Spanning Tree Configuration

About this task

The MSTP (Multiple Spanning Tree Protocol) provides an extension to RSTP to optimize the usefulness of VLANs. MSTP allows for a separate spanning tree for each VLAN group, and blocks all but one of the possible alternate paths within each spanning tree topology.

If there is just one VLAN in the access point managed network, a single spanning tree works fine. However, if the network contains more than one VLAN, the network topology defined by single STP would work, but it is possible to make better use of the alternate paths available by using an alternate spanning tree for different VLANs or groups of VLANs.

A MSTP supported deployment uses multiple MST regions with multiple MSTI (MST instances). Multiple regions and other STP bridges are interconnected using one single CST (common spanning tree). MSTP includes all of its spanning tree information in a single BPDU (Bridge Protocol Data Unit) format. BPDUs are used to exchange information bridge IDs and root path costs. Not only does this reduce the number of BPDUs required to communicate spanning tree information for each VLAN, but it also ensures backward compatibility with RSTP. MSTP encodes additional region information after the standard RSTP BPDU as well as a number of MSTI messages. Each MSTI messages conveys spanning tree information for each instance. Each instance can be assigned a number of configured VLANs. The frames assigned to these VLANs operate in this spanning tree instance whenever they are inside the MST region. To avoid conveying their entire VLAN to spanning tree mapping in each BPDU, the access point encodes an MD5 digest of their VLAN to an instance table in the MSTP BPDU. This digest is used by other MSTP supported devices to determine if the neighboring device is in the same MST region as itself.

To define the spanning tree configuration:

Procedure

Go to Configuration → Devices → System Profile .
The Profile screen displays. This screen lists access point profiles.
Select a profile from the list.

The selected profile's configuration menu displays.
Expand the Network node and select Spanning Tree.

Network - Spanning Tree Screen

Set the following MSTP Configuration parameters:


MSTP Enable	Select this option to enable MSTP for this profile. MSTP is disabled by default, so if requiring different (groups) of VLANs with the profile supported network segment.
Max Hop Count	Define the maximum number of hops the BPDU will consider valid in the spanning tree topology. The available range is from 7 - 127. The default setting is 20.
MST Config Name	Define a 64 character maximum name for the MST region as an identifier.
MST Revision Level	Set a numeric revision value ID for MST configuration information. Set a value from 0 - 255. The default setting is 0.
Cisco MSTP Interoperability	Select either the Enable or Disable radio buttons to enable/disable interoperability with Cisco‘s version of MSTP, which is incompatible with standard MSTP. This setting is disabled by default.
Hello Time	Set a BPDU hello interval from 1 - 10 seconds. BPDUs are exchanged regularly (every 2 seconds by default) and enable supported devices to keep track of network changes and star/stop port forwarding as required.
Forward Delay	Set the forward delay time from 4 - 30 seconds. When a device is first attached to a port, it does not immediately start to forward data. It first processes BPDUs and determines the network topology. When a host is attached the port always goes into the forwarding state, after a delay of while it goes through the listening and learning states. The time spent in the listening and learning states is defined by the forward delay (15 seconds by default).
Maximum Age	Use the spinner control to set the maximum time (in seconds) to listen for the root bridge. The root bridge is the spanning tree bridge with the smallest (lowest) bridge ID. Each bridge has a unique ID and a configurable priority number, the bridge ID contains both. The available range is from 6 - 40. The default setting is 20.

Define the following Port Fast parameters for the profile configuration:


PortFast BPDU Filter	Select Enable to invoke a BPDU filter for this portfast enabled port. Enabling the BPDU filter feature ensures this port channel does not transmit or receive any BPDUs. BPDUs are exchanged regularly and enable the access point to keep track of network changes and to start and stop port forwarding as required. The default setting is disabled.
PortFast BPDU Guard	Select Enable to invoke a BPDU guard for the portfast enabled port. Enabling the BPDU Guard feature means this port will shutdown on receiving a BPDU. Thus, no BPDUs are processed. BPDUs are exchanged regularly and enable the access point to keep track of network changes and to start and stop port forwarding as required. The default setting is disabled.

Define the following Error Disable settings:


Enable Recovery	Select this option to enable a error disable timeout resulting from a BPDU guard. This setting is disabled by default.
Recovery Internal	Define the recovery interval used to enable disabled ports. The available range is from 10 - 1,000,000 seconds with a default setting of 300.

Use the Spanning Tree Instance table to add indexes to the spanning tree topology.
Add up to 16 indexes and use the Priority setting to define the bridge priority used to determine the root bridge. The lower the setting defined, the greater the likelihood of becoming the root bridge in the spanning tree topology.
Use the Spanning Tree Instance VLANs table to add VLAN instance indexes (by numeric ID) and VLANs to the spanning tree topology.
Select the OK button located at the bottom right of the screen to save the changes.
Select Reset to revert to the last saved configuration.