Network OS Administrator’s Guide
Network OS Administrator’s Guide
Network OS v3.0.0
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Configuring STP-Type Protocols : Overview of PVST+ and Rapid PVST+

Overview of PVST+ and Rapid PVST+
A network topology of bridges typically contains redundant connections to provide alternate paths in case of link failures. But since there is no concept of TTL in Ethernet frames, this could result in permanent circulation of frames if there are loops in the network. To prevent loops, a spanning tree connecting all the bridges is formed in real time. The redundant ports are put in a blocking (non-forwarding) state. They are enabled when required.
In order to build a spanning tree for the bridge topology, the bridges must exchange control frames (BPDU – Bridge Protocol Data Unit). The protocols define the semantics of the BPDUs and the required state machine. The first Spanning Tree Protocol (STP) became part of the IEEE 802.1d standard.
But the convergence time of STP is 50 seconds in the case of link failures. This soon became increasingly unacceptable. Keeping the main skeleton of STP the same, the state machine was changed to speed up the convergence time as part of the Rapid Spanning Tree protocol (RSTP). RSTP became part of the standard IEEE 802.1w.
But both STP and RSTP build a single logical topology. A typical network has multiple VLANs. A single logical topology does not efficiently utilize the availability of redundant paths for multiple VLANs. If a port is set to ‘blocked/discarding’ for one VLAN (under STP/RSTP), it is the same for all other VLANs too.
Per-VLAN Spanning Tree Plus (PVST+) protocol runs a spanning tree instance for each VLAN in the network. The version of PVST+ that uses the RSTP state machine is called Rapid-PVST Plus (R-PVST+). R-PVST+ has one instance of spanning tree for each VLAN on the switch.
But PVST+ is not a scalable model when there are many VLANs in the network, as it consumes a lot of CPU power. A reasonable compromise between the two extremes of RSTP and R-PVST+ is the Multiple Spanning Tree protocol (MSTP), which was standardized as IEEE 802.1s and later incorporated into the IEEE 802.1Q-2003 standard. MSTP runs multiple instances of spanning tree which are independent of VLANs. It then maps a set of VLANs to each instance.
NOTE: Brocade Network OS v3.0.0 supports PVST+ and R-PVST+only. The PVST and R-PVST protocols are proprietary to Cisco and are not supported.
To configure PVST+ or R-PVST+, use the protocol spanning-tree pvst and protocol spanning-tree rpvst commands. See the Network OS Command Reference for details.
For example, the script below sets up PVST+ for VLAN 10:
switch(config)#protocol spanning-tree pvst
switch(conf-pvst)#bridge-priority 4096
switch(conf-pvst)#forward-delay 4
switch(conf-pvst)#hello-time 2
switch(conf-pvst)#max-age 7
PVST+ and R-PVST+ guidelines and restrictions
Consider the following items when configuring PVST+ and R-PVST+:
Disabling the tagging of native VLANs is required on STP/RSTP/MSTP switches in standalone mode, otherwise PVST+/R-PVST+ does not converge and forms a loop on the native VLAN. The tagged native VLAN data traffic is ignored. The native vlan untagged data is forwarded.
Disabling the tagging of native VLANs is required on edge ports in fabric cluster mode, otherwise PVST+/R-PVST+ does not converge and forms a loop on the native VLAN. The tagged native VLAN data traffic is ignored. The native vlan untagged data is forwarded.
If a VLAN is configured with tagged ports that do not have PVST+ mode enabled on the interface and are connected to the VDXs, and RSTP is enabled under the VLAN (PVST+), then BPDUs from the tagged ports that are received by the VDX are dropped.

Configuring STP-Type Protocols : Overview of PVST+ and Rapid PVST+