Spanning-tree root

Answered Question
Sep 25th, 2008
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I have 15 switches in my LAN.

Two are root.

Here are config and the sh sp


sehqswi04#sh run

!

spanning-tree mode rapid-pvst

spanning-tree extend system-id

!


sehqswi04#sh sp


VLAN0001

Spanning tree enabled protocol rstp

Root ID Priority 32769

Address 0019.304e.7700

This bridge is the root

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec


sehqswi02#sh run

!

spanning-tree mode rapid-pvst

spanning-tree extend system-id

spanning-tree vlan 1-4094 priority 24576

!


sehqswi02#sh sp


VLAN0002

Spanning tree enabled protocol rstp

Root ID Priority 24578

Address 001a.a1cc.b700

This bridge is the root

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec


Why isnt sehqswi02 the only root?


Regards Jan

Correct Answer by ullasupendran about 8 years 7 months ago

Hi


From this description it seems that the Vlan 1 was created in " sehqswi04" before you give "spanning-tree vlan 1-4094 priority 24576" in "sehqswi02" .You might have created the vlan 2 in sehqswi02 after giving the command mentioned above.That may be the reason why sehqswi02 is not the root for both vlan.If you have option to restart sehqswi04, then sehqswi02 will become root for both vlan due to the lower priority value.

Hope this helps.


Ullas

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Marwan ALshawi Thu, 09/25/2008 - 02:55
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switch 1 root for vlan1

and 2 root for vlan 2

this way u distribute the STP load and convergence amonge vlans each vlan has its own STP instances

!!!


if helpful Rate

Jan Rockstedt Fri, 09/26/2008 - 03:46
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But switch 02 have vlan 1-4064.

spanning-tree vlan 1-4094 priority 24576


So why is switch 04 root for vlan 2?


//Jan


Jon Marshall Thu, 09/25/2008 - 03:17
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Jan


Is 04 switch connected to 02 switch via a trunk link ?


If so then yes 02 should be STP root for vlan 1 as well as the priority of 24576 on 02 for all vlans is better than the 32769 on 04.


Jon

Jan Rockstedt Fri, 09/26/2008 - 03:42
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Jon,


Yes switch 02 is my backbone switch and have only have trunk ports.


Jan

Jon Marshall Fri, 09/26/2008 - 03:49
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Can you


1) post output of "sh spanning-tree vlan 1" off switch 02.


2) Confirm that you haven't cleared vlan 1 off the trunk link between 02 & 04.


Jon

ARUNPRABHU A Thu, 09/25/2008 - 04:02
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Hi Jan,


If your question is Why sehqswi02 is the root for vlan2 then

the answer is your switch sehqswi02 is configured with the priority of 24578 for VLAN2 in STP instead of 32768 by default.


I think your switch sehqswi04 is having default priority value of 32768 for VLAN2, so that it cant able to Become root for VLAN2.

Correct Answer
ullasupendran Fri, 09/26/2008 - 12:24
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Hi


From this description it seems that the Vlan 1 was created in " sehqswi04" before you give "spanning-tree vlan 1-4094 priority 24576" in "sehqswi02" .You might have created the vlan 2 in sehqswi02 after giving the command mentioned above.That may be the reason why sehqswi02 is not the root for both vlan.If you have option to restart sehqswi04, then sehqswi02 will become root for both vlan due to the lower priority value.

Hope this helps.


Ullas

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