Protocol Timers; Port Roles - Cisco Nexus 3000 series Configuration Manual

Understanding Rapid PVST+
Related Topics
Summary of Port States, on page 85

Protocol Timers

The following table describes the protocol timers that affect the Rapid PVST+ performance.
Table 8: Rapid PVST+ Protocol Timers
Variable
Hello timer
Forward delay timer
Maximum age timer

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. 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.
Cisco Nexus 3000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U1(1)
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Description
Determines how often each switch broadcasts BPDUs
to other switches. The default is 2 seconds, and the
range is from 1 to 10.
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.
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.
Configuring Rapid PVST+