We have 2 core cisco Nexus 9K switches and I noticed that one is the STP root for vlans while the other is the active HSRP gateway for vlans. I'm pretty confident that the primary switch should be both the HSRP gateway and the root--correct me if I'm wrong. Also, would changing the priority on the HSRP group make my standby gateway the active one immediately without much interruption or would a reboot be required.
I would make this change after hours of course, but I would like some advice from some of you Cisco gurus.
I agree with you. Usually the switch that is the primary for HSRP, should also be the root switch as well as vPC primary if you have vPC configured. This way it is so much easier to understand and manage the environment. As for changing the priority, you can give a higher priority to the backup HSRP switch, but that switch will not take over as the active HSRP unless you add preemption as well. Also, there is no need for reboot. The other option would be to change the priority on the backup switch and shut down the active HSRP SVI and bring it back online. That way the backup switch will take over but the first option is so much easier.
I tend to agree with @cmarva in a vpc environment my understanding is the vpc primary by default should be the stp root, and if you append the peer switch statement the two VPC switches would have the same STP BID thus to STP they are both seen as one logical switch.