Opnet, Wandl, etc.

We are looking into using a full or partial mesh MPLS overlay for traffic characterization on our network and need a tool to model utilization and simulate failures. At first we won't be doing any real TE.

Does anyone know of any open source tools (maybe similar to Wandl) that have this functionality? I could do what I need with spreadsheets really but it would be more time intensive.

Thanks
-Tom