MST

Why MST?

1 vlan = 1 spanning tree
so if we have 1000 vlans we need 1000 spanning trees which will load CPU

Alternate 802.1Q: All Vlans on 1 spanning tree but no load balancing.

MST:
====

Flexible load-balancing
Need to handle few spanning tree instances

Instance:
---------

-> Vlans are placed into groups and the groups are called instances.

Region:
---------

Group of switches running same mst configuration.

Switch(config)#spanning-tree mode mst
Switch(config)#spanning-tree mst configuration
Switch(config-mst)#name cisco

By default Once mst region creates Instance-0(IST - Internal Spanning Tree) and all vlans are in this instance.

Switch# show spanning-tree mst configuration
Name    []
Revision 0
Instance     Vlans Mapped
----------     ------------------------------------------------
0                 1-4094(IST)
--------------------------------------------------------------

As you create additional instances (and place VLANS into these new instanes), the different bridges need to have a consistent vlan to "instance" mapping

An MST region is a group of switches having the same configuration

MST Region Configuration:(Must be same on all switches)

       -Name(32 bytes)
       -Revision Number(16 bits)
       -Vlan to Instance Table

Instances created manually are called Multiple Spanning Tree Instances(MSTIs).

Upto 65 instances can be created

=> MST ALWAYS SENDS ONLY ONE BPDU WHICH CONTAINS OF THE FOLLOWING DETAILS

->Region Information
         Region Name
         Revision Number
->A Configuration Digest of all VLANS and instances
         
   (Instance - Vlan Information) + Formula = Configration Digest

->One or more MRecords for each MSTI
         -All STP Parameters for the instance like Root BID, Sending BID, Root cost etc
         -MSTIs are RSTP instances that only exist inside the region
         -They don't directly interact with outside of the region.
         -Only one instance has timer related parameters(IST instance)
         -One MST BPDU per interface based on the vlans configured for that interface.