Hi Carl
If you have a resilient network design with dual core and multiple paths around the network it is possible to share the traffic through the network by specifying that one core is root bridge for certain VLANs and the other core is root bridge for the remaining VLANs.
If multiple paths exist and you specify that a single core will be root for all VLANs then all traffic will take the same route through the network and the same ports will be placed in blocking state. This means that certain links will be operating purely as redundant links and will not be utilised unless a failure occurs on the primary links. If you split the roots for your VLANs accross say two devices then the best paths to these devices will be different for each VLAN, and different ports will be in blocking states for different VLANs. This means that the traffic through the network will be more evenly spread, and the resilient links will also be utilised instead of just sitting there incase there is a failure.
Hope this helps
Martin
