User's Manual

6-3
Cisco 3200 Series Wireless MIC Software Configuration Guide
OL-7734-02
Chapter 6 Configuring Spanning Tree Protocol
Understanding 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 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 Interoperability
Cisco bridges are interoperable when STP is enabled and no VLANs are configured. This configuration
is the only one available for the following reasons:
• When STP is disabled, the bridge acts as an access point and disallows association of non-root
bridge.
• The bridge has a single instance of STP in non-VLAN configurations and multiple instances of STP
in VLAN configurations.
• Incompatibilities between single and multiple instances of STP can cause inconsistent blocking of
traffic when VLANs are configured. When the native VLAN is blocked, you can experience bridge
flapping.
Therefore, the best configuration for STP interoperability is when the bridge STP feature is enabled and
VLANs are not configured.
Note When the Cisco bridges are configured as workgroup bridges, they can operate with STP disabled and
allow for associations with access points. However, this configuration is not technically a
bridge-to-bridge scenario.
Bridge Protocol Data Units
The stable, active spanning-tree topology of your network is determined by these elements:
• 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
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:
• 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
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.