Multiple Spanning Tree Protocol

12 important questions on Multiple Spanning Tree Protocol

MST has a blended approach as opposed to STP or PVSTP. What is meant by this?

MST maps one or more VLANs to a single STP tree, called an MST Instance (MSTI).

How is a grouping of MST switches with the same high-level configuration called.

MST Region

MST incorporates a compatibility mechanism which make an MST Region appear like what to external switches?

An MST region appears as a single virtual switch.
  • Higher grades + faster learning
  • Never study anything twice
  • 100% sure, 100% understanding
Discover Study Smart

MST uses a Special STP Instance, which is always the first instance, Instance 0. What is the name of this MST Instance?

Internal Spanning Tree (IST)

MST, using the IST instance of STP, only advertises one set of BPDUs, minimizing STP Traffic, while still providing all information for all MST Instances. Which 2 properties make this possible?

1. The IST Instance runs on all switchports, regardless of VLANs
2. Additional information about other MST Instances are included in the IST BPDU.

The IST always is instance 0. How many MST Instances are supported by default on Cisco Switches?

Cisco supports up to 16 MST Instances. 0 for the IST Instance and 1 - 15 for any other manually configured instances.

MST supports tuning of port cost and port priority. What are the commands to do this?

In interface configuration context:
spanning-tree mst instance-number cost cost
spanning-tree mst instance-number port-priority priority

Why shouldn't VLANs be assigned to the Internal Spanning Tree?

The Internal Spanning Tree (IST) is active on every port in the region. When there is a VLAN active on only one link of a double connection there is a chance the VLAN gets cutoff because of STP.

What can go wrong when pruning VLANs on trunk links when using MST?

If VLANs of the same MST Instance are pruned differently.

What is an MST Region Boundary

Any port that connects to a switch in a different MST Region or that connects to 802.1D or 802.1W BPDUs

What MST feature propogates Common Spanning Tree (CST), which is derived from the IST, at the MST Region Boundary?

PVST Simulation Mechanism

How does the PVST Simulation Mechanism propogate the MST Region to outside it's boundaries?

It uses the IST Topology to send out PVST+/RSTP BPDUs for every VLAN on the boundary link.

The question on the page originate from the summary of the following study material:

  • A unique study and practice tool
  • Never study anything twice again
  • Get the grades you hope for
  • 100% sure, 100% understanding
Remember faster, study better. Scientifically proven.
Trustpilot Logo