D-Link DES-3326S - Manual (Page 336)

DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
336 Understanding and Troubleshooting the Spanning Tree Protocol
Organize the redundant links and tune the port cost parameter
of STP to force those ports to be in the blocking state.
For each VLAN, know which ports should be blocking in a
stable network. A network diagram that shows each physical
loop in the network and which ports break which loops is
extremely helpful.
Minimize the number of ports in the blocking state.
A single blocking port transitioning to the forwarding state at
an inappropriate time can cause a large part of a network to
fail. Limiting the number of blocked ports help to limit the risk
of an inappropriate transition.