Switching and Routing

Re: MSTP instances differences

New Member

MSTP instances differences



What is the expected behavior of a switch network with Multiple MSTP-Instances but with differences of the same instances on different switches:



Switch A


instance 1 = VLAN 1,2,3

instance 2 = VLAN 4,5,6

instance 3 = VLAN 7,8,9


Switch B


instance 1 = VLAN 1,2

instance 2 = VLAN 3,4,5,6

instance 3 = VLAN 7,8,9


Switch C


instance 1 = VLAN 1,2,3,4

instance 2 = VLAN 5,6

instance 3 = VLAN 7,8,9



Thanks for the Answers,


Pete W
Valued Contributor

Re: MSTP instances differences

All switches configured to use MST, exchange an MD5 digest of their VLAN->Instance mappings in their MSTP BPDU. This allows neighbouring switches to determine whether they are in the same MST region. If they are not in the same region (as you describe), then the switches see/treat each other as RSTP neighbours.


Usually, my advice would be to standardise your MST configuration and "ctrl-C ctrl-V" it everywhere - followed by an MD5 hash verification.