In Ethernet switched environments where multiple Virtual 
LANs exist, spanning tree can be implemented  at each 
vlan . and it is named as per VLAN spanning tree (PVST and 
PVST+, which is the default protocol used by Cisco 
switches). Both PVST and PVST+ protocols are Cisco 
proprietary protocols.Since PVST treats each VLAN as a 
separate network, it has the ability to load balance 
traffic (at layer-2) by forwarding some VLANs on one trunk 
and other Vlans on another trunk without causing a Spanning 
Tree loop.
here one point is noted that scinse it is a 
ciscoproprietary so it use isl encapsulation for trunking.

if you want to know that how to do that then here i canot 
expalin how toconfigure that pvst+