另外，vpc role （primary，secondary） 是有配置的role决定，role 值越低越优；如果role值相同，比较system mac，越低越优。
Building a vPC Domain
A vPC domain defines the grouping of switches participating in the vPC. As of today, only two Cisco NEXUS 7000 Series Switches can form a vPC domain.
From a configuration standpoint, vPC domain provides context to define global vPC system parameters.
User enters into vPC domain sub-commands to configure vPC options and features like peer-gateway, peer-swtich and so on.
The process of building a vPC domain involves multiple steps that should be completed in the following order:
1. Globally configure a vPC domain identifier on both vPC devices. The domain ID must be the same on both peer devices.2. Configure vPC peer-keepalive link on both peer devices and ensure that the vPC peer-keepalive link is operational. If not, vPC domain cannot successfully be formed.
3. Configure or reuse an ISL (Inter Switch Link) L2 trunk port-channel between the vPC peer devices. Configure the port-channel as a vPC peer-link on both peer devices and ensure that the port-channel is operational.
4. Configure or reuse port-channels from the access devices to Cisco Nexus 7000 Series forming vPC domain. Then configure a unique logical vPC and join the port-channels across different vPC peer devices.
There are two defined vPC roles: primary and secondary.
vPC role defines which of the two vPC peer devices processes Bridge Protocol Data Units (BPDUs) and responds to Address Resolution Protocol (ARP).
Use role priority command (under vPC domain configuration context) to force vPC role to primary for a dedicated peer device.
ranges from 1 to 65535 and the lowest value will dictate the primary peer device.
In case of tie (same role priority value defined on both peer devices), lowest system mac will dictate the primary peer device.