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The ports associated with a VLAN and VLANs themselves must be contained within a single STG. Not allowing a VLAN to span multiple STGs avoids problems with spanning tree blocking ports and causing a loss of connectivity within the VLAN.
Each untagged port can belong to one and only one STG, while tagged ports can belong to more than one STG. When a tagged port belongs to more than one STG, the spanning tree bridge protocol data units (BPDUs) are tagged to distinguish the BPDUs of one STG from those of another STG. BPDUs from STG 1 are not tagged. The tagged BPDUs are transmitted using a multicast MAC address as tagged frames with a VLAN ID, and you specify the multicast MAC address and the VLAN ID. Because tagged BPDUs are not part of the IEEE 802.1D standard, not all devices can interpret tagged BPDUs.
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