Cisco Nexus 5000 Series Switch CLI Software Configuration Guide, NX-OS 4.0(1a)N1 (OL-16597-01, January 2009)

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Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
OL-16597-01
Chapter 8 Configuring Rapid PVST+
Information About Rapid PVST+
Port Roles
Rapid PVST+ provides rapid convergence of the spanning tree by assigning port roles and learning the
active topology. Rapid PVST+ builds upon the 802.1D STP to select the switch with the highest priority
(lowest numerical priority value) as the root bridge as described in the “Election of the Root Bridge”
section on page 8-5. Rapid PVST+ then assigns one of these port roles to individual ports:
• Root port—Provides the best path (lowest cost) when the switch forwards packets to the root bridge.
• Designated port—Connects to the designated switch, which incurs the lowest path cost when
forwarding packets from that LAN to the root bridge. The port through which the designated switch
is attached to the LAN is called the designated port.
• Alternate port—Offers an alternate path toward the root bridge to the path provided by the current
root port. An alternate port provides a path to another switch in the topology.
• Backup port—Acts as a backup for the path provided by a designated port toward the leaves of the
spanning tree. A backup port can exist only when two ports are connected in a loopback by a
point-to-point link or when a switch has two or more connections to a shared LAN segment. A
backup port provides another path in the topology to the switch.
• Disabled port—Has no role within the operation of the spanning tree.
In a stable topology with consistent port roles throughout the network, Rapid PVST+ ensures that every
root port and designated port immediately transition to the forwarding state while all alternate and
backup ports are always in the blocking state. Designated ports start in the blocking state. The port state
controls the operation of the forwarding and learning processes.
A port with the root or a designated port role is included in the active topology. A port with the alternate
or backup port role is excluded from the active topology (see Figure 8-5).
Table 8-2 Rapid PVST+ Protocol Timers
Variable Description
Hello timer Determines how often each switch broadcasts BPDUs to other switches. The
default is 2 seconds, and the range is from 1 to 10.
Forward delay timer Determines how long each of the listening and learning states last before the
port begins forwarding. This timer is generally not used by the protocol but
is used as a backup. The default is 15 seconds, and the range is from 4 to 30
seconds.
Maximum age timer Determines the amount of time protocol information received on an port is
stored by the switch. This timer is generally not used by the protocol, but it
is used when interoperating with 802.1D spanning tree. The default is 20
seconds; the range is from 6 to 40 seconds.