STP question concerning virtual interfaces on physical ports

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May 9th, 2008
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I was wandering if someone could help me with something I picked up:

I heard that STP creates virtual interfaces on a port (per vlan for PVST, ...), how does this work?

Is there a limitation/...

Could some one be so kind to explain this to me or tell me if this is a bogus story?


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Istvan_Rabai Fri, 05/09/2008 - 07:50
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Hi Sybren,

I don't know of this story.

PVST and PVST+ create a separate instance of STP for each vlan, but this is achieved through sending bpdus within each vlan, not with virtual interfaces.



subharojdahal Thu, 05/15/2008 - 08:03
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Well, It is as simple as normal STP operation. If you configure your switch in PVST STP mode, than the switch create a converged spaning tree on for a each and every VLAN configured in the switch.

Lets say you have 3 switch A, B, C and four VLAN 10,20,30,40. You can configured switch A in PVST mode and change it's priority for VLAN 10 and 20 so that it works as a root switch for VLAN 10 and 20. Then It will create two instance of STP tree each for vlan 10 and 20.

Some of the port in B and C are blocked only for those vlan 10 and 20 if you have redundant links.

Similarly, you can configured switch B to act as root switch for vlan 30 and 40.

PVST allows you to configure load balancing among multiple switches.

I hope it helps you.


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