spanning tree priority

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Jun 22nd, 2009
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Is there a difference between the options:

a) making a switch the root spanning-tree by doing this:

spanning-tree vlan 22 root


b) making a switch the root spanning-tree by

spanning-tree vlan 22 priority 4096 (whereas the other switch is 8192

practically speaking, what is the difference between the two ways of assigning spanning-tree preference?

Also when changing the priority (or making the other switch a ROOT), is there any outage for the traffic to traverse through the other path?

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Edison Ortiz Mon, 06/22/2009 - 09:49
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From the switch documentation:

When you enter the spanning-tree vlan vlan-id root command, the software checks the switch priority of the current root switch for each VLAN. Because of the extended system ID support, the switch sets the switch priority for the specified VLAN to 24576 if this value will cause this switch to become the root for the specified VLAN. If any root switch for the specified VLAN has a switch priority lower than 24576, the switch sets its own priority for the specified VLAN to 4096 less than the lowest switch priority. (4096 is the value of the least-significant bit of a 4-bit switch priority value.)

When using the absolute value, there is no checking for the current STP root, you need to know the value to place there.

Either command may produce traffic outage depending upon the current STP Root.





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