Spanning Tree Configuration - Motorola WiNG 5.6 Reference Manual

Device Configuration 5 - 85

5.2.6.7 Spanning Tree Configuration

Profile Network Configuration
The Multiple Spanning Tree Protocol (MSTP) 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's 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's 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 MST instances (MSTI). Multiple regions and other STP
bridges are interconnected using one single common spanning tree (CST).
MSTP includes all of its spanning tree information in a single Bridge Protocol Data Unit (BPDU) 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:
1. Select the Configuration tab from the Web UI.
2. Select Devices.
3. Select System Profile from the options on left-hand side of the UI.
4. Expand the Network menu and select Spanning Tree.