Cisco Aironet Access Points

Bridges use Spanning Tree Protocol (STP) to eliminate loops in an extended network. If you choose to enable this feature, bridges exchange bridge protocol data unit (BPDU) messages with other bridges to detect loops, and the bridges shut down selected port to stop the loops. Bridges also monitor BPDU messages to detect a failure in the network and change their port status to keep the network intact.

If your network contains only two bridges (in other words, the network has no potential for a loop), you do not need to set up STP. However, if your network uses several bridges and loops are possible, you should set up STP.

You can configure STP to customize the spanning tree used by your bridges or you can use the STP defaults. The bridges on your network exchanges BPDU messages to build a spanning tree using default values. For a thorough overview of the Spanning Tree Protocol, consult Cisco CCNA Exam #640-507 Certification Guide, available from CiscoPress.com. Use the following URL to browse to CiscoPress.com: http://www.ciscopress.com/

Use this setting to enable or disable the STP on the bridge without VLAN.

The list of bridges currently using spanning tree protocol.

Use the Root Configuration settings to influence which bridge is the root bridge in the spanning tree.

Use the priority setting to influence which bridge is designated the root bridge in the spanning tree. When bridges have the same priority setting, STP uses the bridges' MAC addresses as a tiebreaker. The bridge with the lowest priority setting is likely to be designated the root bridge in the tree. Enter a value from 0 to 65535.

This setting determines how long the bridge waits before deciding the network has changed and the spanning tree needs to be rebuilt. For example, with Max Age set to 20, the bridge attempts to rebuild the spanning tree if it does not receive a hello BPDU from the root bridge in the spanning tree within 20 seconds. When you select a Max Age setting, consider the amount of time required for a hello to traverse the network and allow for a few hello BPDU to be lost before the bridge reacts and attempts to change the spanning tree. Enter a value from 6 to 40 seconds.

This setting determines how often the root bridge in the spanning tree sends out a hello BPDU telling the other bridges that the network topology has not changed and that the spanning tree should remain the same. Enter a value from 1 to 10 seconds.

This setting determines how long the bridge's ports stay in the listening and learning transition states if there is a change in the spanning tree. For example, before changing a port to forwarding, the bridge puts the port into listening state for the duration of the Forward Delay, and no packets are forwarded; when the Forward Delay has elapsed, the bridge puts the port into learning state for the duration of the Forward Delay, and no packets are forwarded. After the listening the learning periods, the bridge changes the port to forwarding. The listening and learning periods help prevent loops during spanning-tree changes. Enter a value from 4 to 300 seconds.

These settings apply to individual ports on the bridge. Use these settings to adjust the status of individual ports on the bridge.

The path cost indicates the relative efficiency of a port's network link. A port with a high path cost is less likely to become a bridge's root port. Enter a value from 1 to 65535 for the Ethernet and root radio.

Use the priority setting to influence whether STP designates a port as a bridge's root port. A port with a low priority setting is more likely to become a bridge's root port. Enter a value from 0 to 255 for the Ethernet and root radio.

When you enable STP on the bridge, the Ethernet and radio interfaces go through the blocking state and the transitory states of listening and learning. Spanning tree stabilizes each interface at the forwarding or blocking state. An interface moves through these states:

The number of interfaces in blocking state. An interface in the blocking state does not participate in frame forwarding. After initialization, a BPDU is sent to the bridge's Ethernet and radio ports. A bridge initially functions as the spanning tree root until it exchanges BPDUs with other bridges. This exchange establishes which bridge in the network is the spanning tree root. If there is only one bridge in the network, no exchange occurs, the forward-delay timer expires, and the interfaces move to the listening state. An interface always enters the blocking state when you enable STP.

An interface in the blocking state performs as follows:

The number of interfaces in listening state. The listening state is the first state an interface enters after the blocking state. The interface enters this state when the spanning tree determines that the interface should participate in frame forwarding.

An interface in the listening state performs as follows:

The number of interfaces in learning state. An interface in the learning state prepares to participate in frame forwarding. The interface enters the learning state from the listening state.

An interface in the learning state performs as follows:

The number of interfaces in forwarding state. An interface in the forwarding state forwards frames. The interface enters the forwarding state from the learning state.