1. Spanning Tree V1.2 Slide 1 of 1 Purpose:
Emphasize: The next few slides discuss the basic function of a bridge/switch.
1. How it learns the location of the hosts by reading the source mac address of incoming frames.
2. How it makes forwarding/filtering decisions.
Three conditions when a switch will flood a frame out on all ports except to the port where the frame came in on:
Unknown unicast address
Broadcast frame
Multicast frame
3. How STP is used to avoid loops in a switched/bridged network.

2. Objectives Understand the origin of loop and the solution
Understand the working principle of STP

3. Course Outline Origin of loop and solution Working principle of STP
STP configuration

4. Redundant Topology Server/host X Router Y Segment 1 Segment 2
Slide 1 of 1
Purpose:
Emphasize: Layer 2 has no mechanism (like a TTL) to stop loops.
Segment 2

5. Host X sends a Broadcast frame
Broadcast Storms
Server/host X
Router Y
Segment 1
Broadcast
Switch A
Switch B
Slide 1 of 3
Purpose:
Emphasize: Broadcast frames are flooded.
Segment 2
Host X sends a Broadcast frame

6. Switch B forward Broadcast frame back to Switch A
Broadcast Storms
Server/host X
Router Y
Segment 1
Broadcast
Switch A
Switch B
Slide 2 of 3
Purpose:
Emphasize:
Segment 2
Switch B forward Broadcast frame back to Switch A

7. Broadcast Storms
Server/host X
Router Y
Segment 1
Switch A
Broadcast
Switch B
Slide 3 of 3
Purpose:
Emphasize: Layer 2 has no TTL mechanism to stop looping frames.
Segment 2
Switches continue to propagate broadcast traffic over and over

8. Multiple Frame Copies Host X sends an unicast frame to router Y
Server/host X
Router Y
Segment 1
Switch A
Switch B
Slide 1 of 2
Purpose:
Emphasize: This slide assumes Router Y Mac address has not been learned by Switch A and Switch B yet so the unknown unicast frame to Router Y will be flooded.
Segment 2
Host X sends an unicast frame to router Y
Router Y MAC address has not been learned by either switch yet

9. Multiple Frame Copies
Segment 1
Segment 2
Server/host X
Router Y
Unicast
Switch A
Switch B
Slide 2 of 2
Purpose:
Emphasize:
Router Y will receive two copies of the same frame

10. MAC Database Instability
Segment 1
Segment 2
Server/host X
Router Y
Unicast
Switch A
Switch B
Port 0
Port 1
Slide 1 of 2
Purpose:
Emphasize: This slide assumes Router Y Mac address has not been learned by Switch A and Switch B yet so the unknown unicast frame to Router Y will be flooded.
Host X sends an unicast frame to Router Y
Router Y MAC Address has not been learned by either Switch yet
Switch A and B learn Host X MAC address on port 0

11. MAC Database Instability
Segment 1
Segment 2
Server/host X
Router Y
Unicast
Switch A
Switch B
Port 0
Port 1
Slide 2 of 2
Purpose:
Emphasize:
Frame to Router Y is flooded
Switch A and B incorrectly learn Host X MAC address on port 1

12. Multiple Loop Problems
Server/host
Workstations
Loop
Broadcast
Slide 1 of 1
Purpose:
Emphasize:
Complex topology can cause multiple loops to occur
Layer 2 has no mechanism to stop the loop

13. Solution: Spanning-Tree Protocolx
Block
Slide 1 of 1
Purpose:
Emphasize: A looped topology is often desired to provide redundancy, but looped traffic is undesirable. The Spanning Tree protocol was originally designed for bridges. Today, it is also applied to LAN switches and routers operating as a bridge. STP ensures all bridged segments are reachable but any points where loops occur will be blocked.
Provides a loop free redundant network topology by placing certain ports in the blocking state

14. Course Outline Origin of loop and solution Working principle of STP
STP configuration

15. Spanning-Tree Operations
One root bridge per network
One root port per non-root bridge
One designated port per segment x
Designated port (F)
Root port (F)
Non-designated port (B)
Root bridge
Non-root bridge
SW Y
100baseT
10baseT
SW X
F: forwarding B: blocking
Slide 1 of 1
Purpose:
Emphasize: The three general rules when dealing with STP:
1. One root bridge per network. The root is the bridge with the lowest Bridge ID. All the ports on the root bridge are designated ports (forwarding).
2. For every non root bridge, there is a root port (forwarding). The root port is the port with the lowest accumulated path cost to the root bridge.
3. For every segment, there is only one designated port. The designated port forwards traffic for the segment. The designated port has the lowest accumulated path cost to the root bridge.

16. Spanning-Tree Protocol Root Bridge Selection
Switch X
Default priority (8000 hex)
MAC 0c
Switch Y
Default priority (8000 hex)
MAC 0c
BPDU
Slide 1 of 1
Purpose:
Emphasize: By default, the switch with the lowest MAC address will be the Root bridge.
Note: The Catalyst switches support an instance of Spanning tree per vlan. Each vlan will use an unique mac address for Spanning tree purpose. On the Cat 1900, the address it uses for Spanning tree are the Mac address on the various ports. VLAN is discussed in the next chapter.
The IEEE 802.1d spec specify for a 16 bit priority field. The Cat 1900 switch only supports the 802.1d stp. The default priority on the Cat 1900 is 32,768 in decimal or 8000 in hex, the midrange value.
BPDU contain the following fields:
Protocol ID version
Message type
Flags
Root ID
Cost of path
Bridge ID
Port ID
Message age
Max age
Hello time
Forward delay
BPDU = Bridge protocol data unit (default = sent every 2 seconds)
Root bridge = Bridge with the lowest bridge ID
Bridge ID = Bridge priority + bridge MAC address
In the example, which switch has the lowest bridge ID?

17. BPDU DMA SMA L/T LLC Header Payload
Purpose: This slide begins the presentation on the mechanics of the Spanning-Tree Protocol.
Emphasize:
The contents of the BPDU. A significant field not explained in the text is the flags field, which contains the topology change and topology change acknowledgement. The root ID, bridge ID, and port ID are all made up of two fields. The first portion is the priority. Two bytes are used for the priority for the root ID and the bridge ID, one byte is used for the port ID. This is actually the port priority that is modified later in the section.
Introduce the basics of the Spanning-Tree Protocol here. We will then look at each field in depth.
Transition: The first major step of spanning tree is the election of the root bridge.

18. Spanning-Tree Protocol Path Cost
Link Speed Cost (reratify IEEE spec) Cost (previous IEEE spec)
10 Gbps
1 Gbps
100 Mbps
10 Mbps
Slide 1 of 1
Purpose:
Emphasize: There are two cost calculation method. The Cat1900 uses the older method. The new method is designed to accommodate the higher gigabits ethernet speed.
Note:
Port Priority is used to determine which path has preference when path costs are equal. (Example, when you have two parallel links connecting two switches together). The default port priority is 128.
Port aggregation protocol and EtherChannels are not taught in this class.
Fast EtherChannel is supported by the Cat 1900 switch.

19. Spanning-Tree: 100baseT 100baseT Can you figure out:
Switch Z
Mac 0c
Default priority 32768
Port 0
100baseT
Port 0
Port 0
Switch X
MAC 0c
Default priority 32768
Switch Y
MAC 0c
Default priority 32768
Port 1
Port 1
Slide 1 of 1
Purpose:
Emphasize:
Have the students try to work this out.
The next slide shows the answers.
100baseT
Can you figure out:
Which is the root bridge?
Which are the designated, non-designated, and root ports?
Which are forwarding or blocking ports?

20. Spanning-Tree: 100baseT Designated port (F) Root port (F)
Switch Y
MAC 0c
Default priority 32768
Switch X
MAC 0c
Port 0
Port 1
Switch Z
Mac 0c
100baseT
Designated port (F)
Root port (F)
Non-designated port (BLK)
Slide 1 of 1
Purpose:
Emphasize: Switch X and Y has the same path cost to the root bridge. The DP for the bottom segment is on Switch X because Switch X has a lower bridge ID than Switch Y.

21. Port state Port state capability D: data B: BPDU Disabled N/A D or B
Blocking
N/A D, receive B
no learning address
Listening
N/A D , receive and send B
Learning
learning address
Forwarding
receive and send D and B
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D: data B: BPDU

22. Port States Transfer Disabled Listening Blocking Forwarding Learning
1）port enabled
2）port disabled
3）elected as RP or DP
4） elected as RP or DP
5）Forward Delay
（1）
（2）
（1，2）
（4）
（5）
（3）
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23. Spanning-Tree Port States
Spanning-tree transitions each port through several different state:
Blocking
(20 sec)
Listening
(15 sec)
Learning
Forwarding
Slide 1 of 1
Purpose:
Emphasize: Using the default STP timers setting, the times it takes to go from the blocking state to the forwarding state is 50 sec ( ).

24. Spanning-Tree Recalculation
Switch Y
MAC 0c
Default priority 32768
Switch X
MAC 0c
Port 0
Port 1
10baseT x
100baseT
Root Bridge
Designated port
Root port (F)
Non-designated port (BLK)
BPDU
MAXAGE x
Root bridge
Slide 2 of 2
Purpose:
Emphasize: What will happen if Switch X fails?
Switch Y will detect the missing BPDU from Switch X in 20 seconds (max age timer) then recalculate STP.
After the network converged, Switch Y will be the root bridge, and all its port will transit to the forwarding states (DP) 30 seconds after the max age timer expires.

25. Spanning Tree Practice
MAC=0000.8c
Switch A
Default priority=32768
MAC=0000.8c
Switch B
MAC=0000.8c
Switch C
To enable Spanning Tree on a port : stp enable port et (This command enables Spanning Tree on port one of the line card in slot one)
To enable Spanning Tree on a Line Card: stp enable port et.1.(1-8)
Spanning Tree parameters which are configurable:
Bridge Priority = stp set bridging priority [num]
Port Priority = stp set port [port list] priority [num]
Port Cost = stp set port [port list] port-cost [num]
Hello Times = stp set Switching hello-time [num]
Maximum Age = stp set bridging max-age [num]
MAC=0000.8c
Switch D
MAC=0000.8c
Switch E

26. Course Outline Origin of loop and solution Working principle of STP
STP configuration

27. SSTP Configuration
zte# configure
zte(config)# spanning-tree enable
zte(config)# spanning-tree mode sstp
The default mode of ZXR10 T160G/T64G is MSTP. Whichever mode configured can be compatible and interconnected with other two modes .

28. RSTP
RSTP (Rapid Spanning Tree Protocol) provides higher convergence speed than STP (i.e. SSTP mode).
When the network topology is changing, the status of old redundant switch port can be transferred (From Discard to Forward) quickly in the case of point-to-point connection.
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29. RSTP Port Type STP port type RSTP port type Designated Port Root Port
Disabled Port
Alternate Port
Backup Port

30. RSTP Port State STP port state RSTP port state Disabled Discarding
Blocking
Listening
Learning
Forwarding
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31. Alternate And Backup Ports

32. Speed Up
Edge Port
Handshake

33. MSTP
10,20,30
10,20
20,30 10 30 20
10,30

34. MSTP
The concept of instance and VLAN mirroring are added in MSTP (Multiple Spanning Tree Protocol); SSTP mode and RSTP mode can both be considered to be instances of MSTP mode, namely, the case that only one instance 0 exists. MSTP mode also provides fast convergence and load balance in VLAN environment.
In SSTP and RSTP modes, there is no such kind of concept as VLAN, there is only one status for each port, namely, the forwarding statuses of ports in different VLANs are consistent. While in MSTP mode, there exist multiple spanning tree instances, the forwarding statuses of ports are different in different VLANs. Multiple independent subtree instances can be formed inside MST region to achieve load balance.
By default, MSTP is enabled

35. Per-VLAN STP And Instance
VLAN 10 topology
VLAN 20 topology
VLAN 30 topology
VLAN  Instance
Default Instance 0

36. Interface Loop Protection
ZXR10(config)#interface fei_1/1
ZXR10(config-if)#switchport mode trunk
ZXR10(config-if)#switchport trunk vlan 1-2
ZXR10(config-if)#exit
ZXR10(config)#loop-detect interface fei_1/1 enable
ZXR10(config)#loop-detect interface fei_1/1 vlan 1-2 enable
Switch A
Fei_1/1
ZXR10(config)#show loop-detect interface-detail fei_1/1
isUp isMonitor isLoop isProtected
enable enable no disable
npNum portNum reopenTime counter
loopVlan vlanRange
Switch B

37. ZTE Ethernet Switch RingA B C D E F
Master
Transit
primary
secondary
To F
LINK DOWN
Theory speed: 50ms
zesr add vlan 4094 xgei_4/1 xgei_4/2 MASTER standard
zesr set healthtime 100 failtime 300
zesr add vlan 4094 xgei_4/1 xgei_4/2 TRANSIT standard
而传统多级直联的星型以太网，在网络响应时间、保护机制以及对组播应用的支持方面还存在很大不足，难以支持包括IPTV等业务在内的多业务承载。
以太环网作为一种城域以太网技术，解决了传统数据网保护能力弱、故障恢复时间长等问题，理论上可以提供50ms的快速保护特性，同时兼容传统的以太网协议，是城域宽带接入网优化改造的一种重要的技术选择和解决方案。
以太环网的重要特点是采用环形拓扑结构，天然具有支持组播应用的优势，传统网络在组播分支节点的每一个下联端口上均需要进行组播流的复制，而以太环网则只需在整个环网上复制一次组播流，从而大大提高了组播数据流复制的有效性。

38. Summarization
Loop is easily formed whenever there is a redundant design
LAN switch runs STP to resolve loop problem
STP uses BPDU to exchange information and make decision
Ports that enable STP will transfer to different states according to current topology changing

39. Questions
How are unicast and broadcast packets processed by LAN switch?
How many STP port states there are? When and how will they transfer?
How is root bridge elected? And how about root port?
How to speed up the convergence?
What is the role of BPDU?

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site survey,
network planning and design,
on-site supervision,
project management,
infrastructure construction,
system debugging,
on-site training
project acceptance implementation, etc.
We have Captured our expertise in a Flexible Portfolio of Turnkey Services
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