Confirmation on how to configure the root bridge, rapid pvst

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May 29th, 2009
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Folks, on a given 3750 switch I do 'show vlan' and I see vlan 1,3,4 and 7.


spanning-tree mode is rapid-pvst.


Can you confirm if I do:

"#spanning-tree vlan 1,3,4,7 root primary"


is the right way to make this is the root brige. I am just wondering what happens if I assign a new vlan... then it means I would need to add to the spanning-tree vlan <> root command. This sounds strange.

Correct Answer by Jon Marshall about 8 years 1 month ago

Marlon


There are 2 ways to assign root -


1) spanning-tree vlan 1,3,4,7 root primary


2) spanning-tree vlan 1,3,4,7 priority where priority value is usually 8192


I usually use the second option but there is no reason not to use the first. Be aware that option 1) does not recalculate if a switch with a lower priority is added eg.


if you run the 1) command and the priority is set to 16384 and then a switch is added with a priority of 8192 to the network then the switch with 8192 will be STP root for the relevant vlans.


With either 1) or 2) you need to add new vlans when they are created.


Jon

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Correct Answer
Jon Marshall Fri, 05/29/2009 - 13:50
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Marlon


There are 2 ways to assign root -


1) spanning-tree vlan 1,3,4,7 root primary


2) spanning-tree vlan 1,3,4,7 priority where priority value is usually 8192


I usually use the second option but there is no reason not to use the first. Be aware that option 1) does not recalculate if a switch with a lower priority is added eg.


if you run the 1) command and the priority is set to 16384 and then a switch is added with a priority of 8192 to the network then the switch with 8192 will be STP root for the relevant vlans.


With either 1) or 2) you need to add new vlans when they are created.


Jon

news2010a Fri, 05/29/2009 - 14:16
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Thanks. One more verification:

Imagine I have total of 8 switches on the network.


So if I set "Sw1" as spanning-tree vlan 1,3,4,7 priority 8192, then I need to go to all other 7 switches and set 'spanning-tree vlan 1,3,4,7 priority 16384.


is that right?

Jon Marshall Fri, 05/29/2009 - 14:22
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Marlon


"hen I need to go to all other 7 switches and set 'spanning-tree vlan 1,3,4,7 priority 16384"


No you don't. You choose one other switch and configure that as the secondary root and that is it. You do not need to do anything on the other 6 switches. By default the STP priority on a switch is 32768 or 32768 + vlan ID if using extended system-id.


So the switch with priority of 8192 is the root and the switch with the 16384 priority is the secondary root ie. there in case of failure of root switch.


Jon

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