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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 13 Understanding and Configuring STP
Overview of STP
Election of the Root Bridge
For each VLAN, the switch with the highest bridge priority (the lowest numerical priority value) is
elected as the root bridge. If all switches are configured with the default priority value (32,768), the
switch with the lowest MAC address in the VLAN becomes the root bridge.
The spanning tree root bridge is the logical center of the spanning tree topology in a switched network.
All paths that are not required to reach the root bridge from anywhere in the switched network are placed
in spanning tree blocking mode.
A spanning tree uses the information provided by BPDUs to elect the root bridge and root port for the
switched network, as well as the root port and designated port for each switched segment.
STP Timers
Table 13-3 describes the STP timers that affect the performance of the entire spanning tree.
Creating the STP Topology
The goal of the spanning tree algorithm is to make the most direct link the root port. 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 optimal according to link speed. 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.
In Figure 13-1, Switch A is elected as the root bridge. (This could happen if the bridge priority of all the
switches is set to the default value [32,768] 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. By increasing the STP port priority (lowering the numerical value) of the ideal switch so that it
becomes the root bridge, you force a spanning tree recalculation to form a new spanning tree topology
with the ideal switch as the root.
Table 13-3 Spanning Tree Protocol Timers
Variable Description
hello_time Determines how often the switch broadcasts hello messages to other
switches.
forward_time Determines how long each of the listening and learning states will last before
the port begins forwarding.
max_age Determines the amount of time that protocol information received on a port
is stored by the switch.
