008 stp principle issue1.3bx1

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ODA040005 STP

Principle

ISSUE 1.3

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


References

� Clause 8 of IEEE Std 802.1D, 1998

Edition

� Clause 17 of IEEE Std 802.1D, 2004

Edition

� Ethernet Switch Operation Manual


Upon completion of this course, you

will be able to:

� Understand STP Principle

� Understand RSTP Principle.


Chapter 1 STP Principle

Chapter 2 RSTP Principle


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


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


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


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.


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


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 Type

Flag Bit

Root Bridge ID

Root Path Cost

Designated Bridge ID

Designated Port ID

Message Age

1

1

1

8

4

8

2

2

Max Age

Hello Time

Forward Delay

2

2

2


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


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


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


An Instance Of Receiving And Calculating BPDU

� Select Port4 as the root ，select 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

32，，，，0，，，，32 23，，，，18，，，，123 23，，，，14，，，，321 23，，，，14，，，，100 23，，，，15，，，，80

root

23，，，，15，，，，81

B81

23，，，，15，，，，81


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.


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

23，，，，18，，，，123

Port 1


Port 5

blocking

23，，，，14，，，，321 23，，，，15，，，，80 23，，，，15，，，，81

root

32，，，，0，，，，32

23，，，，15，，，，81 23，，，，15，，，，81

B81


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 to （23，16，81）

Port 1


Port 5

23，，，，15，，，，80 23，，，，16，，，，81

root

23，，，，16，，，，81 32，，，，0，，，，32 23，，，，18，，，，123

23，，，，16，，，，81 23，，，，16，，，，81

B81


Processing Of Link Fault (III)


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

(81，0，81) till a BPDU of a higher priority is received from any port

Port 1


Port 5

81，，，，0，，，，81 81，，，，0，，，，81 81，，，，0，，，，81 81，，，，0，，，，81 81，，，，0，，，，81

B81


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


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 the

forwarding status. This time should at least be twice the forward delay


Port Status

Port Ability

Not receive/send any messageDisabled

Blocking

Listening

Learning

Port Status

Forwarding

Not receive/forward data, receive but not

transfer BPDUs, and not learn addresses

Not receive/forward data, receive and

transfer 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


Port Status Transition

Disabled

Listening

Blocking

Forwarding

Learning

1）interface enabled

2）interface disabled

3）interface is selected as root port or designated port

4）interface is selected as backup interface ( blocking)

5）Forward Delay

（（（（1））））（（（（2））））

（（（（1，，，，2））））

（（（（1，，，，2））））

（（（（1，，，，2））））（（（（1））））

（（（（2））））

（（（（4））））

（（（（4））））（（（（5））））

（（（（4））））

（（（（5））））（（（（3））））


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


Transmission Of Topology Change Message

ROOT

1

3

2

44

5 5

Topology change notice message

Topology change response message

Topology change message


Disadvantages of STP

� Recovery time is too long: two Forward Delay

� Network diameter should be less than 7.


Questions

� What is the working mechanism of STP?

� How to select the best BPDU?


Chapter 1 STP Principle

Chapter 2 RSTP Principle


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


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


STP vs. RSTP (Port States)

STP port state RSTP port state

Disabled Discarding

Blocking Discarding

Listening Discarding

Learning Learning

Forwarding Forwarding


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


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


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


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


Questions

� Tell the port state of RSTP ?

� Describe the three improvements of RSTP in simple words ?


� The functions of STP & RSTP

� Basic principles of STP & RSTP

Summary

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008 stp principle issue1.3bx1

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designated port id root

root path cost of best

path loop

smallest path cost