Ch. 4 - Multiple Spanning Tree (MST)

Question 1

What is PVST?

Answer 1

Per VLAN STP – allows you to make different STP topologies for different VLANs

Question 2

What is MST?

Answer 2

Multiple Spanning Tree - Allows switches to map one or more VLANs to one STP tree (called an MST Instance)

Question 3

What do switches in an MST region look like to another switch outside of that region?

Answer 3

The MST region looks like one virtual switch, for compatibility

Question 3

What do you call a group of switches with the same high-level MST config?

Answer 3

An MST region

Question 4

How many MST instances can you have for Cisco?

Answer 4

16 max (0-15)

Question 5

In MST, what is the first instance called?

Answer 5

Instance 0 is called the internal spanning tree (IST)

Question 6

Does MST support setting port cost & priority per MSTI?

Answer 6

Yes

Question 7

What are a couple common MST configurations?

Answer 7

1) Assigning access VLANs to the IST
2) Trunk link pruning

Question 8

What issues can assigning access VLANs to the IST cause? How do you resolve this?

Answer 8

This can block ports unexpectedly.

To resolve:

1) Move the VLAN to another MSTI
2) Allow VLANs associated w/the IST on all trunk links

Question 9

What issues can trunk link pruning cause with MSTIs? How do you resolve this?

Answer 9

Pruning = removing VLANs from specific trunk links to force VLANs to go a certain route. For load balancing

If you prune VLANs not in the same MSTI, the MSTI topology won’t change, even if the VLANs on the trunk do.

To resolve: only prune all the VLANs in the same MSTI for a trunk link