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Monday, November 14, 2011

Spanning- tree priority with VPC

We are deploying a pair of Nexus 3064 switches in a VPC and they will handle all of the Layer 3 Routing and switching for a small data center. Do we need to test their spanning tree priority the same if they will be configured as a VPC? Example:Primary - 4096, secondary - 4096.

Having the same bridge priority is not a requirement to maintain vPC consistency. Even if you had the highest bridge priority in the secondary, your vPC primary would forward BPDUs in vPC domain. quoted from "Cisco NX-OS Virtual PortChannel: Fundamental Design Concepts..." "vPC by default ensures that only the primary switch forwards BPDUs on vPCs. This modification is strictly limited tovPC member ports. As a result, the BPDUs that may be received by the secondary vPC peer on a vPC port areforwarded to the primary vPC peer through the peer link for processing. Note: Non-vPC ports operate like regular spanning-tree ports. The special behavior of the primary vPC memberapplies uniquely to ports that are part of a vPC."

Please click here for the Design Guide for Cisco NX-OS
Please click here for spanning tree design guidelines

Having the same (highest) priority would also be no problem because in the non-vPC context (i.e. classic spanning tree topology) there would be a tie-breaker (lowest MAC) to elect the root.

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