Common Spanning Tree (CST)

Common Spanning Tree (CST)
The IEEE 802.1Q is referred to as the
Common Spanning Tree (CST)
. It is also called the Mono-
Spanning Tree because it uses only one spanning tree instance regardless of the size of the
switched layer 2 network.
The CST runs on all VLANs by default, and all switches are involved in the election process
to find the root bridge. The switches then form an association with that root bridge. Typically,
using CST does not allow for the optimization of the root bridge placement.
There are some advantages to CST. With one STP instance, there are fewer BPDUs consuming
bandwidth. Because there is only one instance of STP in the network, there is less STP processing
performed by the switches.
Normally, the disadvantages outweigh the advantages in a larger network. With a single root
bridge, the path that has been calculated as the best cost to the root bridge might not be the most
efficient for some users to send their data. Another disadvantage of CST is that the STP topology
increases in size to make sure all ports in the network are found. This can cause delays in the
update and convergence times if the network topology is too large