Setting Up Spanning Tree Protocol (STP)

All switches from Cisco ship with Spanning Tree Protocol (STP) enabled by default, but if STP is misconfigured for any reason, you can easily enable it. To enable spanning tree, connect to your switch and type spanning-tree mode <selected mode> while in Global Configuration mode.

The following table lists some of the differences among the different versions of spanning tree. The current version of the IOS supports the following modes and defaults to PVST mode.

Switch2> enable
Switch2# configure terminal 
Switch2(config)#spanning-tree mode ?
  mst         Multiple spanning tree mode
  pvst        Per-Vlan spanning tree mode
  rapid-pvst  Per-Vlan rapid spanning tree mode

Comparison of STP Versions

STP Version	IEEE Identifier	Failover Interval
STP	802.1D	30–50 seconds
RSTP (Rapid)	802.1w	6 seconds (3 Hello intervals)
MSTP or MST (Multiple)	802.1s or 802.1Q-2005	6 seconds (3 Hello intervals)
PVST (Per VLAN)	Cisco Proprietary	6 seconds (3 Hello intervals)
R-PVST (Rapid)	Cisco Proprietary	6 seconds (3 Hello intervals)

After you enable a version of STP on your switch, you need to configure some of the options for the STP, with the biggest option being one of the fast technologies. One the most popular of these options is PortFast,.

Switch2> enable
Switch2# configure terminal 
Switch2(config)#spanning-tree ?
  backbonefast  Enable BackboneFast Feature
  etherchannel  Spanning tree etherchannel specific configuration
  extend        Spanning Tree 802.1t extensions
  loopguard     Spanning tree loopguard options
  mode          Spanning tree operating mode
  mst           Multiple spanning tree configuration
  pathcost      Spanning tree pathcost options
  portfast      Spanning tree portfast options
  uplinkfast    Enable UplinkFast Feature
  vlan          VLAN Switch Spanning Tree

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Source:http://www.dummies.com/how-to/content/setting-up-spanning-tree-protocol-stp.html