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?

Francisco.

Comments

  • ok i noticed by issuing the command "sh spanning-tree mst 1", the port fa0/13 between sw1 & sw4 has a better cost so i changed the cost on fa0/21 to 1 and that worked.. the root port is now fa0/21. 

    what i havent taken in to consideration is the end-end cost. "sh spanning-tree mst 1" only show the local cost not the end-end on the port fa0/21. "show spanning-tree mst detail" does show the end to end cost and fa0/21 was higer than fa0/13 so i changed fa0/21 to be lower and all is working as expected. :)

     

    Francisco.

  •  

    There are two varibale that will allow you to change the root port election,

    1: Port-cost

    2: Port-priority

    You change the cost when you are going away from the root,

    You change the priority when you are going toward the root

    See below topology,

     

    SW1--------------------SW2

    |                               |

    |                               |

    |                               |

    SW4--------------------SW3

    if you want SW4 goes through SW3 not directly to SW1, you have two options in this case

    1: on SW1 change the port cost on the port connected to direct to SW4 something higher than through SW1 -> SW2 -> SW3 - > SW4

    2: Change the port-priority on SW4 on the port connecting SW4 to SW1 directly.

     

    HTT

    Best Regards,

    Manouchehr

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