vPC NX-OS vs vPC ACI: What’s The Difference

What is a vPC (Virtual Port Channel)

A virtual port channel (vPC) allows links that are physically connected to two different ACI leaf nodes to appear as a single port channel to a third device (i.e., network switch, server, any other networking device that supports link aggregation technology).

Please refer to this article about vPC basics:

vPC as a feature didn’t change from NX-OS to ACI, but, the implementation of the components like vPC peer-link, CFS and Keep-Alive are different.

vPC Implementation in ACI

vPCs consist of two ACI leaf switches designated as vPC peer switches. Of the vPC peers, one is primary and one is secondary. The system formed by the switches is referred to as a vPC domain.
The following behavior is specific to the ACI vPC implementation:

• In NXOS mode, The vPC peer switches are connected through a link called peer link, also known as Multichassis EtherChannel trunk (MCT).

• In ACI, No dedicated peer-link between the vPC peers, instead the fabric itself serves as the MCT:
  

The following are the Key difference:

CFS → ZMQ:

• Peer Reachability protocol : ZMQ is utilized in place of CFS.
  ☑ ZMQ is an open-source high-performance messaging library that uses TCP as transport.
  ☑ This library is packaged as libzmq on the switch and linked into each application that needs to communicate with vPC peer.

Physical Peer-link → URIB:

• Peer-reachability is not handled via a physical peer-link; instead, routing triggers are used to detect peer reachability.
  ☑ The vPC Manager registers with URIB for peer route notifications.
  ☑ When ISIS discovers a route to the peer, URIB notifies vPC manager, in turn attempts to open ZMQ socket with the peer.
  ☑ When the peer route is withdrawn by ISIS, the vPC manager is again notified by URIB, and it brings the MCT link down.

Reference: Cisco