Configuring Spanning Tree Parameters
MST General Overview
OmniSwitch AOS Release 7 Network Configuration Guide
June 2013
page 6-19
•
Using explicit Spanning Tree commands to define the MSTP configuration is required. Implicit
commands are for configuring STP and RSTP. See
“Using Spanning Tree Configuration Commands”
for more information.
•
STP and RSTP use a 16-bit port path cost (PPC) and MSTP uses a 32-bit PPC. When the protocol is
changed to MSTP, the bridge priority and PPC values for the flat mode CIST instance are reset to their
default values.
•
It is possible to configure the switch to use 32-bit PPC value for all protocols (see the
command page for more information). If this is the case, then the PPC for the CIST is not
reset when the protocol is changed to/from MSTP.
•
This implementation of MSTP is compliant with the IEEE 802.1Q 2005 standard and thus provides
interconnectivity with MSTP compliant systems.
Migrating from Per-VLAN Mode to Flat Mode MSTP
As previously described, the per-VLAN mode is an Alcatel-Lucent proprietary implementation that
applies one Spanning Tree instance to each VLAN. For example, if five VLANs exist on the switch, then
their are five Spanning Tree instances active on the switch, unless Spanning Tree is disabled on one of the
VLANs.
Note the following when converting a per-VLAN mode STP/RSTP switch to flat mode MSTP:
•
Making a backup copy of the switch
boot.cfg
file before changing the protocol to MSTP is highly
recommended. Having a backup copy makes it easier to revert to the non-MSTP configuration if
necessary. Once MSTP is active, commands are written in their explicit form and not compatible with
previous releases of Spanning Tree.
•
Using MSTP requires changing the switch mode from per-VLAN to flat. When the mode is changed
from per-VLAN to flat, ports still retain their VLAN associations but are now part of a single, flat
mode
Spanning Tree instance that spans across all VLANs. As a result, a path that was forwarding traffic in
the per-VLAN mode transitions to a blocking state after the mode is changed to flat.
•
Once the protocol is changed, MSTP features are available for configuration. Multiple Spanning Tree
Instances (MSTI) are now configurable for defining data paths for VLAN traffic. See
for more information.
•
Note that STP/RSTP use a 16-bit port path cost (PPC) and MSTP uses a 32-bit PPC. When the
protocol is changed to MSTP, the bridge priority and PPC values for the flat mode CIST instance are
reset to their default values.
•
It is possible to configure the switch to use 32-bit PPC value for all protocols (see the
command page for more information). If this is the case, then the PPC for the CIST is not
reset when the protocol is changed to/from MSTP.
•
This implementation of MSTP is compliant with the IEEE 802.1Q 2005 standard and thus provides
interconnectivity with MSTP compliant systems.