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I am still understanding a bit about this priority for spanning-tree on VLANs. I have a SWITCH A that is the root of a VLAN. It trunks through SWITCH B with no ports on that vlan. SWITCH B trunks to SWITCH C which does have access on that vlan.

If SWITCH A is priority 0. What is SWITCH B and SWITCH C?

Is SWITCH C 4096? If so is SWITCH B 8192?

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    The switch priorities are whatever you configure them to be, otherwise they are 32768 as a default.
    – Ron Maupin
    Mar 28, 2023 at 22:02
  • @RonMaupin I understand that but if your passing a vlan through a switch that doesn't have access ports for it does the pass through switch need a higher priority than the one that does down the line? Does SWITCH B need a higher Priority than Switch C?
    – JukEboX
    Mar 28, 2023 at 22:07
  • No. The BPDUs will determine the direction of the root switch, You cannot get better than '0' so the priorities of the other switches do not matter; only the root matters.
    – Ron Maupin
    Mar 28, 2023 at 22:12

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STP and RSTP are entirely VLAN agnostic. Any bridge priority is the one you have configured on that bridge - the default is 32768 (8). Which VLANs and port memberships you configure is of no relevance.

The same is true for MSTP unless you configure multiple instances (and possibly regions). Each instance (in a region) allows a participating bridge to set different bridge and port priorities.

Cisco's (R)PVST(+) is a somewhat different beast in that it generally creates an independent spanning tree for each VLAN. The bridge and port priorities work the same however.

In no case does one bridge's priority change any of the others. The only bridge priority that is really relevant is the lowest value; the others become only relevant when the root bridge is removed from the network and another one needs to be elected.

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  • Can you explain root bridge?
    – JukEboX
    Mar 30, 2023 at 17:36
  • A switch is a bridge, so the root bridge is your root switch - the one with the lowest STP priority value and the one that roots the spanning tree that is created.
    – Zac67
    Mar 30, 2023 at 18:49

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