STP can be configured to operate in an environment with multiple VLANs
In Per-VLAN Spanning Tree (PVST) versions of STP, there is a root bridge elected for each spanning tree instance
If all ports on all switches are members of VLAN 1 (The Default), then there is only one spanning tree instance
This makes it possible to have different root bridges for different sets of VLANs. STP operates a separate instance of STP for each individual VLAN
Different Root Bridges