some strange cost factor coming in mstp

Hello,

The key question here is whether your ISPs provide a transparent Layer2 interconnection between your switches 1 and 2, i.e. whether the ISP cloud behaves like a "cross-over cable" connecting the switches 1 and 2 together, and whether you can pass MSTP BPDUs through this Layer2 interconnect. If your ISPs do this then configuring what you require is simply a matter of modifying the MSTP costs just like you did. However, if the ISPs do not provide such transparent interconnection between switches 1 and 2 then it is simply not possible to accomplish this using MSTP because there is no Layer2 loop, and hence MSTP has nothing to act on - it must leave all ports unblocked for all instances in a loop-free environment.

Can you confirm that the interconnection between switches 1 and 2 as provided by the ISP cloud is a Layer2-transparent VPN capable of passing all your VLANs and MSTP communication?

Best regards,

ty,
its not isp network. Its our own network one connected with ofc and other with wireless (transparent bridge). There is already a stp loop-protection enabled and there is no port in abnormal state.

@hashbang Hi,
it would be helpful if you can upload the simple topology and the mstp configuration.

stp region-configuration
region-name R1
instance 1 vlan 101 to 199
instance 2 vlan 200 to 299
instance 3 vlan 500 to 599
instance 4 vlan 600 to 699
instance 5 vlan 700 to 799
instance 6 vlan 1500 to 1550            
instance 7 vlan 99 to 100 2000 4001
active region-configuration

interface GigabitEthernet0/0/8
description Cu
port link-type trunk
port trunk allow-pass vlan 100 614 617 620 to 621 2000
stp loop-protection

the configuration indicates the instances of the MSTP and the VLANs binding with the instances. But the 20199 is the cost for instance 0, you'd better check the path from the S5720 to the STP root switch.