008 STP Principle ISSUE1x1

7/29/2019 008 STP Principle ISSUE1x1

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HUAWEI TECHNOLOGIES CO., LTD. All rights reserved

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ODA040005 STP

Principle

ISSUE 1.3


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HUAWEI TECHNOLOGIES CO., LTD. All rights reserved Page 1

Spanning Tree Protocol ( STP ) is applied to

loop network to block some undesirable

redundant paths with certain algorithms and

prune the network into a loop-free tree, thereby

avoiding the proliferation and infinite cycling of

the packet in the loop network


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References

Clause 8 of IEEE Std 802.1D, 1998

Edition

Clause 17 of IEEE Std 802.1D, 2004

Edition

Ethernet Switch Operation Manual


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Chapter 1 STP Principle

Chapter 2 RSTP Principle


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Applications Of Transparent Bridge

Expand LAN scale

Free and dynamic learning of site address information

Problem: frames or packets might be forwarded circularly

and continuously, resulting in network congestion


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Broadcast In A Network With Loops

LAN 1

LAN 2

1

1 1 1

2 2 23 33

Site A

Site B

B1 B2 B3


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Why We Need Spanning Tree Protocol?

Remove path loop that might exist in the bridging network by blocking

redundant links

Activate redundant backup links to restore network connection when the

current active path fails

ROOTLAN B

LAN C

LAN D

LAN E

LAN A


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Basic Principle Of Spanning Tree Protocol

Transmits BPDUs (Bridge Protocol Data Unit )among network bridges and

do the following jobs:

Select the root.

Calculate the shortest path.

Select designated bridge.

Select a root port.

Select designated ports.


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Contents Of BPDU

The content of BPDU is organized as (Root ID, Root Path Cost, Designated

Bridge ID, Designated Port ID)

Root ID: Identifier of root

Root Path Cost: The smallest path cost from the designated bridge to the

root

Designated Bridge ID : Identifier of the designated bridge

Designated Port ID :Identifier of the designated port


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Configuration Message Format

DMA: Destination MAC Address

SMA: Source MAC Address

L/T: Frame length

LLC Header: Fixed link head of

configuration message

Payload: BPDU Data

DMA LLC HeaderSMA L/T Payload

Domain Byte

Protocol ID 2

Protocol Version

BPDU TypeFlag Bit

Root Bridge ID

Root Path Cost

Designated Bridge ID

Designated Port IDMessage Age

1

11

8

4

8

22

Max Age

Hello Time

Forward Delay

2

2

2


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Construct BPDU

Select the best BPDU from all the BPDUs to construct its own BPDU.

Select RootID: the RootID of the BPDU with the highest priority

Calculate Root Path Cost:

Root Path Cost is the sum of the Root Path Cost of best BPDU and

the cost of the port that receives this BPDU

Select Root Port:

The root port is the one that receives the best BPDU

Select designated port: the ports used to forward the packets

Send the new BPDU from a designated port


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How To Determine The Best BPDU?

The best BPDU has the high priority

Assume there are two configuration messages C1 and C2, then:

According to Root ID

If above is the same, According to the Root Path Cost

If above are the same, According to the Designated Bridge ID

If above are the same, According to the Port ID

Principle : the smaller, the better


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An Instance Of Receiving And Calculating BPDU

Port 1

Port 2Port 3 Port 4

Port 5

B81

LAN A

B23

Root

B80B100B321

B32

B123

B102


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An Instance Of Receiving And Calculating BPDU

Select Port4 as the rootselect Port1 and Port2 as designated ports, and

meanwhile block Port3 and Port5.

Send new BPDU from Port1 and Port2: (23, 15, 81)

Root ID = 23

Root Path Cost = 14+1 = 15

Root Port ID = Port4

Designated Bridge ID=81

Port 1

Port 2 Port 3 Port 4

Port 5

blocking

blocking

32032 2318123 2314321 2314100 231580

root

231581

B81

231581


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What To Do If Links Fail?

Hello Time

The bridge cyclically sends BPDU from a designated port at the period of

Hello Time.

Message Age and Max Age

BPDUs stored at a port have a Message Age field, which grows

proportionally with time.

If there is not BPDU received until the Message Age equals to the Max

Age, the bridge will regard it as failure on the links connected to this port.


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Processing Of Link Fault (I)

When the message of the BPDU of Port4 is timeout, this BPDU is

abandoned, and a new spanning tree are calculated, so as to select Port3

as the new root port, while the BPDU of bridge 81 remains unchanged

2318123

Port 1


Port 5

blocking

2314321 231580231581

root

32032

231581 231581

B81


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Processing Of Link Fault (II)


abandoned, and a new spanning tree are calculated, so as to select Port5 as

the new root port, while the BPDU of bridge 81 changes to231681

Port 1


Port 5

231580231681

root

23168132032 2318123

231681 231681

B81


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Processing Of Link Fault (III)


abandoned, and it is taken itself as the root bridge to send the BPDU

(81081) till a BPDU of a higher priority is received from any port

Port 1


Port 5

81081 81081 8108181081 81081

B81


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Temporary Loops Or Disconnection

If the ports at the Forwarding status have not realized that they should stop

forwarding, temporary loops might exist.

If ports blocked have not realized that they should start forwarding, it might

lead to temporary disconnection.

ROOT

ROOT


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How to avoid temporary loops?

Forward Delay: Maximum delay for configuration message to be broadcasted

to the whole network

Design interim status: ports at the interim status merely learns address info

of sites, but does not forward any data

Ports enter the interim status from the blocked status after the Forward

Delay, then enter the Forwarding status after the Forward Delay

Ports will go through a certain delay to change from the blocking status to theforwarding status. This time should at least be twice the forward delay


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Port Status

Port Ability

Not receive/send any messageDisabled

Blocking

Listening

Learning

Port Status

Forwarding

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

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

Not receive/forward data, receive and

transfer BPDUs, and start to learn addresses

Receive and forward data, receive and

transfer BPDUs, and learn addresses


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Port Status TransitionDisabled

Listening

Blocking

Forwarding

Learning

1interface enabled

2interface disabled

3interface is selected as root port or designated port

4interface is selected as backup interface ( blocking)

5Forward Delay

1 2

1

2

12

121

2

4

4 5

4

53


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LAN A

Two Timers about the MAC Address Table

There are two timers used to update the MAC address info:

A bigger timer is used when the topology is stable

A smaller timer is used when the topology changes

LAN B

LAN C

LAN E

LAN D

ROOT

B1

B2

B3

B4


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Transmission Of Topology Change MessageROOT

1

3

2

44

5 5

Topology change notice message

Topology change response message

Topology change message


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Disadvantages of STP

Recovery time is too long: two Forward Delay

Network diameter should be less than 7.


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Questions

What is the working mechanism of STP?

How to select the best BPDU?


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Chapter 1 STP Principle

Chapter 2 RSTP Principle


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Rapid Spanning Tree Protocol

The RSTP evolved from the STP, and shares the same basic

concepts;

RSTP has all functions of STP;

RSTP can bring down the recovery time interval

when the network topology structure changes


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STP vs. RSTP (Port Types)

STP types of port RSTP types of port

Designated Port Designated Port

Root Port Root Port

Disabled Port Disabled Port

Alternate Port

Backup Port


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STP vs. RSTP (Port States)

STP port state RSTP port state

Disabled Discarding

Blocking Discarding

Listening Discarding

Learning Learning

Forwarding Forwarding


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Improvement Of Rapid Spanning Tree (I)

A new root port can immediately enter the Forwarding status, if the original

root port has entered the blocked status, and the designated port of the peer

connected to the new root port is in the Forwarding status.

LAN A

TO ROOT

LAN A

LAN A LAN A

F F

F

LAN A LAN A

F F

F

TO ROOT

Designated Port

Root PortBlocked Port Root Port Blocked Port

Designated Port Designated Port Designated Port


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Improvement Of Rapid Spanning Tree (II)

The designated port can make a handshake with the connected

bridge so as to enter the Forwarding status quickly

LAN B

LAN A

F

1

23

4Handshake request

Handshake response

Designated Port

Root Port


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Attention Handshakes must occur on point to point links.

LAN A

LAN B LAN C

LAN A

F

F F

Non Point to Point Links Handshake Diffusion

Designated Port

Designated Port Designated Port


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Improvement Of Rapid Spanning Tree (III)

The edge ports, i.e., those directly connected to terminals but

not to other bridges can directly enter the forwarding status,

without any delay.

LAN B LAN C

LAN A

Edge Port

LAN DF

TO ROOT

Blocked PortDesignated Port


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Questions

Tell the port state of RSTP ?

Describe the three improvements of RSTP in simple words ?


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The functions of STP & RSTP

Basic principles of STP & RSTP

Summary

Slide Modifyiving Notice(stp)


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008 STP Principle ISSUE1x1

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