Election Of The Root Bridge; Stp Protocol Timers; Creating The Spanning Tree Topology - Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual

Understanding How STP Works
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Election of the Root Bridge

For each VLAN, the network device with the highest bridge ID (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 switch will be elected as
the root bridge. Configuring a higher value increases the probability; a lower value decreases the
probability.
The STP root bridge is the logical center of the 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 Layer 2 network, to elect the root port leading to the root bridge, and to determine the designated
port for each Layer 2 segment.

STP Protocol Timers

Table 17-2
Table 17-2
Variable
Hello timer
Forward delay timer
Maximum age timer

Creating the Spanning Tree Topology

In Figure
devices 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 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.
Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
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The shortest distance to the root bridge is calculated for each network device based on the path cost.
A designated bridge for each LAN segment is selected. This is the network device closest to the root
bridge through which frames are forwarded to the root.
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.
describes the STP protocol timers that affect STP performance.
STP Protocol Timers
Description
Determines how often the network device broadcasts hello messages to other
network devices.
Determines how long each of the listening and learning states last before the
port begins forwarding.
Determines the amount of time protocol information received on an port is
stored by the network device.
17-1, Switch A is elected as the root bridge because the bridge priority of all the network
Chapter 17 Configuring STP and MST
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