Port Roles - Cisco Nexus 7000 Series Configuration Manual

Configuring Rapid PVST+ Using Cisco NX-OS
Variable
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 device with the highest switch priority
(lowest numerical priority value) as the root bridge. Rapid PVST+ assigns one of these port roles to individual
ports:
• Root port—Provides the best path (lowest cost) when the device forwards packets to the root bridge.
• Designated port—Connects to the designated device that has the lowest path cost when forwarding
packets from that LAN to the root bridge. The port through which the designated device 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 device 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 device has two or more connections to a shared LAN segment. A backup port provides
another path in the topology to the device.
• 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.
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
Description
Determines the amount of time that protocol
information received on a port is stored by the
network device. This timer is generally not used by
the protocol, but it is used when interoperating with
the 802.1D spanning tree. The default is 20 seconds;
the range is from 6 to 40 seconds.
Rapid PVST+
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