Difference between “Treat as Virtual IP Address” and “Make this IP Address Primary” in Cisco ACI

 

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🧠 Cisco ACI Demystified: “Treat as Virtual IP Address” vs “Make this IP Address Primary”

In the world of Cisco ACI, Bridge Domains (BDs) are the backbone of Layer 2 networking. But when configuring subnets within a BD, two deceptively similar options often confuse engineers:

• ✅ Make this IP Address Primary
• 🌐 Treat as Virtual IP Address

Let’s break down what each of these means, when to use them, and how they impact your ACI fabric.

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🔹 What is “Make this IP Address Primary”?

This option is used to define the default gateway for endpoints within the Bridge Domain.

✅ Key Characteristics:

• Only one primary IP per BD.
• Used for routing traffic between subnets or to external networks.
• Responds to ARP requests from endpoints.
• Can be advertised externally if route advertisement is enabled.

📌 When to Use:

• In single-site ACI deployments.
• When you want the fabric to act as the default gateway for endpoints.
• For standard BD configurations where no multi-site or stretched fabric is involved.

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🔹 What is “Treat as Virtual IP Address”?

This option is designed for multi-site or stretched fabric deployments where you want a consistent gateway IP and MAC address across multiple locations.

🌐 Key Characteristics:

• Requires a Virtual MAC address.
• Enables Common Pervasive Gateway (CPG) functionality.
• Ensures seamless endpoint mobility across sites.
• Can coexist with a primary IP in the same BD.

📌 When to Use:

• In multi-pod or multi-site ACI environments.
• When you need Layer 3 gateway consistency across data centers.
• For active-active data center designs.

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🔁 Side-by-Side Comparison

Feature	Make this IP Primary	Treat as Virtual IP Address
Default Gateway Role	✅ Yes	✅ Yes (in multi-site)
Number per BD	One	Multiple (with virtual MAC)
Requires Virtual MAC	❌ No	✅ Yes
Use Case	Single-site routing	Multi-site gateway consistency
Supports Endpoint Mobility	❌ Limited	✅ Seamless
Route Advertisement	✅ Yes (if enabled)	✅ Yes (if enabled)

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🧪 Real-World Example

Imagine you have two data centers—DC1 and DC2—connected via ACI Multi-Site. You want VMs to move between them without changing their default gateway.

• You’d configure the same subnet in both sites.
• Use “Treat as Virtual IP Address” with a shared virtual MAC.
• This ensures the gateway IP and MAC remain consistent, avoiding disruptions.

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🧩 Final Thoughts

Both options serve critical but distinct purposes. Choosing the right one depends on your ACI topology and traffic flow requirements. For most single-site deployments, “Make this IP Address Primary” is sufficient. But for advanced, distributed environments, “Treat as Virtual IP Address” is your go-to for seamless mobility and high availability.