Spanning tree protocol summary

Spanning tree protocol ：
Enterprise network three-tier architecture —》 redundancy ----》 Line redundancy —》 Second floor bridging loop
Cause problems ：
1、 Broadcast storm
2、 MAC The address table scrolls — On a switch , The same MAC Addresses can only map unique interfaces ; But the same interface can map multiple different MAC Address ;
3、 Duplicate copies of the same data frame
4、 above 3 Two conditions eventually lead to the overload of the equipment , Causes restart protection

Make trees ： In a layer 2 switching network , Generate a tree structure , The blocking part of logic interfaces , So that there is only one path from root to all nodes ; When the best path fails , Automatic opening of partially blocked ports , To realize the function of line backup ;
The spanning tree is in the process of generation , We should try to generate a star structure , And the shortest path tree ;
There are algorithms ： 802.1D PVST PVST+(CISCO) RSTP(802.1w) MSTP(802.1S)

One 、802.1D There is only one spanning tree instance in a switching network ;
Switch to switch BPDU— Bridge protocol data unit – Data sent and received through communication and interaction between switches

To configure BPDU— Only the root bridge can send , In the initial state of the switching network , All switches define local as root bridge , Conduct BPDU Sending of ; So that all switches in the network receive BPDU, And then compare based on the parameters in the data , Elect the root bridge ; All non root bridges no longer send BPDU, It's just receiving and forwarding the root bridge BPDU; cycle 2s send out ,hold time 20s;

TCN— Topology change message （ It's also BPDU）： After local switch link failure ,STP Converge again , In order to quickly refresh all switches in the whole network MAC surface , Will be distributed to all local STP Interface send TCN（ In the tag bit TCN Location 1）, Neighbor exchange received TCN after , First mark it with ACK Bit for reply , For reliable transmission of messages ; After the TCN Forward level by level to the root bridge , Reply by root bridge TC Message to all switches ; Make all switches temporarily MAC The aging time of the table is modified to 15s（ default , Forwarding delay ）

The election — Root bridge Root port Designated port Non specified port （ Blocking port ）

【1】 Root bridge – In a spanning tree instance , There is only one switch for root;
BPDU Medium Bridge ID To decide
Bridge ID= Bridge priority （0-65535 public ） Default 32768 + MAC Address （ Only exist svi The switch with the interface has mac Address , If there are multiple mac The selected value is the smallest ）
The election of the root bridge Compare priorities first , Xiaoyou ; If the priority is the same , Compare mac, The value is small and excellent ;

【2】 Root port — On every non root bridge , There is and only one interface ; The local interface closest to the root bridge （ The shortest 、 Star type ）, Receive from the root bridge BPDU, Forward user traffic （ The interface is not blocked ）

The rules ：
1、 Compare after sending from root bridge , Minimum when entering through this interface cost value ;
2、 Entry direction cost Same value , Compare the end-to-end equipment of the interface BID, Xiaoyou
3、 Opposite end BID Also the same , Compare the interfaces of the end-to-end devices PID; First, the priority is small , If the priorities are consistent , Small number
4、 Connected to opposite end PID Also the same , More local PID, Xiaoyou ;
PID= port ID Interface priority （0-240, step 16, Default 128） Interface number

【3】 Designated port , In every segment there is STP On the physical link of , There is and only one ; Forward from the root bridge BPDU, At the same time, it can forward user traffic （ Don't block ）; By default, all interfaces on the root bridge are the specified ports ;
1、 Compare after sending from root bridge , When entering this link through this interface cost Minimum value （ Go out ）
2、 If out to cost Same value , Must be local BID, Xiaoyou ;
3、 Local BID identical , More local PID;
4、 Local PID, identical , Block the port directly ;
【4】 Non specified port （ Blocking port ） When all the above roles are elected , The remaining interfaces without any roles are unspecified ;
The interface logic is blocked , You can actually receive information , But don't forward ;

cost value ： Different bandwidth Different exist cost
802.1d standard ： 802.1T standard
10M = 100 1000M= 20000
100M=19 100M=200000
1000M=4
10000M=2

100000M=1

[SWA]stp pathcost-standard ? By default, Huawei uses 802.1t standard
dot1d-1998 IEEE 802.1D-1998
dot1t IEEE 802.1T
legacy Legacy

In the generation protocol , At least the root bridge should interfere with the convergence layer ;

Interface status ：
down： No, BPDU Send and receive , Once you can BPDU The transceiver enters the next state
Listen ： mandatory 15s; All switches perform BPDU Send and receive , Vote for all the characters ; The role of the interface is to enter the blocking state directly without the specified port ;
If the specified port and root port enter the next state ;
Study ： mandatory 15s; Specify the port and root port to learn all the interfaces connected to the device MAC Address , Generate MAC surface ; Then enter the next state
state ;
forward ： Refers to the entry of the end port and the root port , Can forward user message ;
Blocking ： Logical blocking ;
notes ： Only after the interface enters the forwarding state , To forward data messages for users , Previous 30s No data can be forwarded ;

Convergence time ：
Initial convergence —30s = 15 Listen +15s Study
Structural changes ：
Direct connection detection exists ： There is a local blocking port , If other ports are disconnected , The blocking port immediately enters 15 It's listening （ The election ）; If the result is
Enable , Then it will go back to 15s Study — total 30s
No direct connection detection ： There are no blocking ports locally , If a port is disconnected , Will send suboptimal BPDU（ Take the local as the root ） Switch to other neighbors , Other switches ignore this data , Conduct 20s hold time timing , At that time, block the interface to enter 15s Listen ,15s Study = total 50s

802.1D shortcoming ：
1、 Slow convergence
2、 Low link utilization

802.1 Configuration command ：
[sw1]stp mode stp It is amended as follows 802.1d Algorithm , At present, Huawei defaults to MSTP;
[sw1]stp priority 4096 Modify bridge priority

[sw1-GigabitEthernet0/0/1]stp cost ? Modify interface cost value
INTEGER<1-200000000> Port path cost

[sw1-GigabitEthernet0/0/1]stp port priority ? Modify interface priority
INTEGER<0-240> Port priority, in steps of 16
Two 、PVST cisco private be based on vlan Spanning tree protocol
At every vlan Inside , There is a tree , The working principle of each tree is the same as 802.1d Agreement ; Different vlan Of BPDU The difference is in priority ;
priority =4096 Multiple +vlan id You can only modify 4096 Multiple backup , And can only be modified to 4096 Integral multiple of
Support only trunk The trunk road is packaged as ISL（cisco Private encapsulation ）

3、 ... and 、PVST + stay PVST The basis of , compatible 802.1q Of trunk encapsulation ; And some acceleration ;
Port acceleration （ Enter the layer to connect the user interface ） Uplink acceleration - For direct connection detection Backbone acceleration — For suboptimal BPDU
shortcoming ：1、 Slow convergence （ Incomplete acceleration ） 2、 There are many trees （ only cisco There are separate chips , Friends can't load ）

Four 、 Fast spanning tree
cisco Of RSTP — be based on vlan Fast spanning trees - One vlan A tree pvst+ The upgrade
public RSTP（802.1w） — The whole switching network is a tree 802.1d The upgrade
The principle of speed ：
1、 Canceled the timer , It's when a state is done , Go straight to the next state ;
2、 Segmented synchronization , Step by step convergence between two devices ; Use request and consent tags ; It depends on the second 1 And the 6 position
3、BPDU My life is 6s;hello time 2s;
4、 Speed up the port （ Edge interface ）、 Uplink acceleration 、 Backbone accelerated integration
5、 compatible 802.1d and PVST, but 802.1d and PVST The second of the tag bits is not used 1-6 position , So it can't converge quickly ; So if there is a device in the network that doesn't support fast convergence , Then other devices that enable fast convergence can't be fast ;
When tcn When the message appears , There's no need to wait for the root bridge to BPDU, You can refresh the local cam surface ;

Bear in mind : The interface defaults to half duplex , Even if allowed RSTP, Still based on slow 802.1D Algorithm to converge ;
[sw1]stp mode rstp
Edge interface — Used to connect to PC The interface of , Once set to edge interface ; There will be no more BPDU Sending of , And don't do it STP Convergence of , Direct forwarding status ; But if the interface receives a peer request BPDU, Will lose edge properties , Normal convergence again ;
[sw1]interface GigabitEthernet 0/0/1
[sw1-GigabitEthernet0/0/1]stp edged-port enable

[sw1]stp priority ? Modify bridge priority
INTEGER<0-61440> Bridge priority, in steps of 4096

[sw1]stp root ? Quickly define the root bridge role
primary Primary root switch
secondary Secondary root switch

[sw1-GigabitEthernet0/0/1]stp port priority ? Modify interface priority
INTEGER<0-240> Port priority, in steps of 16

[sw1-GigabitEthernet0/0/1]stp cost ? Modify interface cost
INTEGER<1-200000000> Port path cost

5、 ... and 、MSTP/MST/802.1S Huawei devices use this protocol by default
It inherits the basis of fast spanning tree ; Will be multiple vlan Put in a group , A spanning tree based on each group ;
Differences between different groups BPDU The priority = 4096 Multiple + Group number
[r1]stp mode mstp
Group exists by default 0, And all vlan It is in this group by default ; priority = 32768+0
grouping
[sw1]stp enable
[sw1]stp region-configuration
[sw1-mst-region]region-name a All equipment shall be in one group
[sw1-mst-region]instance 1 vlan 1 to 5
[sw1-mst-region]instance 2 vlan 6 to 10
[sw1-mst-region]active region-configuration Activate the current configuration （ The command must be configured ）
Bear in mind ： If a group will be created , But in this group vlan, No... Was created on this switch , At the same time, there is no for this vlan Service Interface ; The group will have no information ; The packet information of all devices in the whole switching network must be completely consistent ;
Define local as group 1 The taproot of , Group 2 The backup root of
stp instance 1 root primary Change the priority to 0
stp instance 2 root secondary Change the priority to 4096

[sw1]stp instance 1 priority ?
INTEGER<0-61440> Bridge priority, in steps of 4096

[sw1]interface GigabitEthernet 0/0/1
[sw1-GigabitEthernet0/0/1]stp instance 1 cost ?
INTEGER<1-200000000> Port path cost

[sw1-GigabitEthernet0/0/1]stp instance 1 port priority ?
INTEGER<0-240> Port priority, in steps of 16
https://blog.csdn.net/xieyunc/article/details/82785340