Allen-Bradley 1783-WAPAK9, User Manual

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304 Rockwell Automation Publication 1783-UM006A-EN-P - May 2014
Chapter 9 Configuring Spanning Tree Protocol
Election of the Spanning-
tree Root All access points in the Layer 2 network participating in STP gather information
about other access points in the network through an exchange of BPDU data
messages. This exchange of messages results in these actions:
•
The election of a unique spanning-tree root for each spanning-tree
instance
•
The election of a designated access point for every LAN segment
•
The removal of loops in the network by blocking Layer 2 interfaces
connected to redundant links
For each VLAN, the access point with the highest access point priority (the
lowest numerical priority value) is elected as the spanning-tree root. If all access
points are configured with the default priority (32768), the access point with the
lowest MAC address in the VLAN becomes the spanning-tree root. The access
point priority value occupies the most significant bits of the access point ID.
When you change the access point priority value, you change the probability that
the access point is elected as the root access point. Configuring a higher value
decreases the probability; a lower value increases the probability.
The spanning-tree root is the logical center of the spanning-tree topology. All
paths that are not needed to reach the spanning-tree root from anywhere in the
network are placed in the spanning-tree blocking mode.
BPDUs contain information about the sending access point and its ports,
including access point and MAC addresses, access point priority, port priority,
and path cost. STP uses this information to elect the spanning-tree root and root
port for the network and the root port and designated port for each LAN
segment.
Spanning-tree Timers This table describes the timers that affect the entire spanning-tree performance.
Table 88 - Spanning-tree Timers
Variable Description
Hello timer Determines how often the access point broadcasts hello messages to other access
points.
Forward-delay timer Determines how long each of the listening and learning states last before the
interface begins forwarding.
Maximum-age timer Determines the amount of time the access point stores protocol information received
on an interface.