PVST with DOT1Q Query

Unanswered Question
May 14th, 2007
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Can I run PVST if I am using Dot1Q encapsulation ?

I was going through the documents that says

"Common Spanning Tree

The IEEE 802.1Q standard specifies how VLANs are to be trunked between switches. It also

specifies only a single instance of STP that encompasses all VLANs. This instance is referred to

as the Common Spanning Tree (CST). All CST BPDUs are transmitted over trunk links using the

native VLAN with untagged frames.

Having a single STP for many VLANs simplifies switch configuration and reduces switch CPU

load during STP calculations. However, having only one STP instance can cause limitations, too.

Redundant links between switches will be blocked with no capability for load balancing. Conditions

also can occur that would cause CST to mistakenly enable forwarding on a link that does not

carry a specific VLAN, whereas other links would be blocked.

Per-VLAN Spanning Tree

Cisco has a proprietary version of STP that offers more flexibility than the CST version. Per-VLAN

Spanning Tree (PVST) operates a separate instance of STP for each individual VLAN. This allows

the STP on each VLAN to be configured independently, offering better performance and tuning

for specific conditions. Multiple spanning-trees also make load balancing possible over redundant

links when the links are assigned to different VLANs. One link might forward one set of VLANs,

while another redundant link might forward a different set.

Because of its proprietary nature, PVST requires the use of Cisco Inter-Switch Link (ISL) trunking

encapsulation between switches. In networks where PVST and CST coexist, interoperability

problems occur. Each requires a different trunking method, so BPDUs are never exchanged

between STP types."

But When I am seeing on my Network, I can see multiple Instances running with different BID for different VLAN's though I am using 802.1Q trunk .

Just to further clarify that this is a distribution Switch where we have configured trunks to all my Access Switches with respective Vlan on them. All these have also been tagged with NATIVE.

Thanks for help


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Overall Rating: 4 (1 ratings)
ankbhasi Mon, 05/14/2007 - 21:56
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  • Cisco Employee,

Hi Arshad,

By default all new age switches and new IOS on Cisco switches support PVST+ which works for interopertability and also works on dot1q trunks.

Cisco developed PVST+ in order to allow running several STP instances (even over an 802.1Q network).

PVST is not supported on dot1q links.




*Pls rate all helpfull post

arshad.usmani Mon, 05/14/2007 - 23:46
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Hi Ankur

I know that PVST+ supports interopertability . BUt as per the documentation I need to enable this

"spanning-tree mode pvst+ " -- correct ?

In my network, I have not configured the above comamnd

spanning-tree mode pvst is only command that is configured.


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