vPC Traffic Flows
The vPC configurations are optimized to help ensure that traffic through a vPC-capable system is symmetric. In Figure 2-9, for example, the flow on the left (from Core1 to Acc1) reaching a Cisco Nexus switch (Agg1 in the figure) from the core is forwarded toward the access layer switch (Acc1 in the figure) without traversing the peer Cisco Nexus switch device (Agg2). Similarly, traffic from the server directed to the core reaches a Cisco Nexus switch (Agg1), and the receiving Cisco Nexus switch routes this traffic directly to the core without unnecessarily passing it to the peer Cisco Nexus device. This process occurs regardless of which Cisco Nexus device is the primary HSRP device for a given VLAN.

vPC Dual-Control Plane
While still operating with two separate control planes, vPC helps ensure that the neighboring devices connected in vPC mode see the vPC peers as a single spanning tree and LACP entity. For this to happen, the system has to perform IEEE 802.3ad control-plane operations in a slightly modified way (which is not noticeable to the neighbor switch). IEEE 802.3ad specifies the standard implementation of port channels. Port channel specifications provide LACP as a standard protocol, which enables negotiation of port bundling. LACP makes misconfiguration less likely, because if ports are mismatched, they will not form a port channel. Switch MAC addresses are normally used by spanning tree and LACP, respectively—the bridge ID field in the spanning tree BPDU and as part of LACP LAGID. In a single chassis, they use the systemwide MAC address for this purpose.
For systems that use vPCs, a systemwide MAC address would not work because the vPC peers need to appear as a single entity. To meet this requirement, vPC offers both an automatic configuration and a manual configuration of the system ID for the vPC peers.
The automatic solution implemented by vPC consists of generating a system ID composed of a priority and a MAC address, with the MAC derived from a reserved pool of MAC addresses combined with the domain ID specified in the vPC configuration. The domain ID is encoded in the last octet and the trailing 2 bits of the previous octet of the MAC address.
By configuring domain IDs to be different on adjacent vPC complexes (and to be identical on each vPC peer complex), you help ensure the uniqueness of the system ID for LACP negotiation purposes. You also help ensure that the spanning tree BPDUs use a MAC address that is representative of the vPC complex.