Cisco Nexus 5000 Series - Understanding Rapid PVST

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1-6

Cisco Nexus 5000 Series Switch CLI Software Configuration Guide

OL-16597-01

Chapter 1 Configuring Rapid PVST+

Information About Rapid PVST+

Figure 1-2 Spanning Tree Topology

When the spanning tree topology is calculated based on default parameters, the path between source and

destination end stations in a switched network might not be ideal. For instance, connecting higher-speed

links to a port that has a higher number than the current root port can cause a root-port change. The goal

is to make the fastest link the root port.

For example, assume that one port on Switch B is a fiber-optic link, and another port on Switch B (an

unshielded twisted-pair [UTP] link) is the root port. Network traffic might be more efficient over the

high-speed fiber-optic link. By changing the STP port priority on the fiber-optic port to a higher priority

(lower numerical value) than the root port, the fiber-optic port becomes the new root port.

Understanding Rapid PVST+

This section includes the following Rapid PVST+ topics:

• Overview, page 1-6

• Rapid PVST+ BPDUs, page 1-8

• Proposal and Agreement Handshake, page 1-8

• Protocol Timers, page 1-9

• Port Roles, page 1-10

• Port States, page 1-11

• Synchronization of Port Roles, page 1-13

• Detecting Unidirectional Link Failure, page 1-14

• Port Cost, page 1-15

• Port Priority, page 1-16

Overview

Rapid PVST+ is the IEEE 802.1w (RSTP) standard implemented per VLAN. A single instance of STP

runs on each configured VLAN (if you do not manually disable STP). Each Rapid PVST+ instance on a

VLAN has a single root switch. You can enable and disable STP on a per-VLAN basis when you are

running Rapid PVST+.

Note Rapid PVST+ is the default STP mode for the switch.

187026

RP DP

RP = Root Port

DP = Designated Port

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