How to configure STP root and load balancing using Cisco PVST/PVST+?

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How to configure STP root and load balancing using Cisco PVST/PVST+?

Configuring STP on Cisco switches is straightforward since STP is on by default.  This default ensures loop-free topologies when connecting Cisco Switches and is very useful in Campus and Access LANs.

For our example, we’ll be using the network topology below.

The switches are in a triangle with switchA as ROOT of the STP.  All switch port connections between switches are using an ISL trunk and are configured using the port default. SwitchC port fa0/1 is blocking.

SWITCH BRIDGE ID VLAN SWITCH BRIDGE ID VLAN SWITCH BRIDGE ID VLAN
A 32769 000f.2323.6800 1 A 32770 000f.2323.6800 2 A 32771 000f.2323.6800 3
B 32769 0011.5c00.ba80 1 B 32770 0011.5c00.ba80 2 B 32771 0011.5c00.ba80 3
C 32769 0011.925f.aa80 1 C 32770 0011.925f.aa80 2 C 32771 0011.925f.aa80 3 

sh vlan

VLAN Name Status Ports
---- -------------------------------- ------- ---------------------------
1 default active Fa0/1, Fa0/2, Fa0/3, Fa0/4
2 Sales active
3 Accounting active

 

interface FastEthernet0/1
 switchport mode dynamic desirable
!
interface FastEthernet0/2
switchport mode dynamic desirable
!
switchA#sh spanning-tree


VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     000f.2323.6800
             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     000f.2323.6800
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Desg FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    P2p
Fa0/30              Desg FWD 19        128.30   P2p

VLAN0002
  Spanning tree enabled protocol ieee
  Root ID    Priority    32770
             Address     000f.2323.6800
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32770  (priority 32768 sys-id-ext 2)
             Address     000f.2323.6800
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Desg FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    P2p

VLAN0003
  Spanning tree enabled protocol ieee
  Root ID   Priority    32771
             Address     000f.2323.6800
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32771  (priority 32768 sys-id-ext 3)
             Address     000f.2323.6800
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Desg FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    P2p

SwitchA#
SwitchB#sh spanning-tree

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     000f.2323.6800
             Cost        19
             Port        1 (FastEthernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0011.5c00.ba80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Root FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    P2p

VLAN0002
  Spanning tree enabled protocol ieee
  Root ID    Priority    32770
             Address     000f.2323.6800
             Cost        19
             Port        1 (FastEthernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32770  (priority 32768 sys-id-ext 2)
             Address     0011.5c00.ba80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Root FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    P2p

VLAN0003
  Spanning tree enabled protocol ieee
  Root ID    Priority    32771
             Address     000f.2323.6800
             Cost        19
             Port        1 (FastEthernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32771  (priority 32768 sys-id-ext 3)
             Address     0011.5c00.ba80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Root FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    P2p

SwitchB#
SwitchC#sh spanning-tree

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    32769
             Address     000f.2323.6800
             Cost        19
             Port        2 (FastEthernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0011.925f.aa80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Altn BLK 19        128.1    P2p
Fa0/2               Root FWD 19        128.2    P2p

VLAN0002
  Spanning tree enabled protocol ieee
  Root ID    Priority    32770
             Address     000f.2323.6800
             Cost        19
             Port        2 (FastEthernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32770  (priority 32768 sys-id-ext 2)
             Address     0011.925f.aa80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Altn BLK 19        128.1    P2p
Fa0/2               Root FWD 19        128.2    P2p

VLAN0003
  Spanning tree enabled protocol ieee
  Root ID    Priority    32771
             Address     000f.2323.6800
             Cost        19
             Port        2 (FastEthernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32771  (priority 32768 sys-id-ext 3)
             Address     0011.925f.aa80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Altn BLK 19        128.1    P2p
Fa0/2               Root FWD 19        128.2    P2p

SwitchC#

To ensure that SwitchA will always be root for VLAN 1, we reduce it’s priority.  Another switch with a lower MAC address could steal away root by causing a STP election.  The modification reduces the priority to 25477 but otherwise everything else remains the same.

SWITCH BRIDGE ID VLAN SWITCH BRIDGE ID VLAN SWITCH BRIDGE ID VLAN
A 25477 000f.2323.6800 1 A 32770 000f.2323.6800 2 A 32771 000f.2323.6800 3
B 32769 0011.5c00.ba80 1 B 32770 0011.5c00.ba80 2 B 32771 0011.5c00.ba80 3
C 32769 0011.925f.aa80 1 C 32770 0011.925f.aa80 2 C 32771 0011.925f.aa80 3 
SwitchA(config)#spanning-tree vlan 1 root primary
SwitchA(config)#end
SwitchA#sh spanning-tree

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     000f.2323.6800
             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     000f.2323.6800
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Desg FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    P2p
Fa0/30              Desg FWD 19        128.30   P2p

VLAN0002
  Spanning tree enabled protocol ieee
  Root ID    Priority    32770
             Address     000f.2323.6800
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32770  (priority 32768 sys-id-ext 2)
             Address     000f.2323.6800
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Desg FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    P2p

VLAN0003
  Spanning tree enabled protocol ieee
  Root ID    Priority    32771
             Address     000f.2323.6800
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32771  (priority 32768 sys-id-ext 3)
             Address     000f.2323.6800
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Desg FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    P2p

SwitchA#
SwitchB#sh spanning-tree

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     000f.2323.6800
             Cost        19
             Port        1 (FastEthernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0011.5c00.ba80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Root FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    P2p

<—–Output Omitted—–>

SwitchC#sh spanning-tree

VLAN0001
  Spanning tree enabled protocol ieee
  Root ID    Priority    24577
             Address     000f.2323.6800
             Cost        19
             Port        2 (FastEthernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0011.925f.aa80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Altn BLK 19        128.1    P2p
Fa0/2               Root FWD 19        128.2    P2p
<-----Output Omitted----->
Now to get load balancing across the unused link, we modify the STP priority for VLAN 2 on switchB.   SwitchA is root for all VLANs presently, thus all data follows the same path and does not use the link between switchBand switchC.

SWITCH BRIDGE ID VLAN SWITCH BRIDGE ID VLAN SWITCH BRIDGE ID VLAN
A 25477 000f.2323.6800 1 A 32770 000f.2323.6800 2 A 32771 000f.2323.6800 3
B 32769 0011.5c00.ba80 1 B 25478 0011.5c00.ba80 2 B 32771 0011.5c00.ba80 3
C 32769 0011.925f.aa80 1 C 32770 0011.925f.aa80 2 C 32771 0011.925f.aa80 3 
SwitchB(config)#spanning-tree vlan 2 root primary
SwitchB(config)#end

SwitchB#sh spanning-tree

<-----Output Omitted----->
VLAN0002
  Spanning tree enabled protocol ieee
  Root ID    Priority    24578
             Address     0011.5c00.ba80
             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     0011.5c00.ba80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 15

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Desg FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    P2p

>—–Output Omitted—–>

SwitchA#sh spanning-tree

<-----Output Omitted----->

VLAN0002
  Spanning tree enabled protocol ieee
  Root ID    Priority    24578
             Address     0011.5c00.ba80
             Cost        19
             Port        1 (FastEthernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32770  (priority 32768 sys-id-ext 2)
             Address     000f.2323.6800
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Root FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    P2p

>—–Output Omitted—–>

SwitchC#sh spanning-tree

<-----Output Omitted----->
VLAN0002
  Spanning tree enabled protocol ieee
  Root ID    Priority    24578
             Address     0011.5c00.ba80
             Cost        19
             Port        1 (FastEthernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32770  (priority 32768 sys-id-ext 2)
             Address     0011.925f.aa80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Root FWD 19        128.1    P2p
Fa0/2               Altn BLK 19        128.2    P2p
<-----Output Omitted----->

We can try to optimize our load balancing to ensure each VLAN takes a slightly different STP tree in the topology by making switchC the STP root for VLAN3.

SWITCH BRIDGE ID VLAN SWITCH BRIDGE ID VLAN SWITCH BRIDGE ID VLAN
A 25477 000f.2323.6800 1 A 32770 000f.2323.6800 2 A 32771 000f.2323.6800 3
B 32769 0011.5c00.ba80 1 B 25478 0011.5c00.ba80 2 B 32771 0011.5c00.ba80 3
C 32769 0011.925f.aa80 1 C 32770 0011.925f.aa80 2 C 25479 0011.925f.aa80 3 
SwitchC(config)#spanning-tree vlan 3 root primary
SwitchC(config)#end
SwitchC#sh spanning-tree

<-----Output Omitted----->
VLAN0003
  Spanning tree enabled protocol ieee
  Root ID    Priority    24579
             Address     0011.925f.aa80
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    24579  (priority 24576 sys-id-ext 3)
             Address     0011.925f.aa80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 15

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Desg FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    P2p

 

<-----Output Omitted----->
SwitchA# sh spanning-tree
<-----Output Omitted----->
VLAN0003
  Spanning tree enabled protocol ieee
  Root ID    Priority    24579
             Address     0011.925f.aa80
             Cost        19
             Port        2 (FastEthernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32771  (priority 32768 sys-id-ext 3)
             Address     000f.2323.6800
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Desg FWD 19        128.1    P2p
Fa0/2               Root FWD 19        128.2    P2p

SwitchA#
switchB# show spanning-tree
<-----Output Omitted----->
VLAN0003
  Spanning tree enabled protocol ieee
  Root ID    Priority    24579
             Address     0011.925f.aa80
             Cost        19
             Port        2 (FastEthernet0/2)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32771  (priority 32768 sys-id-ext 3)
             Address     0011.5c00.ba80
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Fa0/1               Altn BLK 19        128.1    P2p
Fa0/2               Root FWD 19        128.2    P2p

SwitchB#

PVST+ uses 802.1q versus ISL, to enable it on the switches, we modify the trunking encapsulation to use the standards based protocol 802.1q.

interface FastEthernet0/1
 switchport 
 switchport trunk encapsulation dot1.q
 switchport mode trunk

!

interface FastEthernet0/2
 switchport 
 switchport trunk encapsulation dot1.q
 switchport mode trunk

 

In summary, Cisco’s proprietary implementation of STP, PVST/PVST+, allows for load balancing across links by ensuring all links get used if different switches are the STP root switch of different VLANs.

###

<-PREVIOUS What is 802.1D Spanning Tree Protocol?

NEXT-> Why use link aggregation?

/ Articles, Cisco, PVST/PVST+, STP / Tags: , , , , ,

About the Author

Praveen Bahethi

Chief Technical Officer, Shilpa Systems
Washington D.C. Metro Area
Information Technology and Services
Driven leader looking to reduce costs and enhance revenues through the efficient and productive use of Technology Specialties: Managing Technology Teams Developing and Teaching Courseware Using a Comprehensive approach to Converged Infrastructure with insight into Technology, Costs, and People.

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