Spanning Tree operation with VLANs

Depending on the spanning tree option configured on the switch, the spanning tree feature may operate as:
  • A single instance across all ports on the switch regardless of VLAN assignments

  • Multiple instances on a per-VLAN basis.

For single-instance operation, this means that if redundant physical links exist between the switch and another 802.1Q device, all but one link will be blocked, even if the redundant links are in separate VLANs. In this case, you can use port trunking to prevent Spanning Tree from unnecessarily blocking ports (and to improve overall network performance). For multiple-instance operation, physically redundant links belonging to different VLANs can remain open.

Spanning Tree operates differently in different devices. For example, in the (obsolete, non-802.1Q) switch 2000 and the switch 800T, Spanning Tree operates on a per-VLAN basis, allowing redundant physical links as long as they are in separate VLANs. Spanning tree operates differently in different devices.