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
Rapid PVST+ is the default STP mode for the switch.
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.
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
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
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
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
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 duplex
setting of the port. Full-duplex ports are assumed to be point-to-point ports,
while half-duplex ports are assumed to be shared ports.
Edge ports do not generate topology changes, but all other designated
and root ports generate a topology change (TC) BPDU when they either fail to
receive three consecutive BPDUs from the directly connected neighbor or the
maximum age times out. At this point, the designated or root port sends out a
BPDU with the TC flag set. The BPDUs continue to set the TC flag as long as the
TC While timer runs on that port. The value of the TC While timer is the value
set for the hello time plus 1 second. The initial detector of the topology
change immediately floods this information throughout the entire topology.
When Rapid PVST+ detects a topology change, the protocol does the
Starts the TC While timer with a value equal to twice the hello
time for all the non-edge root and designated ports, if necessary.
Flushes the MAC addresses associated with all these ports.
The topology change notification floods quickly across the entire
topology. The system flushes dynamic entries immediately on a per-port basis
when it receives a topology change.
The TCA flag is used only when the switch is interacting with
switches that are running legacy 802.1D STP.
The proposal and agreement sequence then quickly propagates toward the
edge of the network and quickly restores connectivity after a topology change.