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Per VLAN Spanning Tree


All modern Cisco switch support PVST. As name indicates what per VLAN Spanning Tree does is add a VLAN number to the priority of the BPDU headers. e.g., the default priority is 32768; if you run VLAN 10 on your switch then new priority would be 32778. The result of this is you have one Root Bridge per VLAN. If your network has multiple VLAN then you have multiple Root Bridge per VLAN. In that case if you don't change anything, by default same switch will elected as Root Bridge for every single VLAN number.


From the diagram above we have VLAN 10 and 20 running on Switches which are trunked each other. We have two VLAN here that means we have two completely separate network of Spanning Tree running. Now the trunk link has run both VLAN 10 & 20. We already discuss by default priority is 32678, and then new priority would be 326778 for VLAN 10 and 32688 for VLAN 20. If that so then it’s all tied and VLAN 10 only communicate with VLAN 10 and VLAN 20 only Communicated its instance. They would end up with electing same switch for Root Bridge for both VLANs. Let’s see above topology switch4 wins the Root Bridge, it’s may have the lowest mac address and block switch1 port Fa1/1. This will block upper side of the network. That would be the primary link if we left all by default. So if we tuned the priority then Switch1 wins Root for VLAN 20 and Switch3 for VLAN 10. Now we have two separate Root Bridges.


Let's see on the topology if a VLAN 10 packet wants to traverse then it use switch3 path and VLAN 20 user Switch1 path. Because Switch3 block one port for VLAN 20 and Switch1 block for VLAN 10. That means VLAN 10 traffics doesn't goes through Switch1 and VLAN 20 traffic doesn't goes through Switch3.This could load balance effectively.  

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