CCNP

Per VLAN Spanning Tree (PVST) Overview And Basics

pvst

STP is the loop detection mechanism on the switches as you know. But STP has a brother whose name is PVST ( Per VLAN Spanning Tree ) !

Per-VLAN Spanning Tree (PVST) maintains a spanning tree instance for each VLAN configured in the network. It uses ISL Trunking and allows a VLAN trunk to be forwarding for some VLANs while blocking for other VLANs. Since PVST treats each VLAN as a separate network, it has the ability to load balance traffic (at layer-2) by forwarding some VLANs on one trunk and other Vlans on another trunk without causing a Spanning Tree loop.

One root bridge is elected for per VLAN. In PVST header there is a priority value which is “priority + VLAN number”. Default priority value is 32768 + 1 = 32769.

PVST Configuration

SWITCH_A(conf)#spanning-tree VLAN ... root primary
SWITCH_A(conf)#spanning-tree VLAN ... root secondary
SWITCH_A(conf)#spanning-tree VLAN ... priority X

 

PVST config is made as you can see above. You select the primary and secondary VLAN and a priority number and hard code it into the switch.

pvst 2

Let’s verify the SWITCH_A spanning – tree configuration via show command.

SWITCH_A(conf)#show spanning-tree
Root ID Priority 32769
Address 000b:5f00:9880
Port 1 (Fa0/1)

 

**** These values above are the informations of the root bridge.

Bridge ID Priority 32769
Address 000c:854b:ee80

 

**** These values above are the informations of the SWITCH_A

Interface   Role   Sts   Cost
f0/1        Root   Fwd    19
f0/2        Altnt  Blk    19
f0/24       Desig  Fwd    19

 

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