In this paper, we address the combined resource allocation and protocol parameter assignment problem of Ethernet networks using IEEE 802.1S Multiple Spanning Tree Protocol (MSTP). MSTP enables (i) the definition of multiple network regions, where a Common Spanning Tree is set up between all switches, (ii) the set up of additional Spanning Trees inside each region and (iii) the assignment of each traffic VLAN to a different Spanning Tree. We propose an algorithm to compute the appropriate MSTP parameters aiming to optimize the network load balance and minimize service disruption, and show its efficiency through computational results. We study the relationship between load balance and service disruption under single link failures when either a single region or different regions are adopted and for different number of additional Spanning Trees. We show that the single region approach is always better when MSTP is optimally configured and that a very small number of Spanning Trees can obtain near optimal solutions, thus, not penalizing the switching processing overhead.