VPC Traffic Flows

In a vPC traffic flow, communication begins with ARP if the source host does not know the destination MAC address. The source host sends a broadcast ARP request.

The Nexus switch that receives the frame learns the source MAC address on the incoming interface. The ARP broadcast is forwarded across the vPC topology using hashing logic, and MAC information is synchronized between vPC peers through Cisco Fabric Services (CFS).

This ensures both switches maintain consistent MAC tables for proper forwarding.

When an ARP broadcast arrives on a Nexus switch that is part of a vPC, a hashing algorithm selects one active physical link from the vPC port-channel to forward the frame.

This ensures:

• Efficient load distribution

• Prevention of duplicate frame transmission

• Optimal utilization of available links

Hashing is a core mechanism that enables efficient vPC traffic flow.

vPC implements loop-prevention logic.

If a peer switch receives a broadcast frame over the vPC peer-link, it does not forward that frame back out through its local vPC member ports.

This prevents Layer-2 loops while still ensuring that broadcast traffic reaches the intended destination host.

Once the destination host receives the ARP request, it sends an ARP reply containing its MAC address.

The ARP reply is a unicast frame sent back to the original host. During this process, switches update their MAC address tables accordingly, completing the MAC learning process required for proper vPC traffic forwarding.

MAC address synchronization between vPC peers occurs through Cisco Fabric Services (CFS).

When a MAC address is learned on one vPC peer, the information is synchronized with the peer switch. This ensures both switches maintain consistent forwarding tables, allowing traffic to be forwarded correctly even if it enters through different paths.

(Note: MAC learning happens locally first. CFS ensures synchronization. This clarification was required.)

After ARP resolution is complete and MAC addresses are learned, traffic becomes unicast.

Unicast traffic follows the hashing algorithm of the vPC port-channel to select the outgoing physical link. Both vPC peers can actively forward traffic, providing:

• Redundancy

• Load balancing

• Loop-free Layer-2 forwarding

This active-active forwarding model is the key advantage of optimized vPC traffic flow.