
Explanation/Reference:
Spanning Tree instances - 4
RPVST+ BPDU transmitted - 4
Explanation:
Rapid PVST+ is the IEEE 802.1w (RSTP) standard implemented per VLAN. A single instance of STP runs on each configured VLAN (if you do not manually disable STP). Each Rapid PVST+ instance on a VLAN has a single root switch. You can enable and disable STP on a per-VLAN basis when you are running Rapid PVST+.
When a switch transmits a Rapid PVST+ BPDU frame, all switches connected to the VLAN on which the frame is transmitted receive the BPDU. When a switch receives a BPDU, it does not forward the frame but instead uses the information in the frame to calculate a BPDU, and, if the topology changes, initiate a BPDU transmission.
A BPDU exchange results in the following:
One switch is elected as the root bridge.
*

* The shortest distance to the root bridge is calculated for each switch based on the path cost.

* A designated bridge for each LAN segment is selected. This is the switch closest to the root bridge

through which frames are forwarded to the root.
* A root port is selected. This is the port providing the best path from the bridge to the root bridge.

* Ports included in the spanning tree are selected.

Reference: Configuring Rapid PVST+
http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5000/sw/configuration/guide/cli/ CLIConfigurationGuide/RPVSpanningTree.pdf