借官方文档讨论一发：
vPC Upgrade and Downgrade Procedure
In vPC topologies, the two peer switches usually must be upgraded individually. An upgrade on one peer switch does not automatically update the vPC peer switch.
However, Cisco NX-OS Releases 7.0(3)F3(3c) and 7.0(3)F3(4) are not compatible with Cisco NX-OS Release 9.2(x) for vPC peer switches. Both vPC peers must be upgraded simultaneously to Cisco NX-OS Release 9.2(x) to avoid one switch running a 7.0(3)F3(x) release and the other switch running 9.2(x). Optionally, if the switches are being upgraded from Cisco NX-OS Release 7.0(3)F3(4), you can use the following procedure to minimize the traffic impact during upgrade.
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https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus9000/sw/92x/upgrade/guide/b-cisco-nexus-9000-nx-os-software-upgrade-downgrade-guide-92x/b-cisco-nexus-9000-nx-os-software-upgrade-downgrade-guide-92x_chapter_0110.html#id_74490
Peer-link先断，keepalive后断
1）VPC Secondary关闭所有VPC member port和VPC vlan SVI；
2）peer-link和keepalive均恢复之后，被关闭的端口自动恢复。
Keepalive先断，peer-link后断
1）两台设备处于双A状态（vpc peer的角度）；
2）两个VPC peer均会发送BPDU，各自为根；
3）原来的流量可正常转发；
vPC里面角色好像是不可抢占的吧？
vPC Role
Although both vPC switches appear as a single switch to a downstream device, among themselves two vPC switches have clearly defined vPC roles: vPC Primary and vPC Secondary.
vPC roles are non-preemptive, which means a device can be configured as vPC primary, but operate as vPC secondary peer device. This can happen in this scenario:
When the original primary device fails, the secondary vPC device becomes the new primary device.
When the system recovers itself, the previously primary device is now the secondary device and vice versa.
vPC role defines which of the two vPC peer devices processes Bridge Protocol Data Units (BPDUs) and responds to Address Resolution Protocol (ARP) requests. vPC role also defines a set of actions to be taken by vPC primary and vPC secondary in response to vPC peer-link down situation.
https://www.cisco.com/c/en/us/support/docs/ios-nx-os-software/nx-os-software/212589-understanding-vpc-election-process.html
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