Common Spanning Tree CST

The IEEE 802.1Q standard specifies how VLANs are to be trunked between switches. As well, it specifies only a single instance of STP for all VLANs. This instance is referred to as the Common Spanning Tree (CST) or the Mono Spanning Tree (MST). All BPDUs are transmitted over VLAN 1, the management VLAN.

Having a single STP for many VLANs simplifies switch configuration and reduces switch CPU load during STP calculations. However, the STP can cause limitations, too. Redundant links between switches will be blocked with no capability for load balancing. Conditions can also occur that would cause forwarding on a link that doesn't support all VLANs, while other links would be blocked.

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