Concept of vPC in ACI

Concept of vPC in ACI

In Cisco ACI, a Virtual Port Channel (vPC) enables two separate leaf switches to present a unified port channel to a connected endpoint—such as a server, firewall, or another switch that supports link aggregation protocols like LACP.

In this setup, two ACI leaf nodes (e.g., Leaf201 and Leaf202) act as vPC peers, forming a logical construct known as a vPC domain. One of these peers is elected as the primary, while the other assumes the secondary role.

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ACI’s MCT-Based Architecture

Unlike traditional vPC implementations that rely on a dedicated peer-link, ACI leverages the fabric itself to manage synchronization and control-plane communication. This architecture is referred to as Multichassis EtherChannel Trunk (MCT).

🔧 Key Characteristics:

• No physical peer-link is required between Leaf201 and Leaf202.
• Instead, the ACI fabric handles all peer communication and synchronization.
• ZMQ (Zero Message Queue) replaces traditional CFS (Cisco Fabric Services) for messaging between vPC peers.

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How Peer Communication Works in ACI

• ZMQ, a high-performance messaging library using TCP, is embedded as libzmq on each switch.
• Applications that require peer communication (like the vPC manager) use this library to exchange messages.

🔄 Peer Reachability Mechanism:

• The vPC manager subscribes to routing updates via URIB.
• When IS-IS discovers a route to the peer (e.g., Leaf202 sees Leaf201), URIB notifies the vPC manager.
• The manager then attempts to establish a ZMQ socket with the peer.
• If the route is withdrawn (e.g., due to link failure), the vPC manager is notified and the MCT link is brought down accordingly.

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Upgrade Best Practices with vPC

To ensure high availability during fabric upgrades, it's recommended to divide switches into at least two upgrade groups. For example:

• Group A: Leaf201, Leaf203, Spine101
• Group B: Leaf202, Leaf204, Spine102

This strategy ensures that at least one vPC peer remains active during the upgrade, preventing service disruption for connected endpoints.

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Glossary

Term	Description
ACI	Application Centric Infrastructure
vPC	Virtual Port Channel
MCT	Multichassis EtherChannel Trunk
ZMQ	Zero Message Queue
URIB	Unicast Routing Information Base
IS-IS	Intermediate System to Intermediate System
LACP	Link Aggregation Control Protocol

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 VPC Design Options:-

Option 1 -VPC with SAME Leaf interfaces across two leafs with Combined Profiles

Option 2 -  VPC with SAME Leaf interfaces across two leafs with Individual Profiles.

Option 3 -  VPC with DIFFERENT Leaf interfaces across two leafs with Individual Profiles