Spanning tree STG from switch to switch

Question

Does the STG need to match on all linked switches for VLANs to function properly using PVRST? It seems like whenever I notice they are not the same the VLANs that are not the same are not working as they should be.  

Answer 1

Hello,
A couple of comments on this topic. As noted, the STP instance to VLAN number is locally significant and generally does not need to match between switches. One exception to this is for switches in a vLAG pair (similar to Cisco vPC). For a pair of switches sharing a common vLAG tier ID, the STP-instance-to-VLAN-number must match exactly, or unexpected blocking/discarding will occur (I have never seen this cause a loop, just unexpected blocking, and one way to see if this is the case is to use the command “show span block” and look for any interfaces blocking that should not be).  Lenovo switches use an algorithm to assign an instance to a VLAN, and if you add VLANs in the same order on a pair of switches, those VLANs will get the same STP instance. But if you add VLANs in different order on each switch, then they might get a different instance assigned, and then would require manually adjustment on a vLAG pair.  Again, this does not matter on non-vLAGed switches, and can be any instance, but as noted in a previous response, it is always a good idea to have them in sync between switches, just to prevent user confusion.
There is also a draft paper out for Lenovo switching best practices at the following link that covers a lot of such topics:
http://www.redbooks.ibm.com/abstracts/sg248245.html?Open
Thanks, Matt