Allied Telesis AT-8100 Series - Manual (Page 820)

Chapter 56: Rapid Spanning Tree Protocol (RSTP) Procedures
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Designating RSTP as the Active Spanning Tree Protocol
The first step to using RSTP on the switch is to designate it as the active
spanning tree protocol. This is accomplished with the SPANNING-TREE
MODE RSTP command in the Global Configuration mode. Afterwards,
you can configure its settings and enable the protocol. Here is the
command:
awplus> enable
awplus# configure terminal
awplus(config)# spanning-tree mode rstp
Because RSTP is the default active spanning tree protocol on the switch,
you only need to use this command if you activated STP and now want to
change the switch back to RSTP.