STP Spanning Tree Experiment

Lab topology

Experimental demand

All devices running STP
Change a blocked port

Experimental Procedure

1. All devices are running STP, until convergence is completed, observe the state

[SW1]stp mode stp 
[SW2]stp mode stp
[SW3]stp mode stp

2, view the STP state in SW1, SW1 found to be the root bridge

Analysis: Spanning Tree convergence election principle is to compare root bridge ID, root path cost (cost), designated bridge ID (forwards bridge ID), port ID, once compare the size, will determine the role, we will not continue to compare subsequent arguments
　　
　　root bridge ID = switch priority (32768) + MAC address of the
　　root path cost = the non-root switch reaches through which the root for all path cost of the
　　designated bridge ID = switch priority (32768) + MAC address
　　port ID = port priority (128) + port number

[Huawei]dis stp
-------[CIST Global Info][Mode STP]-------
CIST Bridge         :32768.4c1f-cc40-5e35
Config Times        :Hello 2s MaxAge 20s FwDly 15s MaxHop 20
Active Times        :Hello 2s MaxAge 20s FwDly 15s MaxHop 20
CIST Root/ERPC      :32768.4c1f-cc40-5e35 / 0
CIST RegRoot/IRPC   :32768.4c1f-cc40-5e35 / 0
CIST RootPortId     :0.0
BPDU-Protection     :Disabled
TC or TCN received  :13
TC count per hello  :0
STP Converge Mode   :Normal 
Time since last TC  :0 days 1h:4m:16s
Number of TC        :8
Last TC occurred    :Ethernet0/0/1

3. Review the port role and status of each device

SW3 on the look and found E0 / 0/2 interface blocked.

[SW1]dis stp brief 
 MSTID  Port                        Role  STP State     Protection
   0    Ethernet0/0/1               DESI  FORWARDING      NONE
   0    Ethernet0/0/2               DESI  FORWARDING      NONE

[SW2]dis stp brief 
 MSTID  Port                        Role  STP State     Protection
   0    Ethernet0/0/1               DESI  FORWARDING      NONE
   0    Ethernet0/0/2               ROOT  FORWARDING      NONE

[SW3]dis stp brief 
 MSTID  Port                        Role  STP State     Protection
   0    Ethernet0/0/1               ROOT  FORWARDING      NONE
   0    Ethernet0/0/2               ALTE  DISCARDING      NONE

4. Change in the overhead SW3, so that the blocked port is switched to e 0/0/1

Observed, E0 / 0/1 port becomes preparatory port

[Huawei-Ethernet0/0/1]stp cost 400001

[Huawei-Ethernet0/0/1]dis stp brief 
 MSTID  Port                        Role  STP State     Protection
   0    Ethernet0/0/1               ALTE  DISCARDING      NONE
   0    Ethernet0/0/2               ROOT  FORWARDING      NONE

Analysis: To make changes to the blocked port E0 / 0/1, we need to SW3 to SW1 through SW3 SW2 overhead than larger LW1 reach overhead, Huawei's default port cost of 200,000, we connect SW1 of E0 / 0/1 cost of the interface into the line 400001