A colleague (Twitter: @Toonieh) mentioned spanning-tree scalability in a Cisco network. He had an article about this matter. All the credits on this post go to him. I found the article on internet and post it here to be able to find it quickly..
In a Layer 2 looped topology design, spanning tree processing instances are created on each interface for each active VLAN. These logical instances are used by the spanning tree process in processing the spanning tree-related packets for each VLAN. These instances are referred to as active logical ports and virtual ports. Both active logical ports and virtual ports are important values to consider in spanning tree designs because they affect STP convergence time and stability. These values are usually only of concern on the aggregation layer switches because they typically have a larger number of trunks and VLANs configured than other layers in the data center topology.