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New Member

PVST

what are differences between PVST and PVST+?

  • WAN Routing and Switching
1 ACCEPTED SOLUTION

Accepted Solutions

Re: PVST

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

12 REPLIES
Hall of Fame Super Blue

Re: PVST

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

Cisco Employee

Re: PVST

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

New Member

Re: PVST

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!!!

Re: PVST

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

New Member

Re: PVST

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?

Re: PVST

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

New Member

Re: PVST

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?

Re: PVST

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

New Member

Re: PVST

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?

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