Spanning Tree Root Bridge Configuration

Lesson Contents

In the Per VLAN Spanning Tree (PVST) lesson, I explained that we elect a root bridge for each VLAN. In this lesson, I’ll show you different options to configure the root bridge.

Configuration

Here is the topology we’ll use:

We have three switches. On each switch, I configure three VLANs:

SW1, SW2 & SW3
(config)#vlan 10
(config)#vlan 20
(config)#vlan 30

And we’ll configure the interfaces as trunk interfaces:

SW1, SW2 & SW3
(config)#interface range GigabitEthernet 0/0 - 1
(config-if-range)#switchport trunk encapsulation dot1q 
(config-if-range)#switchport mode trunk

Let’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 sec

SW2#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 sec

SW3#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 sec

The priority has a default value of 32768 so without configuring anything, the MAC address is the tie-breaker. In my case, SW2 is the root bridge for all VLANs:

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    P2p

SW2#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

Let’s see what we can do about that. I want to configure my network so it looks like this:

We can change the root bridge for each VLAN with the spanning-tree command. Here are our 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 root

Above, we see two options:

priority: We can manually change the bridge priority.
root: We can configure the switch as root.

What’s the difference between these two parameters? Let’s find out.

The spanning-tree vlan command also accepts ranges of VLANs.

Root Parameter

We’ll start with the root parameter. 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 root

I can configure the switch to become the primary or secondary root bridge. Let’s try primary:

SW1(config)#spanning-tree vlan 10 root primary

Let’s check whether that works:

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    P2p

In the output above, we see that SW1 is now the root bridge for VLAN 10. The priority of SW1 is now 24586.

We can also configure another switch to become the “secondary” root bridge. Let’s try that on SW2:

SW2(config)#spanning-tree vlan 10 root secondary

Let’s check 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    P2p

Above, we see that the priority of SW2 is now 28682. In reality, there is no such thing as a “secondary” root bridge.

What happens is that behind the scenes, Cisco IOS sets a priority when you use the root primary or root secondary parameters. We can verify this by looking at our configuration:

SW1#show running-config | include priority              
spanning-tree vlan 10 priority 24576

SW2#show running-config | include priority
spanning-tree vlan 10 priority 28672

The way this works is that the switch looks at the priority of the current root bridge and then decreases its own priority so that it becomes the new root bridge.

Priority Parameter

We can also configure the priority manually like this:

SW2(config)#spanning-tree vlan 20 priority ?
  <0-61440>  bridge priority in increments of 4096

Let’s try that on SW2 for VLAN 20:

SW2(config)#spanning-tree vlan 20 priority 0

By setting the priority to 0, I’ll have the lowest possible priority. Let’s check whether SW2 is now the root bridge for VLAN 20:

SW2#show spanning-tree vlan 20

VLAN0020
  Spanning tree enabled protocol ieee
  Root ID    Priority    20
             Address     5254.0015.bc74
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    20     (priority 0 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    P2p

Above, we see that the priority is now 20 (priority 0 and sys-id-ext 20). Let’s configure SW3 to become the root bridge for VLAN 30:

SW3(config)#spanning-tree vlan 30 priority 0

Here’s the output of SW3:

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    P2p

And as we can see above, SW3 is the root bridge for VLAN 30. That’s all there is to it.

Conclusion

You have now learned how to:

Verify the current root bridge per VLAN.
Configure the root bridge using the spanning-tree command:
- Using the root parameter.
- Using the priority parameter.

I hope you enjoyed this lesson. If you have any questions please leave a comment.

Forum Replies

narad says:

may i know what u have updated in this lesson compared to previous article ??
colella says:

Hi everyone
what tips or practices do you use to search for root bridges quickly
in a network with 290 switches

thank you

Hello Giovanni

In order to find the root bridge, you can issue the show spanning-tree command, and this will show you the root bridge for each VLAN configured on the switch. You’ll get something like this:

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)

... Continue reading in our forum

lagapidis says:

Hello Michael

The valid values for bridge priorities in STP are between 0 and 61440. Hpwever, the value must be in increments of 4096. So in practice, the valid values for bridge priorities are: 0, 4096, 8192, 12288, …, up to 61440.

Have you ever wondered why they’re in increments of 4096? It’s not a coincidence that the VLAN ID is a 12-bit value, which can represent up to 4096 VLAN IDs. The reason is because for every VLAN you have a different priority.

In the original 802.1D standard for Spanning Tree Protocol, the 2-byte (16-bit) Priority field in the BPD
... Continue reading in our forum

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