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Chapter 8
Configuring Spanning Tree Protocol
The bridge maintains a separate spanning-tree instance for each active VLAN configured on it. A bridge
ID, consisting of the bridge priority and the bridge MAC address, is associated with each instance. For
each VLAN, the bridge with the lowest bridge ID becomes the spanning-tree root for that VLAN.
Bridge Protocol Data Units
The stable, active spanning-tree topology of your network is determined by these elements:
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When the bridges in a network are powered up, each bridge functions as the STP root. The bridges send
configuration BPDUs through the Ethernet and radio ports. The BPDUs communicate and compute the
spanning-tree topology. Each configuration BPDU contains this information:
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When a bridge receives a configuration BPDU that contains superior information (lower bridge 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 bridge, the bridge also forwards it with an updated message to all attached LANs for which
it is the designated bridge.
If a bridge receives a configuration BPDU that contains inferior information to that currently stored for
that port, it discards the BPDU. If the bridge is a designated bridge for the LAN from which the inferior
BPDU was received, it sends that LAN a BPDU containing the up-to-date 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:
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OL-4059-01
The unique bridge ID (wireless bridge priority and MAC address) associated with each VLAN on
each wireless bridge
The spanning-tree path cost to the spanning-tree root
The port identifier (port priority and MAC address) associated with each Layer 2 interface
The unique bridge ID of the wireless bridge that the sending bridge identifies as the spanning-tree
root
The spanning-tree path cost to the root
The bridge ID of the sending bridge
Message age
The identifier of the sending interface
Values for the hello, forward delay, and max-age protocol timers
One bridge is elected as the spanning-tree root.
A root port is selected for each bridge (except the spanning-tree root). This port provides the best
path (lowest cost) when the bridge forwards packets to the spanning-tree root.
The shortest distance to the spanning-tree root is calculated for each bridge based on the path cost.
A designated bridge for each LAN segment is selected. The designated bridge incurs the lowest path
cost when forwarding packets from that LAN to the spanning-tree root. The port through which the
designated bridge is attached to the LAN is called the designated port.
Interfaces included in the spanning-tree instance are selected. Root ports and designated ports are
put in the forwarding state.
All interfaces not included in the spanning tree are blocked.
Cisco Aironet 1400 Series Wireless Bridges Software Configuration Guide
Understanding Spanning Tree Protocol
8-3
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