In this lab, I created two trunk ports on each switch and let spanning-tree run it's default election process.
Sw1(config)#int fa0/11
Sw1(config-if)#switchport trunk encap dot
Sw1(config-if)#switchport mode trunk
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/11, changed state to down
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/11, changed state to up
Sw1(config-if)#int fa0/12
Sw1(config-if)#switchport trunk encap dot
Sw1(config-if)#switchport mode trunk
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/12, changed state to down
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/12, changed state to up
Sw2(config)#int fa0/11
Sw2(config-if)#switchport trunk encap dot
Sw2(config-if)#switchport mode trunk
Sw2(config-if)#int fa0/12
Sw2(config-if)#switchport trunk encap dot
Sw2(config-if)#switchport mode trunk
Running the show spanning-tree command by itself will display all spanning tree information for each vlan. There are 4 vlan on each switch, vlan 1,10,20 and 30. Sw1 won the election and is the root switch for all vlans. There are multiple ways of verifying if the switch is the root switch, one is that the show spanning tree command will display "this is the root bridge", the root id information will match the bridge id information and lastly, all ports will be forwarding state.
Sw1#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0002.1710.2A57
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0002.1710.2A57
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/11 Desg FWD 19 128.11 P2p
Fa0/12 Desg FWD 19 128.12 P2p
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 32778
Address 0002.1710.2A57
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 0002.1710.2A57
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/11 Desg FWD 19 128.11 P2p
Fa0/12 Desg FWD 19 128.12 P2p
VLAN0020
Spanning tree enabled protocol ieee
Root ID Priority 32788
Address 0002.1710.2A57
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32788 (priority 32768 sys-id-ext 20)
Address 0002.1710.2A57
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/11 Desg FWD 19 128.11 P2p
Fa0/12 Desg FWD 19 128.12 P2p
VLAN0030
Spanning tree enabled protocol ieee
Root ID Priority 32798
Address 0002.1710.2A57
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32798 (priority 32768 sys-id-ext 30)
Address 0002.1710.2A57
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/11 Desg FWD 19 128.11 P2p
Fa0/12 Desg FWD 19 128.12 P2p
On Sw2 I displayed the information for vlan 30 only, here we don't see "this is the root bridge", the root information doesn't match the bridge ID and not all ports are forwarding
Sw2# show spanning-tree vlan 30
VLAN0030
Spanning tree enabled protocol ieee
Root ID Priority 32798
Address 0002.1710.2A57
Cost 19
Port 11(FastEthernet0/11)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32798 (priority 32768 sys-id-ext 30)
Address 000C.CF68.DC11
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/11 Root FWD 19 128.11 P2p
Fa0/12 Altn BLK 19 128.12 P2p
I want to manually change the SPF root for vlan 20 and 30 to Sw2. I used the spanning-tree vlan root primary command. There is a option for a secondary root if I wanted to create a backup root.
Sw2(config)#spanning-tree vlan 20 root primary
Sw2(config)#spanning-tree vlan 30 root primary
I can see now that Sw2 is the root switch for vlan 20 and vlan 30.
Sw2#show span vlan 20
VLAN0020
Spanning tree enabled protocol ieee
Root ID Priority 24596
Address 000C.CF68.DC11
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24596 (priority 24576 sys-id-ext 20)
Address 000C.CF68.DC11
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/11 Desg FWD 19 128.11 P2p
Fa0/12 Desg LRN 19 128.12 P2p
Sw2#show span vlan 30
VLAN0030
Spanning tree enabled protocol ieee
Root ID Priority 24606
Address 000C.CF68.DC11
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24606 (priority 24576 sys-id-ext 30)
Address 000C.CF68.DC11
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/11 Desg FWD 19 128.11 P2p
Fa0/12 Desg LRN 19 128.12 P2p
Sw2#show span vlan 30
VLAN0030
Spanning tree enabled protocol ieee
Root ID Priority 24606
Address 000C.CF68.DC11
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24606 (priority 24576 sys-id-ext 30)
Address 000C.CF68.DC11
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/11 Desg FWD 19 128.11 P2p
Fa0/12 Desg FWD 19 128.12 P2p
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