Cisco Systems N3KC3064TFAL3, N3KC3048TP1GE manual Protocol Timers, Port Roles, Variable Description

Configuring Rapid PVST+

Understanding Rapid PVST+

Protocol Timers

The following table describes the protocol timers that affect the Rapid PVST+ performance.

Table 7: 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.

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.

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

Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)

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