Multiple Spanning Tree (MST)

Lessons Discussion
16

Hi Ryan,
Interaction between MST and PVST/Rapid STP can be quite confusing. There are a few key points that are helpful to keep in mind.

  1. MST tries to hide the details of what is going on inside a region to any STP topology outside of that region
  2. MST accomplishes this by using “Virtual Bridge” in that the entire MST region is present as one giant switch to an outside STP topology
  3. MST uses “Instance 0” for the mechanics of this virtual bridge interaction with an external STP domain
  4. Every MST region MUST have an Instance 0–MST does this automatically
  5. MST uses the best Bridge ID of any switch within its region as its “Virtual Bridge” ID.

Basically, what happens when MST interacts with an external STP domain is this: MST looks at all the switches that are part of its region and selects the one with the best bridge ID. When it comes time to elect a root bridge between the MST region and the external STP, MST says, “Here’s the best I’ve got (and presents the previously selected bridge ID, via Instance 0). Can you do any better?” Depending on whether the external STP can beat the Instance 0 bridge ID, the main root bridge (also called the CST Root) will be either inside the MST or outside. It is always best for it to be inside, fyi.

Here’s the interesting bit. MST will engage in telling lies to the external STP domain. Let’s say the Virtual Bridge ID chosen was from a switch deep inside the MST region, and not at the boundary with the external STP domain. When the external STP switch receives information from the directly connected member of the MST region, it thinks it is talking directly to the switch that is deep within MST. The MST boundary switch will pretend that it has the bridge-ID of the switch that was deep inside MST. The external STP switch will never learn the true identity of the actual MST switch it is talking to.

Like I said, this is a complicated topic, and if you want to get into the real nuts and bolts of it, check out a great blog from INE:
http://blog.ine.com/2010/02/22/understanding-mstp/

3 Likes