Stp vs Pvst+

Unanswered Question
Jun 17th, 2009
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On cisco switches pvst+ is by default running.Means as soon as you create any vlan single spanning tree instance starts running fot tht particular vlan.

But if i disable pvst+........then switch runs a Common spanning tree instance for all vlans?


what is the difference between pvst and pvst+?

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Giuseppe Larosa Wed, 06/17/2009 - 00:58
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Hello Alexandr,

PVST was a previous version of PVST+ that had problems in dealing with 802.1Q trunks.


PVST+ is supported since catos 4.x


In modern IOS switches the following STP modes are available:

PVST+

Rapid PVST

MST


the last two uses instances that are rapid as per 802.1W standard.

Rapid PVST is simply using a rapid STP instance for each Vlan.

MST 802.1s uses instances for sets of vlans achieving greater scalability at a price of increased management activity.


note:

actually a prestandard MSTP version exist on some IOS versions/platforms.


the global command is

spanning-tree mode xx


You cannot revert to simple CST 802.1D on cisco switches.

But you can have fall back on a per port basis.


Hope to help

Giuseppe


liuguiqing Wed, 06/17/2009 - 21:49
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PVST was a previous version of PVST+ that had problems in dealing with 802.1Q trunks.



Do you know what problems ?

I have got in trouble with stp ,

but i can't sure it's cause by pvst or not .

Giuseppe Larosa Wed, 06/17/2009 - 23:21
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Hello Liuquiqinq,


PVST+ has a mechanism that allows to support 802.1Q trunks: it uses a proprietary format for STP BPDU that is not processed by 802.1D standard switches and allows them to reach another cisco switch.


for more details


http://www.cisco.com/en/US/products/hw/switches/ps700/products_white_paper09186a00801b49a4.shtml


The older PVST was able to work well only on ISL trunk links.


It is unlike you are using the older PVST because PVST+ is supported since catos 4.x however some problems can happen in a multivendor environment.


Hope to help

Giuseppe



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