What is VPC peering? Is it transitivie ?

 Excellent — this question touches on the core mechanics of VPC Peering and how AWS handles inter-VPC connectivity. Let’s unpack it clearly and precisely.

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🔹 1. What is VPC Peering?

VPC Peering is a networking connection between two Amazon Virtual Private Cloud (VPCs) that enables routing traffic between them using private IP addresses, as if they were part of the same network.

It’s a point-to-point connection that works over the Amazon network backbone, not the public internet.

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🔹 2. Can VPC Peering connect VPCs in different accounts?

✅ Yes, absolutely.
VPC Peering can connect:

• Two VPCs in the same AWS account

• Two VPCs in different AWS accounts

• Even VPCs in different AWS Regions (called inter-region VPC peering)

Requirements:

• Both VPC owners must accept the peering request.

• Proper route tables, security groups, and network ACLs must allow traffic.

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🔹 3. Are there IP range restrictions?

Yes — this is critical.

❌ Overlapping CIDR blocks are not allowed.

• The IP address ranges (CIDR blocks) of the two VPCs must not overlap.

• Example:

  • VPC-A: 10.0.0.0/16

  • VPC-B: 10.1.0.0/16 ✅ OK

  • VPC-B: 10.0.1.0/24 ❌ Overlaps, not allowed

AWS enforces this restriction because overlapping ranges would break routing determinism.

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🔹 4. Is VPC Peering transitive?

🚫 No, VPC Peering is not transitive.

That means:

• If VPC-A is peered with VPC-B

• and VPC-B is peered with VPC-C

Then:

A cannot automatically communicate with C.

You would need to create a separate peering connection between A and C if you want traffic to flow between them.

🧠 Example:

Connection	Works?
A ↔ B	✅
B ↔ C	✅
A ↔ C (via B)	❌ Not transitive
A ↔ C (direct peering)	✅ Works

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🔹 5. Routing Behavior

Once peering is established:

• You must manually update route tables in each VPC to send traffic through the peering connection.

• No implicit routing is created by AWS.

Example:

VPC-A Route Table:
Destination: 10.1.0.0/16 → Target: pcx-xxxxxxx

VPC-B Route Table:
Destination: 10.0.0.0/16 → Target: pcx-xxxxxxx

Where pcx-xxxxxxx is your VPC peering connection ID.

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🔹 6. When to Use Other Options

For large or multi-VPC environments, peering doesn’t scale well — because it’s a mesh model (N×(N-1)/2 connections needed).

👉 In those cases, AWS recommends:

• AWS Transit Gateway (TGW):

  • Supports transitive routing

  • Can connect hundreds of VPCs and on-prem networks

  • Simplifies management and routing

• AWS PrivateLink:

  • For service-specific private connectivity (not full mesh routing)

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🔹 7. Summary Table

Feature	Description
Cross-account support	✅ Yes
Cross-region support	✅ Yes (inter-region peering)
Overlapping CIDRs	❌ Not allowed
Transitive routing	❌ Not supported
Bandwidth cost	Same-region: data transfer cost per GB
Routing	Must be added manually in both route tables
Scalability	Limited — point-to-point only
Alternative	Use AWS Transit Gateway for transitive multi-VPC routing

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✅ Example Summary Diagram

   [VPC-A 10.0.0.0/16]
          |
          | Peering Connection (pcx-1a2b3c)
          |
   [VPC-B 10.1.0.0/16]
          |
          | Peering Connection (pcx-4d5e6f)
          |
   [VPC-C 10.2.0.0/16]

➡ A ↔ B ✅
➡ B ↔ C ✅
➡ A ↔ C ❌ (Needs direct peering or TGW)

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Would you like me to show a Terraform or AWS CLI example of setting up cross-account VPC peering and route configuration? It can help you see exactly how to implement this end-to-end.