Rockwell Automation 1783-WAPxxx Stratix 5100 Wireless Access Point User Manual - Manual (Page 303)

Rockwell Automation Publication 1783-UM006A-EN-P - May 2014 303
Configuring Spanning Tree Protocol Chapter 9
When the access points in a network are powered up, each access point functions
as the STP root. The access points send configuration BPDUs through the
Ethernet and radio ports. The BPDUs communicate and compute the spanning-
tree topology. Each configuration BPDU contains this information:
• The unique access point ID of the wireless access point that the sending
access point identifies as the spanning-tree root
• The spanning-tree path cost to the root
• The access point ID of the sending access point
• Message age
• The identifier of the sending interface
• Values for the hello, forward delay, and max-age protocol timers
When a access point receives a configuration BPDU that contains superior
information (lower access point ID, lower path cost, and so forth), it stores the
information for that port. If this BPDU is received on the root port of the access
point, the access point also forwards it with an updated message to all attached
LANs that are the designated access point.
If a access point receives a configuration BPDU that contains inferior
information that is currently stored for that port, it discards the BPDU. If the
access point is a designated access point for the LAN where the inferior BPDU
was received, it sends that LAN a BPDU containing the updated information
stored for that port. In this way, inferior information is discarded, and superior
information is propagated on the network.
A BPDU exchange results in these actions:
• One access point is elected as the spanning-tree root.
• A root port is selected for each access point (except the spanning-tree
root). This port provides the best path (lowest cost) when the access point
forwards packets to the spanning-tree root.
• The shortest distance to the spanning-tree root is calculated for each access
point based on the path cost.
• The designated access point attached to the LAN is called the designated
port. A designated access point for each LAN segment is selected. The
designated access point incurs the lowest path cost when forwarding
packets from that LAN to the spanning-tree root.
• Interfaces in the spanning-tree instance are selected. Root ports and
designated ports are put in the forwarding state. All interfaces not in the
spanning tree are blocked.