time whentwo configuration messages are exchanged, i.e.,the handshake delay; in the worst case, networkconnectivity is restored only after handshake

starts from one side of the network, and spreadsto bridges at the other edge of the network. Forinstance, when the network diameter is 7, 6handshakes are required.

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What are the differences between spanningtrees and fast spanning trees?

Different in protocol versions

Different in port status change mode

Different in configuration message format

Different in broadcasting modes of topologychange messages

Note: a fast spanning tree also uses a single spanning tree instance in wholeswitching network, and cannot solve the performance deterioration problem

caused by enlargement of network scope. It is recommended to keep thenetwork diameter less than 7

Config re spanning tree

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Configure spanning tree

Spanning tree function is closed in default in a switch; if path loops possibly exist in network,commands should be used to start the spanning tree function

Quidway(config)#spanning-tree enable

If it is certain that no loops exist in connections at a port, then commands can be used to close the

spanning tree function of this port:Quidway(config-if-ethernet0/1)#spanning-tree disable

We can also close the spanning tree function of a switch according to requirements, or start thespanning tree function of a port

on gura e parame ers o

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g pa spanning tree

Configurable parameters of a spanning treeinclude:

Bridge priority

Port priorityPath cost of a link corresponding to the port

Three important timerparameters:HelloTime/Max Age/ForwardDelay

BridgeDiameter of whole switched network

e au va ues o

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Parameter nameDefault

valueDomain Configuration mode

BridgePriority32768

0-61440 (steplength:4096)

Whole officeconfiguration mode

PortPriority128 0-1024 (step length:16)

Interfaceconfiguration mode

PortPathCost

200,000 1-200,000,000Interface

configuration mode

Max Age20s 6-40


Hello Time2s 1-10


Forward Delay15s 4-30


Bridge Diameter7 -----


Note: parameters in the interface configuration mode can beconfigured either at a specific port, or at a group of ports

e ec a su a e roo r ge

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gthrough configuration

Network bridge ID consists of two parts:

BridgePriority+BridgeMacAddress

If all switches in the network are in default

configuration, then according to theconfiguration message contrast rule, the switchwith the smallest MAC address is selected asthe root bridge. However, this switch is maybe

not an ideal root bridge. So, commands can beused to configure the Bridge Priority so as toelect a suitable switch to be the root bridge

Configuration port overhead

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Configuration port overhead

Link rate Recommendedvalue

Recommended domain Value domain

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Configuration port priority

According to the configuration message contrastrule, port ID sometimes will be compared

Port ID consists of two parts:

PortPriority+PortID, the port priority part isconfigrable:

Quidway(config-if-ethernet0/1)#spanning-treepriority [value]

Parallellink

LAN

Multiple ports connected toone network segment

on gure e e o me o

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gthe port

Configuration of hello time requires attention tothe following:

Longer hello time can reduce the overhead of

spanning tree calculations; shorter hello timecan increase the robustness of spanning treeswhen the message loss rate is high.

But longer hello time will cause wrong

judgment of link failure; shorter hello time willcause sending configuration messagesfrequently , thus increasing CPU and networkload.

ax ge o con gura on

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g gport

Configuration of max age requires attention tothe following:

Too long a Max Age will cause link failure to

fail to be detected in time;Too short a Max Age will let the switch wronglytake it as link failure during network congestion,leading to frequent spanning tree

recalculations.Command format

Quidway(config)#spanning-tree maxage [value]

orwar e ay o

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Configuration of forward delay requiresattention to the following:

Too long a Forward Delay will cause too slow a

spanning tree convergence;Too short a Forward Delay will introducetemporary path loops when the topologychanges.

Command format:Quidway(config)#spanning-tree forwarddelay[value]

Note: configuration of parameters above are all in whole globalconfiguration mode , and the unit of time parameter is second.

on gur ng ne wor

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Network diameter is defined as:

Maximum switches passed between any twoterminal devices

Change of network diameter can indirectlyaffect the values of two parameters: Max Ageand Forward Delay. And such impact is moreobjective than direct manual configuration of

the two parametersSo, adding switches to the network can adapt tonetwork status by changing the network

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