Cisco – When connecting 3 switch in Spanning tree, Should I use Switch port or SFP ports

According to the details you explain about your network topology, my understanding is:

• Cisco switch is root of the spanning-tree.
• Port A4 in switch HP 7 is root port and therefore it is in forwarding state
• Port B1 in switch HP 8 is root port and therefore it is also in forwarding state

Now let´s focus in the other link closing the loop, the one between both HP switches. You can have only a designated port and because the cost to root advertised by both HP switches into the link is the same, switches would have to check the lowest sender bridge ID and lowest sender port ID in order to choose the designated port and the blocking port.

So, setting a different costs for ports in the link between HP switches will have no effect in choosing the designated port for this link.

You should run 'port-fast' (in standard terms edge port) in every port not part of your switch core. Even if it is switch.

You should NOT have L2 loop through customer switches.

You should run BPDUGuard and BUM policers all interfaces, customer facing interfaces should be 1/5th or less of core facing limits. Unfortunately limiting unknown unicast often is not supported.

Why running 'port-fast' or edge is crucial is performance of RSTP (and by extension MST) rely on it. How RSTP works is it asks downstream if it can go to forwarding mode, and downstream asks its downstreams until there are no more ports to ask frmo, then the permission propagates back up. Port-fast or edge port is implicit permission from RSTP point-of-view, if you remove this implicit permission, explicit permission must be gotten otherwise it'll fall back to classic STP timers. Which means even one non-portfast port will kill your subsecond RSTP converge.