Hello
What you are seeing here is down to a feature called “pvst simulation” which in my case I have personally experienced between a mixed DELL and CISCO mixed stp domain which caused me some trouble in the past, However this can be any non mst domain doesnt matter what vendor it is.
Dell running mst
Cisco running pvst
The Common/Instance spanning-tree (CIST) root needs to be within the MST or non MST domain and such the stp priority’s need to adhere to this:
CIST Root = MST region: Vlans 2 + of the NON MST region (that is in my case the cisco stp domain) Had to have its stp priority’s less preferred then the CIST root of the MST region (instance 0) so the cisco stp pvst for the vlans 2 + had to have a higher less preferred value.
CIST root = NON MST region: Vlans 2 + of the cisco stp region had to have its stp priority’s less preferred than its own vlan 1
Failure of either of the above would otherwise result in the boundary port between the two stp domain to transition into a “*PVST_Inc” state because the port cannot be a designated /root port at the same time (ie: vlan 1 designated, all other vlans root port)
I have since found this excellent document on “PVST simulation” and have been able to lab this up so now i am very aware of this feature!
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Kind Regards
Paul
