Port cost determines the best path when multiple links are used between two switches and none of the links is a root port. The cost of a link is determined by the bandwidth of a link.
The root port is always the link directly connected to the root bridge, or the shortest path to the root bridge. If more than one link connects to the root bridge, then a port cost is determined by checking the bandwidth of each of link. The lowest-cost port becomes the root port. If multiple links have the same cost, the bridge with the lower advertising bridge ID is used.
A designated port is one that has been determined as having the best cost. A designated port will be marked as a forwarding port.
The convergence time for legacy Spanning Tree Protocol (STP) IEEE 802.1D standard is 30 to 50 seconds. When the network is converging on a topology change, no traffic is forwarded to or from any of the network bridges and switches. In modern networks this convergence time gap is not acceptable and Cisco enhanced the original Spanning Tree Protocol (STP) 802.1D specification with features such as PortFast, UplinkFast and BackboneFast to speed up the convergence time. But these were proprietary enhancements.
The Rapid Spanning Tree Protocol (RSTP) IEEE 802.1W standard is available to address the convergence time gap issue. Rapid Spanning Tree Protocol (RSTP) enables root ports and designated ports to change from the blocking to forwarding port state in a few seconds.
The type of a single instance Spanning Tree Protocol (STP) is known as Common Spanning Tree (CST). Delay in receiving BPDUs is common in large switched Common Spanning Tree (CST) networks. The delay in receiving BPDUs can cause problems like convergence time problems. Per-VLAN Spanning Tree (PVST) is a solution for these problems. Per-VLAN Spanning Tree (PVST) is a Cisco proprietary Spanning Tree Protocol (STP) which operates a separate instance of Spanning Tree Protocol (STP) for each individual VLAN. A separate instance of Spanning Tree Protocol (STP) for each VLAN helps VLAN to be configured independently and also can perform better. Per-VLAN Spanning Tree (PVST) requires Inter-Switch Link (ISL).
Per-VLAN Spanning Tree+ (PVST+) is an extension of the PVST standard. Per-VLAN Spanning Tree+ (PVST+) allows interoperability between CST and PVST in Cisco switches and support the IEEE 802.1Q standard.