If you are familiar with Nexus vPC configuration, you might have been setting different STP priority on the primary and secondary switches so the primary is always a STP root, and have that lined up with, for example, HSRP active node. With vPC+ (ie. running vPC on a pair of switches that participate in fabricpath), the two Nexus switches appear as a single logical switch to both fabricpath cloud and upstream/downstream vPC switches, so it is actually crucial to make sure the upstream/downstream vPC switches receive consistent STP root priority regardless of which path is active. In this article, we demonstrate the importance of setting identical STP priority on the vPC+ peer switches, how the switches react when a superior BPDU is received, and other implications using Cisco Nexus 5000.
