PVST

Answered Question
Jun 5th, 2007

what are differences between PVST and PVST+?

Correct Answer by sundar.palaniappan about 9 years 8 months ago

Dan,


Here's an excerpt from the link below. Just go through the document and hopefully it should clear all your doubts.


http://www.cisco.com/en/US/products/hw/switches/ps663/products_tech_note09186a0080094713.shtml


In order to support the IEEE 802.1Q standard, the existing Cisco STP implementation was extended to become PVST+ by adding support for tunneling across an IEEE 802.1Q mono Spanning Tree region. PVST+ is therefore compatible with both IEEE 802.1Q MST and Cisco PVST protocols and does not require extra commands or configuration. In addition, PVST+ adds verification mechanisms in order to ensure that there is no configuration inconsistency of port trunking and VLAN IDs across switches.


These are some operational highlights of the PVST+ protocol:


PVST+ interoperates with 802.1Q mono Spanning Tree through the so-called Common Spanning Tree (CST) over an 802.1Q trunk. The CST is always on VLAN 1, so this VLAN needs to be enabled on the trunk to interoperate with other vendors. CST BPDUs are transmitted, always untagged, to the IEEE Standard Bridge-Group (MAC Address 01-80-c2-00-00-00, DSAP 42, SSAP 42). For completeness of description, a parallel set of BPDUs are also transmitted to the Cisco shared Spanning Tree MAC address for VLAN 1.


HTH


Sundar

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Jon Marshall Tue, 06/05/2007 - 09:15

Hi


PVST works with Cisco's ISL trunking protocol.


PVST+ is an enhancement to PVST to allow it to work with 802.1q trunking.


HTH


Jon

ankbhasi Tue, 06/05/2007 - 09:17

Hi Dangal,


PVST is basically Cisco Prop and PVST+ is for inter-operatibility. PVST+ provides the same functionality as PVST using 802.1Q trunking technology rather than ISL. PVST+ is an enhancement to the 802.1Q specification .


Now mostly all switches supports PVST+ by default.


HTH


Ankur


*Pls rate all helpfull post



dangal.43 Tue, 06/05/2007 - 09:53

so PVST support ISL only is that correct?

and PVST+ support both ISL and 802.1Q so we can say PVST+ is the open standard. right!!!

sundar.palaniappan Tue, 06/05/2007 - 10:09

It's not an open standard. It's an extension to the Cisco Proprietary PVST protocol to support 802.1q in addition to ISL and it's only supported on Cisco devices.


HTH


Sundar

dangal.43 Tue, 06/05/2007 - 10:13

so sundar PVST and PVST+ both are the cisco propritory one but only difference is they add the 802.1Q support in new version right!!!

then wht is the benifit of doing so as they are not going to help for any other vendor integration?

sundar.palaniappan Tue, 06/05/2007 - 11:08

PVST+ is only supported on Cisco devices but can work with other vendors' products except that it doesn't offer the multiple ST instance support as the other vendors' products don't support this feature. 802.1q supports MST aka Mono Spanning tree. It's nothing but a single spanning tree instance for all the VLANs. In a multi-vendor environment PVST+ architecture maps multiple instances of spanning tree to a single instance that's supported with 802.1q.


Have a look at this link for more info:


http://www.cisco.com/en/US/products/sw/iosswrel/ps1834/products_feature_guide09186a008008019c.html


HTH


Sundar

dangal.43 Tue, 06/05/2007 - 12:41

Sundar i got confuse with "In a multi-vendor environment PVST+ architecture maps multiple instances of spanning tree to a single instance that's supported with 802.1q."


will you please explain me some what in details?

Correct Answer
sundar.palaniappan Tue, 06/05/2007 - 13:29

Dan,


Here's an excerpt from the link below. Just go through the document and hopefully it should clear all your doubts.


http://www.cisco.com/en/US/products/hw/switches/ps663/products_tech_note09186a0080094713.shtml


In order to support the IEEE 802.1Q standard, the existing Cisco STP implementation was extended to become PVST+ by adding support for tunneling across an IEEE 802.1Q mono Spanning Tree region. PVST+ is therefore compatible with both IEEE 802.1Q MST and Cisco PVST protocols and does not require extra commands or configuration. In addition, PVST+ adds verification mechanisms in order to ensure that there is no configuration inconsistency of port trunking and VLAN IDs across switches.


These are some operational highlights of the PVST+ protocol:


PVST+ interoperates with 802.1Q mono Spanning Tree through the so-called Common Spanning Tree (CST) over an 802.1Q trunk. The CST is always on VLAN 1, so this VLAN needs to be enabled on the trunk to interoperate with other vendors. CST BPDUs are transmitted, always untagged, to the IEEE Standard Bridge-Group (MAC Address 01-80-c2-00-00-00, DSAP 42, SSAP 42). For completeness of description, a parallel set of BPDUs are also transmitted to the Cisco shared Spanning Tree MAC address for VLAN 1.


HTH


Sundar

dangal.43 Tue, 06/05/2007 - 13:49

thanks sundar for your informative reply.

so MST and PVST is doing the same function but using different WAY right!!! so is that something like tunneling of PVST data through the CST instance?

dangal.43 Tue, 06/05/2007 - 14:01


hey your replies are so informative and you deserve for expert title and other thing is you are not getting bored by answering one by one thats the best quality i like most in all experts in this forum. As well as member like me who constantly posting questions and querries ;).


sunder will it be possible to have your email id.


looking further for more replies.....



sundar.palaniappan Tue, 06/05/2007 - 14:10

Dan,


Thanks for the appreciation!! I just want to share with you that we also learn some things while trying to help others :)


I am a frequent visitor to this forum and so are a good # of people visiting the forum just to help. Therefore, you are better off posting any queries here and that way anyone who's around at that time can offer you the solution.


HTH


Sundar


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