Enable portfast on ports where the connected hosts are very likely to bring their link up and down (typically end stations that users frequently power cycle). This feature should not be necessary for server ports. It should definitely be avoided on ports that lead to hubs or other bridges. A port that directly transitions to forwarding state on a redundant link can cause temporary bridging loops. The following document has more information related to Spanning Tree Protocol Problems and Design Considerations.
http://www.cisco.com/warp/public/473/16.html.
We can change the root bridge either by changing the priority or using the root command:
CatOS: Issue the set spantree root vlan_id command in order to set the priority of the switch to 8192 for the VLAN or VLANs that the vlan_id specifies. The set spantree priority command provides a second method to specify the root switch.
IOS: To configure a VLAN instance to become the root bridge, enter the spanning-tree vlan vlan_ID root command to modify the bridge priority from the default value (32768) to a significantly lower value.
