Configuring STP for Layer 2 Switching

Goal: There may be communication failures caused by line failures in the second-tier network, and the network interruption caused by a line failure can be eliminated through redundant lines.
However, due to the existence of redundant lines, there may be broadcast storms, continuous duplication of the same frames, and instability of the MAC address table.
• Configure Switch1 as the primary root of vlan1 and Switch2 as the secondary root of vlan1

Program:
In order to ensure that there will be no problems such as broadcast storms in a redundant environment, the Spanning Tree (STP) protocol is introduced. Through the spanning tree protocol, a certain port on the redundant line can be placed in the blocking (BLOCKING) state to prevent broadcast storms. When a line fails, the blocked port automatically enters the forwarding (FORWARDING) state to ensure the network The unobstructed.
The network topology is shown below:

Steps:
Step 1: Configure the ports connected to the three switches as trunk ports

tarena-sw1(config)#interface range f0/12 -13
tarena-sw1(config-if-range)#switchport mode trunk
tarena-sw2(config)#interface range f0/12, f0/23
tarena-sw2(config-if-range)#switchport mode trunk
tarena-sw3(config)#interface range f0/13 ,f0/23
tarena-sw3(config-if-range)#switchport mode trunk

Step 2: Adjust the priority and set tarena-sw1 as the root bridge
The only basis for the root bridge is the smallest BID. BID is divided into two parts: priority + MAC address. When comparing BIDs, the priority is compared first, and only the MAC addresses are compared if the priorities are the same.
The priority value range is 0 to 65535, and the default value is 32768. When viewing the priority, even the default value is not 32768, because the priority of the switch adopts the system priority + VLAN number, so the default priority of VLAN1 viewed is 32769 (system priority 32768 + VLAN number 1).

tarena-sw1(config)# spanning-tree vlan 1 priority 28672 
tarena-sw1(config)#exit
tarena-sw1#show spanning-tree 
VLAN0001
  Spanning tree enabled protocol ieee
  Root ID Priority 28673 //The default priority is 32768
             Address     0060.478B.607B
             This bridge is the root
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    28673  (priority 24576 sys-id-ext 1)
             Address     0060.478B.607B
             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  20
Interface        Role   Sts   Cost      Prio.Nbr   Type
-----------    ------  --- --------   --------  ----------------------
Fa0/13           Desg    FWD    19        128.13     P2p
Fa0/12           Desg    FWD    19        128.12     P2p

According to the results, the Root ID part refers to the root bridge information, and the Bridge ID part refers to the information of the currently operating switch. If the two are consistent, it means that the currently operating switch is the root bridge.

Layer 3 switching configuration STP

Goal: Configure Switch1 as the secondary root of vlan1 and Switch2 as the secondary root of vlan1.
Configure Switch1 as the secondary root of vlan2, and Switch2 as the secondary root of vlan2

Program:
The network topology as shown in the figure below:

Steps:
Step 1: Configure the ports connected to the four switches as trunk ports, and create vlan2 respectively

SM1(config)#vlan 2
SM1(config)#exit
SM1(config)#interface range fastEthernet 0/1-3
SM1(config-if-range)#switchport trunk encapsulation dot1q 
SM1(config-if-range)#switchport mode trunk 
SM2(config)#vlan 2
SM2(config)#exit
SM2(config)#interface range fastEthernet 0/1-3
SM2(config-if-range)#switchport trunk encapsulation dot1q 
SM2(config-if-range)#switchport mode trunk
Switch1(config)#vlan 2
Switch1(config)#exit
Switch1(config)#interface range fastEthernet 0/1-2
Switch1(config-if-range)#switchport mode trunk
Switch2(config)#vlan 2
Switch2(config)#exit
Switch2(config)#interface range fastEthernet 0/1-2
Switch2(config-if-range)#switchport mode trunk

Step 2: Set SM1 as the secondary root of vlan1's primary root vlan2, and set SM2 as the secondary root of vlan2's primary root vlan1 and check

SM1(config)#spanning-tree vlan 1 root primary 
SM1(config)#spanning-tree vlan 2 root secondary
SM1#show spanning-tree 
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 00D0.972A.43E5
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24577 (priority 24576 sys-id-ext 1)
Address 00D0.972A.43E5
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/2 Desg FWD 19 128.2 P2p
Fa0/3 Desg FWD 19 128.3 P2p
Fa0/1 Desg FWD 19 128.1 P2p
VLAN0002
Spanning tree enabled protocol ieee
Root ID Priority 24578
Address 00E0.F9CE.7424
Cost 19
Port 3(FastEthernet0/3)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 28674 (priority 28672 sys-id-ext 2)
Address 00D0.972A.43E5
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/2 Desg FWD 19 128.2 P2p
Fa0/3 Root FWD 19 128.3 P2p
Fa0/1 Desg FWD 19 128.1 P2p
SM2(config)#spanning-tree vlan 1 root secondary 
SM2(config)#spanning-tree vlan 2 root primary 
SM2#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 24577
Address 00D0.972A.43E5
Cost 19
Port 3(FastEthernet0/3)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 28673 (priority 28672 sys-id-ext 1)
Address 00E0.F9CE.7424
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/2 Desg FWD 19 128.2 P2p
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/3 Root FWD 19 128.3 P2p
VLAN0002
Spanning tree enabled protocol ieee
Root ID Priority 24578
Address 00E0.F9CE.7424
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24578 (priority 24576 sys-id-ext 2)
Address 00E0.F9CE.7424
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/2 Desg FWD 19 128.2 P2p
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/3 Desg FWD 19 128.3 P2p