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Cisco IOS Software Configuration Guide for Cisco Aironet Access Points
OL-11350-01
Chapter8 Configuring Spanning Tree Protocol
Understanding Spanning Tree Protocol
The access point maintains a separate spanning-tree instance for each active VLAN configured on it. A
bridge ID, consisting of the bridge priority and the access point MAC address, is associated with each
instance. For each VLAN, the access point with the lowest access point ID becomes the spanning-tree
root for that VLAN.
350 Series Bridge Interoperability
Cisco Aironet 1300 and 350 Series 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 350 series bridge acts as a 350 series access point and disallows
association of non-root bridges, including non-root 350, 1200, and 1240 series access points.
•The 350 series bridge supports only a single instance of STP in both non-VLAN and VLAN
configurations, while the 1300 series 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 350 and 1300 series access point
STP feature is enabled and VLANs are not configured.
Note When the 350 and 1300 series access points 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.
Access Point/Bridge Protocol Data Units
The stable, active spanning-tree topology of your network is determined by these elements:
•The unique access point ID (wireless access point priority and MAC address) associated with each
VLAN on each wireless access point
•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 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