Hello,
"The Loop Protect enable/disable settings for an MSTI port should match those for the CIST port."
Answer: The MSTI port is the port in which the MSTP Instance is configured but the CIST port in this case will be the underlying SID 0. So when you configure SID 1 and 2 and align VLAN 5,15 to SID 1 and then VLAN 10,20 to SID 2. You will then configure 'set spantree lp ge.x.x sid 1' on all ports that belong to that VLAN and the same goes for SID 2 for any port that belongs to those correlating VLANs. But that is only configuring for the MSTI, to match the CIST port, you must also configure Loop Protect on SID 0 for all of these same ports to ensure proper operation in MSTP.
So in a way this does require you to be very specific about how you prepare and apply your configuration or you may be unknown results if done improperly.
Please let me know if that answers your question.
Thank you and have a great day!
Regards,
Allen George
Technical Support Engineer, GTAC / Extreme Networks