Differences between PVST and PVST+

CCIE, the beginning!

Behind the simple “plus” in PVST+ lurk quite subtle details that can make the difference between the two concepts very fuzzy, so the goal of this post is to give you a very brief explanation and I hope enough simple to grasp about PVST and PVST+ and their relationship with the standard IEEE 802.1q:

IEEE 802.1q standard

PVST (Per VLAN Spanning Tree) Cisco proprietary

PVST+ Cisco proprietary

BPDU transported over native VLAN untagged (cannot differentiate between different VLANs), therefore support natively only one single instance of STP for all VLAN, MST (Mono Spanning Tree).

(-) Not interoperable and less flexible approach.

(+) Improve the limitation of 802.1d STP (created before VLAN) by supporting one separate instance for each VLAN, using ISL trunk only.

(-) Still not interoperable with IEEE 802.1q that supports only one STP instance.

(+) Modification of PVST: allow PVST over standard IEEE 802.1q:

1) – PVST+…

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