PVST and MST difference

Answered Question
Jul 26th, 2007

What is difference between PVST and MST?

I have this problem too.
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Correct Answer by sundar.palaniappan about 9 years 4 months ago

In PVST mode the bridge priority is configured individually for every VLAN. In MST mode the bridge priority is set for the MST instance, which can include multiple VLANs, and any change to the bridge priority of an MST instance should make all the VLANs that are part of the MST group inherit the same attribute(s).

Here's a small capture from my lab switch that I hope helps.

SW1-3550(config)#spanning-tree vlan ?

WORD vlan range, example: 1,3-5,7,9-11

SW1-3550(config)#spanning-tree vlan 10 priority 32768

SW1-3550(config)#spanning-tree mst ?

WORD MST instance range, example: 0-3,5,7-9

configuration Enter MST configuration submode

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

max-hops Set the max hops value for the spanning tree

SW1-3550(config)#spanning-tree mst 1 ?

priority Set the bridge priority for the spanning tree

root Configure switch as root

SW1-3550(config)#spanning-tree mst 1 priority ?

<0-61440> bridge priority in increments of 4096

SW1-3550(config)#spanning-tree mst 1 priority 32768

HTH

Sundar

Correct Answer by sundar.palaniappan about 9 years 4 months ago

You would change the bridge priority for the MST instance and not vlan4 in MST setup. Hence, it would affect all 3 vlans in this case.

HTH

Sundar

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Overall Rating: 5 (6 ratings)
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ankbhasi Thu, 07/26/2007 - 08:49

Hi Dangal,

In very simple terms as the name states PVST is only capable of running spanning tree per vlan but MST has the capability of running spanning tree for group of vlans for eg: vlan 1,2,3 can run one single instance of spanning tree as compared to PVST where each vlan will have its own spanning tree instance.

Have a look at this link and the section where to use MST will clear your views

http://www.cisco.com/warp/public/473/147.html#where

HTH

Ankur

*Pls rate all helpfull post

dangal.43 Thu, 07/26/2007 - 10:18

You mean to say i can have multiple VLAN in single instance of MST but for PVST i must have to go for the seperate instance for seperate VLAN, right!!!!! what are the other difference?

and what is difference between PVST and PVST+?

other question about tunneling ISL in 802.1Q can i have it in deployment? I mean to say as we are doing Q in Q tunneling then can we have tunneling of ISL in dot1Q, vice versa?

Amit Singh Thu, 07/26/2007 - 10:56

The major difference between PVST and PVST+ is that, PVST+ is backward compatible with IEEE 802.1d CST spanning-tree. Another difference is that PVST only supports ISL trunking while PVST+ support 802.1Q trunk.

Q in Q tunneling only works with 802.1Q encapsulation its not supported on ISL trunks.

Since all VLANs in an MSTP instance share the same STP topology, a topology change in one VLAN will affect all VLANs within that particular instance.While a topology change in PVST will not effect the other vlans.

Please see the link below for more understanding

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml#pvst

HTH,please rate if it does.

-amit singh

dangal.43 Thu, 07/26/2007 - 11:51

Amit,

will you please explain me this in detail:

"Since all VLANs in an MSTP instance share the same STP topology, a topology change in one VLAN will affect all VLANs within that particular instance.While a topology change in PVST will not effect the other vlans. "

I am not getting properly how MST will use single VTP topology?

Jon Marshall Thu, 07/26/2007 - 12:11

Hi

MST uses a single STP topology or more correctly MST groups together multiple vlans into one STP instance and MST can contain multiple instances.

Key thing here is that any topology change in one of the vlans has an effect on all the vlans within that instance.

PVST however runs an instance of STP for every single vlan so a change in one vlan is isolated to that vlan.

HTH

Jon

sundar.palaniappan Thu, 07/26/2007 - 12:24

Hope this helps.

CST - One STP instance for all VLANs

MST - One STP instance for a group of VLANs

PVST - One STP instance for every VLAN

As you can see STP topology change in one of the VLANs that make up the MST instance would affect the whole MST instance.

HTH

Sundar

dangal.43 Thu, 07/26/2007 - 13:44

sundar you mean to say is:

Lets say we have vlan 2,3 and 4. if we change the root bridge for vlan 4 then root bridge for vlan 2 and 3 will also change.....???

Correct Answer
sundar.palaniappan Thu, 07/26/2007 - 13:50

You would change the bridge priority for the MST instance and not vlan4 in MST setup. Hence, it would affect all 3 vlans in this case.

HTH

Sundar

dangal.43 Thu, 07/26/2007 - 14:42

sundar will you please explain me in little bit detail about the same?

Correct Answer
sundar.palaniappan Thu, 07/26/2007 - 15:07

In PVST mode the bridge priority is configured individually for every VLAN. In MST mode the bridge priority is set for the MST instance, which can include multiple VLANs, and any change to the bridge priority of an MST instance should make all the VLANs that are part of the MST group inherit the same attribute(s).

Here's a small capture from my lab switch that I hope helps.

SW1-3550(config)#spanning-tree vlan ?

WORD vlan range, example: 1,3-5,7,9-11

SW1-3550(config)#spanning-tree vlan 10 priority 32768

SW1-3550(config)#spanning-tree mst ?

WORD MST instance range, example: 0-3,5,7-9

configuration Enter MST configuration submode

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

max-hops Set the max hops value for the spanning tree

SW1-3550(config)#spanning-tree mst 1 ?

priority Set the bridge priority for the spanning tree

root Configure switch as root

SW1-3550(config)#spanning-tree mst 1 priority ?

<0-61440> bridge priority in increments of 4096

SW1-3550(config)#spanning-tree mst 1 priority 32768

HTH

Sundar

dangal.43 Fri, 07/27/2007 - 06:10

I got your point sundar thanks for explanation. so here instance will have their root bridge and that rootbridge is common for the all vlan belong to that insatnce, right!!!!!

Now one more question lets say if i have vlan3,4 and 5 in MST instance 1. If i will change the priority of VLAN 3 and trying to influance the root bridge election then what will happen? root bridge will change for only vlan3 and it will be out of instance or it will affect the whole instance root bridge.....

dangal.43 Fri, 07/27/2007 - 08:22

one more thing what is the difference between MST instance and MST regions?

During the reading of configuration guide of 3550 i found some different work called IST CST which play the role in MST functionality so what is that exactly, i tried to understand it but i am not able to get it.

sundar.palaniappan Fri, 07/27/2007 - 16:06

Changing the bridge priority of vlan3 will have no effect. Vlan3 will continue to be part of the MST instance and will inherit the attributes of that instance that includes the bridge priority.

HTH

Sundar

sundar.palaniappan Fri, 07/27/2007 - 16:39

MST Region: An MST region consists of interconnected bridges that have the same MST configuration. There is no limit on the number of MST regions in the network.

MST Instance: A group of VLANs mapped to same MST STP configuration.

Have a look at this link to clear your queries about IST/MST/CST etc.

http://cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a00807b075f.shtml

HTH

Sundar

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