Alcatel-Lucent 6850-48 Guida Di Rete
Using 802.1Q 2005 Multiple Spanning Tree
MST Interoperability and Migration
OmniSwitch AOS Release 6 Network Configuration Guide
September 2009
page 12-13
Migrating from 1x1 Mode to Flat Mode MSTP
As previously described, the 1x1 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.
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 1x1 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 will make 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.
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 1x1 to flat. When the mode is changed from 1x1
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 1x1 mode
may transition to a blocking state after the mode is changed to flat.
instance that spans across all VLANs. As a result, a path that was forwarding traffic in the 1x1 mode
may transition 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
for more information.
• Note that STP/RSTP use a 16-bit port path cost (PPC) and MSTP uses a 32-bit PPC. When the proto-
col is changed to MSTP, the bridge priority and PPC values for the flat mode CIST instance are reset to
their default values.
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.