MSTP

Multiple Spanning Tree Protocol (MSTP) was first specified in IEEE 802.1s and is standardized in IEEE 802.1Q. MSTP enables multiple VLANs to be mapped to the same spanning-tree instance, reducing the number of spanning-tree instances needed to support a large number of VLANs.

. MSTP provides multiple forwarding paths for data traffic and enables load balancing. It improves the fault tolerance of the network because a failure in one instance, or forwarding path, does not affect other instances.

MSTP Instances: An STP process (which defines a root bridge, root port, designated port and blocking port for a group of switch)  that can be share by multiple VLANs

We will show in our example

Now instead of having 9 different spanning-tree protocol instances running we have 2 instances running

.

You will use MSTP is when you have a lot of VLANs. In  large enterprise corporation.
You wont need MSTP if you have a local network with few VLANs.

All MST instances are numbered 0 through 4094. Instance 0 is a special instance for the region, known as the internal spanning tree (IST).The IST is the only spanning-tree instance that sends and receives BPDUs. 

MSTP is compatible  with other versions of spanning-tree.If for example,  another neighbor switch  is running rapid pvst+ or pvst+ , it can exchange message with that switch

MSTP Region: A group of switches sharing the following configuration attributes:

• region name
• revision number 
• mapping table. 

 All MST instances within the same region share the same protocol timers, but each MST instance has its own topology parameters, such as root bridge ID, root path cost, and so on. By default, all VLANs are assigned to the IST. We will see that at the end of this exercise

 Three differences with VTP:

1.  Unlike VTP . when we made a switch that was propagated to the other switches, if we enter a change into one instance, that change need to be enter in each switch.
2. Revision Number: With VTP we had  a revision number  that automatically  incremented and say :"This is the latest version of information".In MSTP we have it as well , but it doesn't increment automatically 
3. Switches that exchange MSTP information , they don't exchange the entire mapping table

MST Configuration

Let's do some configuration in our personal lab...Please check the table so you will understand what are we going to try to achieve

1.-We will configure the VLAN  in each Switch (please copy and paste in each one)

conf t
spanning-tree mst configuration
instance 1 vlan 110,450
instance 2 vlan 460
end

2.-Now we will apply the primary and secondary root on switch B and C
conf t
spanning-tree mst 1 ?
spanning-tree mst 1 root primary
spanning-tree mst 2 root secondary
end

3.-We will enable MSTP (Copy and paste in each switch)

conf t
spanning-tree mode ?
spanning-tree mode mst
end

4.- We will verify our configuration: show spanning-tree summary

5.- Another way to verify configuration: show spanning-tree mst configuration

6.- We will assigning a name Ccnp_Switch and a revision number 1 on each switch (copy and paste)
conf t
spanning-tree mst configuration
?
name Ccnp_Switch
revision 1
end

 7.-We will issue the same previous command show spanning-tree mst configuration

8.- We will issue the command show spanning-tree vlan 110 on switch B

we see that our priority 28674 is not as low the default of the root bridge 24578
we will check that on switch C

9.- When we are exchanging information with other switches , we are not sending the whole mapping table , we are sending a hash Digest of the table
We will issue the command show spanning-tree mst configuration digest

With that command and checking the Name , Revision number and Digest we will know that the switches belongs to the same MSTP region as we explained before



We suggest that with MSTP , the switches must be interconnected with trunks, and don't prune any VLAN out of those trunks because it can lead to connectivity issues  with MSTP