Hello Anil,
if you are running PVST+ or Rapid PVST and your switches have a limitation of 128 Per Vlan STP instances and you have more then 128 Vlans you are potentially in trouble:
some Vlans will have the command
no spanning-tree Vlan X
no spanning-tree vlan y
and if in PVST+/ Rapid PVST mode these commands are meaningful and say that no STP instance is running for Vlan X and Vlan Y.
It is not clear how the "exceeding Vlans" are chosen the last created in VLAN database are a likely choice but I am not sure about this.
What is more important is that any switch or switch stack can make different choices about the "exceeding Vlans" the VLANs without STP and this would increase the impact and probability of issues:
just at access layer what if two ports belonging to Vlan X are connected together by accident ? Because no STP BPDUs are sent or received the switch has no means to detect the loop.
The only alternative to MST is the following one:
implement manual list of allowed Vlans on each uplink both sides if access ports are not spread over all the Vlans just allow the needed ones and the management VLANs
Then for each access layer device ensure that STP is active for VLANs with access ports associated to them and allowed on the uplinks.
As you see it is a long work and requires frequent check and update.
On the other side moving to MST solves the scalability issue but requires proper planning to create subsets of Vlans served by different MST instances.
To be noted also not existing Vlans can be associated to an MST instance.
This work is important because MST is not so plug and play as PVST is. A change on a single switch creates a new MST region.
Things have became easier with VTP version 3 that can propagate the VLANs to MST instances associations from primary server to all switches.
Hope to help
Giuseppe
