1.6.a PVST+, RPVST+, MST
common spanning tree
the original iteration of 802.1q defined a single instance of spanning tree regardless of the amount of vlans; a common tree for the entire network. when a path is blocked due to convergence, as it will be, there is but one path for the vlans to traverse toward their destination.
this is hardly fair for the multiple vlans that could potentially take more than one path if given the opportunity.
so the common tree does not allow for load balancing although this is ultimately less cpu intensive.
by definition mst supports multiple trees similar to pvst, which supports an instance per vlan. however, unlike pvst, a reduction in the total amount of instances can be achieved by balancing, grouping, vlans together and shipping them across different paths; many vlans;