MST root port cost & priority change
SW4 is directly connected to SW1 and SW1 is the MST root bridge and the port that directly connect SW1 & SW4 is the root port. no issue right.
But if there is a request to change the root on sw4 to go via the transit layer 2 between sw3 & sw4 instead by changing the port cost & priority, i dont think it will work as sw4 is already directly connected already to the root bridge! it will not make any difference by modifying the port cost & priority on the transit layer 2 links between sw4 & sw3 to be better, what i noticed is that the root port on sw4 is always choosen to the the directly connected port to the root.
I have tried the above on the switching lab for example Vol 1 Lab 1:35, changing the STP priority to modify layer 2 transit for MST instance 1 from sw4 to sw1 to uses the last link between sw4 & sw3. I have modified the last port between sw4 & sw3 with a better priority & cost much lower that the direct links between sw1 & sw4, sw4 root port is still via sw1 even though the port cost & priority is better via sw3.
What do you think guys?