MST - Multiple Spanning Tree
Do not let MST get to you, nor should you ignore it. This is something that is relatively simple to understand and not as hard to configure as you might think. However, it IS something that you might not see every day or use in your workplace. And if you don't use something, it can be really difficult to remember.
First of all - use NOTEPAD. You are going to want to plan this out in notepad and then copy and paste it into your switches. You will want to visit each switch once and crank out the config in 4 easy paste steps (one for each switch).
Here's the scenario that I will use as an example:
Configure MST. Instance 1 should service VLANs 1 through 10. Instance 2 should service VLANs 11 through 20 and Instance 3 should service VLANs 21 through 30. You have 4 switches with trunk links between them in any manner that may be previously specified. SW1 is primary and SW2 is secondary root bridge for Instance 1. SW2 is primary and SW3 is secondary root bridge for Instance 2. SW3 is primary and SW4 is secondary root bridge for Instance 3.
So for all of them you are going to --
#go into mst configuration mode
#configure the instances
#turn on mst
config t
spanning-tree mst configuration
instance 1 vlan 1-10
instance 2 vlan 11-20
instance 3 vlan 21-30
spanning-tree mode mst
#then list your switch-specific configurations
#SW1
spanning-tree mst 1 root primary
#SW2
spanning-tree mst 1 root secondary
spanning-tree mst 2 root primary
#SW3
spanning-tree mst 2 root secondary
spanning-tree mst 3 root primary
#SW4
spanning-tree mst 3 root secondary
Now you have what you need to copy and paste. Go to the first switch and copy/paste the common part, then copy/paste the specific part. Move to the next switch. Rinse and repeat. ; ) Once you get to SW4, you can go back to SW1 and you should be ready to verify.
For something like this, you should be able to just use "show spanning-tree" and eyeball everything that you need to see. It will show you each instance and if the bridge is root and what ports are in what state.
From there you may be asked to set cost and priority. Read the scenario carefully. There's a difference between setting the cost/priority for that connection and setting it for that connection on only one instance. Another thing you will want to remember is that when you set the cost, use the ? to refresh your memory on the allowable values. Also do a "sh spanning-tree" to see the current values. For MST the cost has a range of 1 to 200000000; with the default value being derived from the bandwidth of the interface. That means that a FastEthernet interface will have a default of 200000. Look at the other interfaces and take a peek before you 'tweak' them. This is very different from the cost of 19 that you normally see with PVST.
Always look before you configure. Before I configure any interface, I always enter the interface configuration mode and then "do show int fa0/x" for whatever interface I'm configuring. This way I have the initial configuration in front of me before I change it. You might also see something there that doesn't belong - and you might need to fix that in order to make your configuration work properly.
For more information on MST, see the Technical White Paper for MST 802.1s - Multiple STP. The White Paper explains the protocol itself and how it works. It is good for getting an understanding of the process. However, the Configuration Guide is the document that goes into the configuration commands and all their variations and uses. The White Paper is necessary for the Written Exam and the Configuration Guide is necessary for the Lab Exam.