Cisco N5010P-N2K-BE, Руководство по настройке программного обеспечения

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8-7
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
OL-16597-01
Chapter 8 Configuring Rapid PVST+
Information About Rapid PVST+
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).
Note
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 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.
Note
We recommend that you configure all ports connected to a host as edge ports. See
Chapter 10, “Configuring STP Extensions,” for more information on STP port types.
•
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 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 following:
•
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.