Election Of The Root Bridge; Creating The Spanning Tree Topology - Cisco Nexus 7000 Series Configuration Manual

STP
• A root port is elected. This port provides the best path from the bridge to the root bridge.
• Ports included in the spanning tree are selected.

Election of the Root Bridge

For each VLAN, the network device with the highest bridge ID (that is, the lowest numerical ID value) is
elected as the root bridge. If all network devices are configured with the default priority (32768), the network
device 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 device 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 Layer 2 network. All paths that
are not needed to reach the root bridge from anywhere in the Layer 2 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 that leads to the root bridge, and to determine the designated port for each
Layer 2 segment.

Creating the Spanning Tree Topology

By increasing the priority (lowering the numerical value) of the ideal network device so that it becomes the
root bridge, you force an STP recalculation to form a new spanning tree topology with the ideal network
device as the root.
In this figure, switch A is elected as the root bridge because the bridge priority of all the network devices is
set to the default (32768) and switch A has the lowest MAC address. However, due to traffic patterns, the
number of forwarding ports or link types, switch A might not be the ideal root bridge.
Figure 5: Spanning Tree Topology
When the spanning tree topology is calculated based on default parameters, the path between the 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.
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
94
Configuring Rapid PVST+ Using Cisco NX-OS