Spanning tree in VPC

[Rishav Garg]

Hi All,

I have a topology like two vpc peer connected to down catalyst switch 3750 with VPC 51. My left switch is primary in VPC and other is secondary.

So acc. to Theory only primary switch would generate BPDU not secondary switch. But if down catalyst or Secondary switch will be root switch in Spanning tree.

Will primary switch still generate the BPDU's? 

[Suresh Vinayagam Subramanian · Accepted solution]

Hi Garg,

In VPC environment , In simple term regardless of the Spanning-tree root, VPC primay always generate BPDU and seconday device only rely that bpdu and never generate itself.

For vPC ports only the vPC primary switch runs the STP topology for those vPC ports. In other words, Spanning Tree Protocol for vPCs is controlled by the vPC primary peer device, and only this device generates then sends out Bridge Protocol Data Units (BPDUs) on Spanning Tree Protocol designated ports. This happens irrespectively of where the designated Spanning Tree Protocol root is located. STP on the secondary vPC switch must be enabled but it doesn’t dictate vPC member port state. vPC secondary peer device proxies any received Spanning Tree Protocol BPDU messages from access switches toward the primary vPC peer device . Both vPC member ports on both peer devices always share the same STP port state (FWD state in a steady network).

HTH
Regards,
VS.Suresh.
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[Rishav Garg]

Thanks Suresh for great answer.

But if i enable spanning tree port type Network on VPC member ports. Then Secondary peer device start generating BPDU's and advertise its own bridge ID to Down host swich.

Then Can you explain how down switch react to that because in this scenario down switch recieve two different designated ID's one from Primary and other from Secondary on same port channel which is logical one port.

How it gonna handle this?