Motorola WiNG 5.5 Reference Manual page 113

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 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 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.
19. Refer to the PortFast field to define the following:
Enable PortFast
Figure 5-15 Ethernet Ports - Spanning Tree tab
PortFast reduces the time taken for a port to complete STP. PortFast must only be
enabled on ports on the wireless controller which are directly connected to a server/
workstation and not to another hub or controller. PortFast can be left unconfigured on
the access point.
Select this option to enable drop-down menus for both the Enable PortFast BPDU Filter
and Enable PortFast BPDU Guard options. This setting is disabled by default.
Device Configuration 5 - 27