Brocade Mobility RFS Controller System Reference Guide (Supporting software release 5.5.0.0 and later) - Manual (Page 458)

446 Brocade Mobility RFS Controller System Reference Guide
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The Multiple Spanning Tree Protocol (MSTP) provides an extension to STP 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 a spanning tree configuration:
1. Select Configuration > Profiles > Network.
2. Expand the Network menu to display its submenu options.
3. Select Spanning Tree.