Differences between PVST and PVST+

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+ native VLAN BPDUs are transported (merged) in IEEE native VLAN (CST) untagged using IEEE STP MAC 0180.0CCC.CCCD 01-80-C2-00-00-00 2)– In addition to that, PVST+ native VLAN is send a second time tunneled over IEEE 802.1q using special multicast MAC 0100.0CCC.CCCD (Shared spanning Tree, SSTP): Untagged, if PVST+ native VLAN=VLAN 1. (figure1) Tagged (coded with TLV, containing VLAN ID), if native VLAN other than VLAN 1. (figure2) This is used exclusively for consistency check. Besides, the error “PVID-inconsistency” is generated if PVST+ BPDU is received on a VLAN different from the one it was generated from. 3)– non-native VLAN BPDUs always tunneled over IEEE 802.1q using special multicast MAC 0100.0CCC.CCCD (Shared spanning Tree, SSTP), tagged (coded with TLV, containing VLAN ID)
– Make sure the native VLAN in PVST+ regions, communicating together through IEEE 802.1q, is the same. – Whatever the complexity of the IEEE 802.1q network, only costs at the borders with PVST+ regions counts for PVST+-IEEE 802.1q native VLAN (VLAN 1 by default) cooperate with PVST, PVST+ and MSTI (802.1s). – PVST (ISL) instances are mapped one-to-one with PVST+ (802.1q) instances.

Figure 1: PVST+ native VLAN is VLAN 1

Figure 2: PVST+ native VLAN is different from VLAN 1

Here is what to retain :

802.1q (standard) supports only one single instance of STP

• native VLAN (Common Spanning Tree)  – (let’s say channel 1)
• one STP instance – (let’s say channel 2)

PVST (Cisco proprietary)

• Support one STP instance per each VLAN
• uses ISL trunk only.
• Doesn’t support 802.1q

PVST+ (Cisco proprietary) Enhance PVST capabilities by allowing to transport PVST over 802.q :

• native VLAN over “Common Spanning Tree” (over channel 1)
• Each per-VLAN STP is encapsulated using a special Multicast MAC and transported (over channel 2)