By default, in Cisco Catalyst switches, a single STP instance is enabled for each configured VLAN. This is referred to as Per VLAN Spanning Tree Plus (PVST+), which simply means a single Spanning Tree instance for every individual VLAN.
NOTE: PVST+ is a Cisco proprietary protocol that supports 802.1Q. It is an extension of PVST, which is also a Cisco proprietary protocol that supports ISL. PVST is described in detail in the CCNA guide. There is no explicit configuration command required to enable PVST+. A sample PVST+ frame is illustrated in Figure 3-42 below:
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