User's Manual

Understanding Rapid PVST+
Rapid PVST+ Overview
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+.
Rapid PVST+ is the default STP mode for the switch.Note
Rapid PVST+ uses point-to-point wiring to provide rapid convergence of the spanning tree. The spanning tree
reconfiguration can occur in less than 1 second with Rapid PVST+ (in contrast to 50 seconds with the default
settings in the 802.1D STP).
Rapid PVST+ supports one STP instance for each VLAN.Note
Using Rapid PVST+, STP convergence occurs rapidly. Each designated or root port in the STP sends out a
BPDU every 2 seconds by default. On a designated or root port in the topology, if hello messages are missed
three consecutive times, or if the maximum age expires, the port immediately flushes all protocol information
in the table. A port considers that it loses connectivity to its direct neighbor root or designated port if it misses
three BPDUs or if the maximum age expires. This rapid aging of the protocol information allows quick failure
detection. The switch automatically checks the PVID.
Rapid PVST+ provides for rapid recovery of connectivity following the failure of a network device, a switch
port, or a LAN. It provides rapid convergence for edge ports, new root ports, and ports connected through
point-to-point links as follows:
• Edge ports—When you configure a port as an edge port on an RSTP switch, the edge port immediately
transitions to the forwarding state. (This immediate transition was previously a Cisco-proprietary feature
named PortFast.) You should only configure on ports that connect to a single end station as edge ports.
Edge ports do not generate topology changes when the link changes.
Enter the spanning-tree port type interface configuration command to configure a port as an STP edge
port.
We recommend that you configure all ports connected to a host as edge ports.Note
• Root ports—If Rapid PVST+ selects a new root port, it blocks the old root port and immediately transitions
the new root port to the forwarding state.
• Point-to-point links—If you connect a port to another port through a point-to-point link and the local
port becomes a designated port, it negotiates a rapid transition with the other port by using the
proposal-agreement handshake to ensure a loop-free topology.
Rapid PVST+ achieves rapid transition to the forwarding state only on edge ports and point-to-point links.
Although the link type is configurable, the system automatically derives the link type information from the
Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
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Configuring Rapid PVST+
Understanding Rapid PVST+