In the Per VLAN Spanning-Tree (PVST) tutorial, I discussed how we elect a root bridge for each VLAN. In this tutorial, we'll explore various options for configuring the root bridge for your switched environment.
As you can see in the above diagram, we have three switches, and on each switch, we'll configure three VLANs:
Here is our output:
Let's see what we can do about this. I want to now configure my network so it looks like this:
To change the root bridge for each VLAN using the spanning-tree command, you have several options. Here's how you can do it:
1. Configuration
Here is our topology:
As you can see in the above diagram, we have three switches, and on each switch, we'll configure three VLANs:
Configurations for SW1, SW2, and SW3
SW1, SW2 & SW3
(config)#vlan 10
(config)#vlan 20
(config)#vlan 30Let's check the bridge IDs for our switches:
SW1#show spanning-tree bridge detail
VLAN0001
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 5254.001a.935a
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
VLAN0010
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 5254.001a.935a
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
VLAN0020
Bridge ID Priority 32788 (priority 32768 sys-id-ext 20)
Address 5254.001a.935a
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
VLAN0030
Bridge ID Priority 32798 (priority 32768 sys-id-ext 30)
Address 5254.001a.935a
Hello Time 2 sec Max Age 20 sec Forward Delay 15 secSW2#show spanning-tree bridge detail
VLAN0001
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 5254.0015.bc74
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
VLAN0010
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 5254.0015.bc74
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
VLAN0020
Bridge ID Priority 32788 (priority 32768 sys-id-ext 20)
Address 5254.0015.bc74
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
VLAN0030
Bridge ID Priority 32798 (priority 32768 sys-id-ext 30)
Address 5254.0015.bc74
Hello Time 2 sec Max Age 20 sec Forward Delay 15 secSW3#show spanning-tree bridge detail
VLAN0001
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 5254.001d.e6bb
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
VLAN0010
Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 5254.001d.e6bb
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
VLAN0020
Bridge ID Priority 32788 (priority 32768 sys-id-ext 20)
Address 5254.001d.e6bb
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
VLAN0030
Bridge ID Priority 32798 (priority 32768 sys-id-ext 30)
Address 5254.001d.e6bb
Hello Time 2 sec Max Age 20 sec Forward Delay 15 secThe default priority value for a switch is 32768, which means that if no other configuration is applied, the MAC address becomes the tiebreaker in the root bridge election. In this scenario, SW2 ends up being the root bridge for all VLANs.
+What is the significance behind the number 32768 being used for a switch's default priority? Why such a number?
SW2#show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 32778
Address 5254.0015.bc74
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 5254.0015.bc74
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0 Desg FWD 4 128.1 P2p
Gi0/1 Desg FWD 4 128.2 P2p SW2#show spanning-tree vlan 20
VLAN0020
Spanning tree enabled protocol ieee
Root ID Priority 32788
Address 5254.0015.bc74
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 5254.0015.bc74
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0 Desg FWD 4 128.1 P2p
Gi0/1 Desg FWD 4 128.2 P2pSW2#show spanning-tree vlan 30
VLAN0030
Spanning tree enabled protocol ieee
Root ID Priority 32798
Address 5254.0015.bc74
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 5254.0015.bc74
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0 Desg FWD 4 128.1 P2p
Gi0/1 Desg FWD 4 128.2 P2p 
To change the root bridge for each VLAN using the spanning-tree command, you have several options. Here's how you can do it:
1. Set the Bridge Priority: You can modify the priority of the switch for a specific VLAN. A lower priority value increases the chances of becoming the root bridge.
Switch(config)# spanning-tree vlan [VLAN_ID] priority [PRIORITY_VALUE]For example, to set the priority for VLAN 10 to 20480:
Switch(config)# spanning-tree vlan 10 priority 204802. Specify the Root Bridge: You can explicitly designate a switch as the root bridge for a specific VLAN using the following command:
Switch(config)# spanning-tree vlan [VLAN_ID] root primaryThis command sets the switch as the root bridge if it has the lowest bridge ID among all switches for that VLAN. For example, to make the switch the root for VLAN 20:
Switch(config)# spanning-tree vlan 20 root primaryAlternatively, to configure another switch as the root bridge for a specific VLAN, use:
Switch(config)# spanning-tree vlan [VLAN_ID] root secondary3. View Current Configurations: To see the current Spanning-Tree configuration and the root bridge for each VLAN, use the command:
Switch# show spanning-tree4. Adjusting the MAC Address: In situations where you need to maintain the same priority but want to change the root bridge, you can change the switch's MAC address (not common, but possible):
Switch(config)# mac-address [NEW_MAC_ADDRESS]These commands help to manage the STP topology effectively, allowing for optimized load balancing and redundancy across the entire network.
If we run the command spanning-tree vlan 10 ? Appending the question mark ('?'), you can view additional options:
SW1(config)#spanning-tree vlan 10 ?
forward-time Set the forward delay for the spanning tree
hello-time Set the hello interval for the spanning tree
max-age Set the max age interval for the spanning tree
priority Set the bridge priority for the spanning tree
root Configure switch as rootAbove, we see two options:
- priority: We can manually change the bridge priority.
- root: We can configure the switch as root.
Let me ask you this: what's the difference between these two parameters?
Let's find out!
‼ The spanning-tree vlan command also accepts ranges of VLANs.
2. Root Parameter
The root parameter in the STP refers to the designated switch that serves as the central point of the Spanning-Tree topology for each VLAN. This switch is called the "root bridge." The root bridge plays a critical role in managing the STP process to ensure that there are no loops in the network. It serves as the reference point for all other switches, which calculate their roles based on their distances from this central switch.
The election process for the root bridge is based on bridge priority and MAC address. By default, all switches have a priority of 32768, and in cases where multiple switches share the same priority, the switch with the lowest MAC address becomes the root. This mechanism ensures a systematic approach to establishing the root bridge, but network administrators can influence this outcome.
With protocols like Per VLAN Spanning-Tree (PVST), it is possible to have different root bridges for different VLANs. This capability allows for optimized load balancing across the network since one switch can serve as the root for one VLAN while another switch serves as the root for a different VLAN. Such flexibility can significantly improve network performance (although may allow for more broadcast storms).
You can manually configure a switch to become the root bridge for your specific VLAN by adjusting its priority using commands like spanning-tree vlan [VLAN_ID] priority [PRIORITY_ID] or by designating it as a primary root bridge. The choice of the root bridge is crucial, as it can greatly impact the efficiency and redundancy of the network. A strategically chosen root bridge minimizes the number of blocked ports and enhances the overall traffic flow.
In summary, the root parameter signifies the switch that is elected to manage the Spanning-Tree topology for a VLAN. This role is vital for influencing traffic flow and ensuring optimal performance in any network.
With that being said, let's check our options:
SW1(config)#spanning-tree vlan 10 root ?
primary Configure this switch as primary root for this spanning tree
secondary Configure switch as secondary rootI can configure the switch to become the primary or secondary root bridge. Let's go with making it a primary:
SW1(config)#spanning-tree vlan 10 root primaryNow, let's check and see if that worked:
SW1#show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 24586
Address 5254.001a.935a
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24586 (priority 24576 sys-id-ext 10)
Address 5254.001a.935a
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 15 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0 Desg FWD 4 128.1 P2p
Gi0/1 Desg FWD 4 128.2 P2pIn the output above, we can see that SW1 has been designated as the root bridge for VLAN 10, with its priority set to 24586. To further enhance network reliability and load balancing, we can configure another switch, SW2, to act as the "secondary" root bridge. This can be achieved by adjusting the priority on SW2, ensuring it has a higher priority value than SW1 but still lower than the default value of 32768.
To configure SW2 as the secondary root bridge, you would typically enter the following command in the configuration mode:
SW2(config)#spanning-tree vlan 10 root secondary
To configure SW2's priority, use the following command:
SW2(config)#spanning-tree vlan 10 priority 28672
Let's check and see what this command does:
SW2#show spanning-tree vlan 10
VLAN0010
Spanning tree enabled protocol ieee
Root ID Priority 24586
Address 5254.001a.935a
Cost 4
Port 1 (GigabitEthernet0/0)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 28682 (priority 28672 sys-id-ext 10)
Address 5254.0015.bc74
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0 Root FWD 4 128.1 P2p
Gi0/1 Desg FWD 4 128.2 P2pIn the above output, we see that the priority of SW2 has been set to 28682; however, it's important to note and understand that there is no official designation for a "secondary" root bridge in spanning-tree protocols. Instead, when you configure a switch with the root primary or root secondary parameters in Cisco IOS, the system automatically adjusts the priority values accordingly behind the scenes, so to speak.
To verify this, we can examine the configuration of SW2. By using the command show running-config, we can see how the priority has been set when the root primary or root secondary command was issued and applied. This will show us the specific priority values assigned to the switch, confirming that the underlying mechanisms is designed to facilitate the selection of a backup root bridge without explicitly labeling it as such.
This way, while the concept of a secondary root bridge is useful for understanding redundancy, the actual implementation is handled through priority settings that determine which switch will assume the root role in the event of a failure.
Let's verify this by looking at our running-configuration:
SW1#show running-config | include priority
spanning-tree vlan 10 priority 24576SW2#show running-config | include priority
spanning-tree vlan 10 priority 28672The way this mechanism operates is that the switch evaluates the priority of the current root bridge and then lowers its own priority to ensure it can become the new root bridge. When a switch is configured with the root primary or root secondary commands, it calculate a priority that is either lower than the existing root bridge's priority, or set to a specific value designed to ensure it takes over the root role when necessary.
By doing this, the switch effectively positions itself as the preferred candidate for the root bridge in the Spanning-Tree topology. If the current root bridge fails or becomes unreachable, the switch with the adjusted priority will automatically be elected as the new root, maintaining network stability and minimizing disruptions. This dynamic adjustment is essential for ensuring redundancy and optimizing traffic flow within your network.3. Priority Parameter
SW3#show spanning-tree vlan 30
VLAN0030
Spanning tree enabled protocol ieee
Root ID Priority 30
Address 5254.001d.e6bb
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 30 (priority 0 sys-id-ext 30)
Address 5254.001d.e6bb
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/0 Desg FWD 4 128.1 P2p
Gi0/1 Desg FWD 4 128.2 P2pAs the output above shows, we can see that SW3 is now the root bridge for the VLAN 30. That's really all there is to it, folks!
Conclusion
In this tutorial, you have learned how to:
- Verify the current root bridge for each VLAN.
- Configure the root bridge using the show spanning-tree
command by utilizing the root parameter.
- Adjust the root bridge configuration using the priority parameter.
I hope you found this tutorial informative. If you have any questions or need further clarification, please feel free to drop us a comment below!
Happy STP Root Bridging!
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Thanks for reading: Spanning-Tree Protocol (STP) Root Bridge Configuration Tutorial, Sorry, my English is bad:)