D-Link DES-3326 User Manual - page 390
Manual is about: 24-Port Fast Ethernet Plus 2-Port Gigabit Module Layer 3 Switch
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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
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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.