STP - MST CST and CIST

In the context of Spanning Tree Protocol (STP), there are several constructs and terminologies that are used to refer to the various components of MST. These include the CST and the CIST, which are described below:

CST (Common Spanning Tree)

  • The CST is a single spanning tree that covers all the switches in the entire network, regardless of VLANs.
  • CST treats the entire Layer 2 network as one spanning tree instance, ignoring the VLANs, which is how traditional Spanning Tree Protocol (STP) works.
  • In the context of MST, CST is primarily a legacy concept that MSTP builds upon. The CST is typically implemented by the IST (Internal Spanning Tree), which operates within a single MST region.

CIST (Common and Internal Spanning Tree)

  • The CIST is an extension of the CST and is crucial in the MSTP environment.
  • It consists of the Common Spanning Tree (CST) that connects all MST regions in the network, plus the Internal Spanning Tree (IST) within each MST region.
  • The IST runs inside an MST region and ensures that the region behaves as a single bridge with respect to the outside world. Within the region, the IST manages VLAN traffic according to the specific spanning tree instances (MSTIs) configured within the region.
  • The CIST, therefore, provides a unified spanning tree structure across the entire network, including across different MST regions, while still allowing for multiple spanning trees (MSTIs) to be configured within each region.

https://networklessons.com/spanning-tree/multiple-spanning-tree-mst/

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