Election Of The Root Bridge; Creating The Spanning Tree Topology - Cisco AP775A - Nexus Converged Network Switch 5010 Configuration Manual

Information About Rapid PVST+
• The shortest distance to the root bridge is calculated for each switch based on the path cost.
• A designated bridge for each LAN segment is selected. This is the switch closest to the root bridge
• A root port is selected. This is the port providing the best path from the bridge to the root bridge.
• Ports included in the spanning tree are selected.
Related Topics
•

Election of the Root Bridge

For each VLAN, the switch with the highest bridge ID (that is, the lowest numerical ID value) is elected as
the root bridge. If all switches are configured with the default priority (32768), the switch with the lowest
MAC address in the VLAN becomes the root bridge. The bridge priority value occupies the most significant
bits of the bridge ID.
When you change the bridge priority value, you change the probability that the switch will be elected as the
root bridge. Configuring a lower value increases the probability; a higher value decreases the probability.
The STP root bridge is the logical center of each spanning tree topology in a network. All paths that are not
needed to reach the root bridge from anywhere in the network are placed in STP blocking mode.
BPDUs contain information about the transmitting bridge and its ports, including bridge and MAC addresses,
bridge priority, port priority, and path cost. STP uses this information to elect the root bridge for the STP
instance, to elect the root port leading to the root bridge, and to determine the designated port for each segment.

Creating the Spanning Tree Topology

In the following figure, Switch A is elected as the root bridge because the bridge priority of all the switches
is set to the default (32768) and Switch A has the lowest MAC address. However, due to traffic patterns,
number of forwarding ports, or link types, Switch A might not be the ideal root bridge. By increasing the
priority (lowering the numerical value) of the ideal switch so that it becomes the root bridge, you force an
STP recalculation to form a new spanning tree topology with the ideal switch as the root.
Figure 21: 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
OL-16597-01
through which frames are forwarded to the root.
Rapid PVST+ BPDUs, page 156
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
Election of the Root Bridge
153