5.3 VLANs and STP
5.3.2 Common STP
The IEEE 802.1Q standard uses an approach to VLANs referred to as mono or common Spanning Tree. The STP runs on VLAN 1, which is the default VLAN. All switches elect a single root bridge and form an association with that root bridge.

In the Figure, a single root bridge has been elected for the red and green VLANs. Notice that the root bridge is optimized for the red VLAN, but some of the green VLAN users must take a less-than-optimal path to get to their file servers. A CST approach does not allow for optimization of the root-bridge placement per VLAN.
CST advantages are as follows:

  • Fewer BPDUs consuming bandwidth
  • Less processing overhead on the switch

CST disadvantages are as follows:

  • Single root bridge, a scenario that may mean a less-than-optimum path for some devices
  • Spanning-Tree topology increases in size to encompass all ports in the switch fabric; this feature can lead to longer convergence times and more frequent reconfiguration