So i know this in stp domain if you want to influence traffic you. Eed to increase the port cost on the undesirable port for the desired vlan or mst instance and it worked for me,but on a non root bridge , no luck on root bridge i tried cost port-priority,my question is , is the traffic for certain vlan goes through the root bridge for that vlan?how to influence paths on root bridge let vlan x use path A instead of path B

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    – Ron Maupin
    Dec 23, 2021 at 17:05

1 Answer 1


You modify a port's priority on a bridge to change the selection of the root port - by default, the lowest port number is selected.

On the root bridge, there are no root ports and setting priorities doesn't change anything (unless another root bridge is eventually selected).

It's up to the other switches to select their root port based on port metrics and priorities. Port cost is used first to select the root port, and for equal-cost ports their priorities are used (lower value=higher priority). If costs and priorities are equal, the lowest port number wins.

Once those lower switches actually utilize their root port, the root bridge moves their MAC addresses to the selected ports and traffic flows that way (since layer-2 path selection uses MAC addresses generally, for MAC-based networks).

  • Let me see if i got your point in order to influence the root bridge port selection i have to change priority on the next hop (desired switch) Mar 9, 2021 at 8:36
  • Yes - port priority (for equal cost ports). I've added the metrics to the answer.
    – Zac67
    Mar 9, 2021 at 9:13

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