Configuring Spanning Tree Parameters MST General Overview
OmniSwitch AOS Release 8 Network Configuration Guide December 2017 page 6-19
• 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 spantree path-
cost-mode 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 OmniSwitch 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. 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 “How MSTP
Works” on page 6-12 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 spantree path-
cost-mode 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.
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