STP and VLAN tagging

Discussion in 'Cisco' started by uofitorn, Mar 29, 2005.

  1. uofitorn

    uofitorn Guest

    Hello everyone,
    I have two switches that are tagged and trunked together with two
    connections. I want to manually configure the STP priorities on the
    VLANs that are on these switches to ensure a nice deterministic
    convergence. I understand that each VLAN runs its own instance of STP
    and is seperately configured at the VLAN level. But how do I configure
    the priority for the VLAN that is trunked between the switches? Do I
    configure the same priority in each switches' configuration for the
    VLAN that is on both switches?

    uofitorn, Mar 29, 2005
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  2. uofitorn

    Brad Guest

    The switch with the lowest priority will be the root switch. Once
    you've figured out which switch you want to be the root then use the
    "set spantree priority" command on it to make it less than the default
    of 32768 (assuming the other switches haven't been changed). It's a
    good idea also to have a backup root switch.
    Cisco uses Per-VLAN spanning tree (PVST). The initial version of .1q
    specified using a single instance of STP for all VLANs but they may
    have added support for multiple instances. Maybe someone else knows
    Brad, Mar 29, 2005
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  3. uofitorn

    Scooby Guest

    It depends on how you want it to work. For my purposes, I want a single
    switch to be the root bridge. Therefore, I set the priority on that switch
    and that is all that needs to be done. For example:

    spanning-tree vlan 10,20,30,40,50,60 priority 4096

    If you don't set this on a switch, it will use the default prio+mac address
    for the bridge and will calculate automatically. Setting the same priority
    for a vlan between the switches would be the same as not setting the
    priority at all. If you have certain areas that you want to have separate
    root bridges, set the priority for the particular vlans on that device.

    That said... With only two switches, you don't really need spanning-tree
    unless you are using two uplinks for redundancy. Even so, I'm not sure I
    would even care about the root bridge as it really wouldn't make any

    Hope that helps,

    Scooby, Mar 29, 2005
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