AWS Transit Gateway: Managing Overlapping VPC CIDR Routes

Select Language:

If you have two Virtual Private Clouds (VPCs) with the same CIDR range, like both being 10.0.0.0/16, and you haven’t set up a static route, your network could face problems. Specifically, the Transit Gateway won’t be able to split traffic evenly or balance the load between the two VPCs because it can’t tell the difference between the two paths. This means routing becomes unpredictable and can result in traffic not reaching its destination properly.

To fix this, you need a clear plan to tell your network how to handle traffic between VPCs with overlapping IP ranges. Here are some simple solutions:

– Use a static route that points to the preferred VPC when the traffic is headed for that destination.
– Make one of the CIDR blocks more specific by using a longer prefix (longest prefix match), guiding traffic more precisely.
– Implement Network Address Translation (NAT) to modify the IP addresses, allowing both VPCs to be reachable at the same time despite overlapping ranges.

A practical and reliable solution for overlapping CIDRs involves using Private NAT Gateways. Here’s how you can do it:

First, assign a new, unique IP address range for each VPC to handle transit traffic. For example, give VPC A an address block like 192.168.1.0/24 and VPC B 192.168.2.0/24. This step ensures there’s no overlap in the ranges used for transit traffic.

Next, deploy a Private NAT Gateway in each VPC. These gateways act like translators that convert the IP addresses during communication.

Now, when an instance in VPC A wants to communicate with VPC B, it sends the traffic to the unique address (say, 192.168.2.x). The NAT Gateway in VPC B receives this traffic and translates the destination IP back to the original overlapping IP range (like 10.0.x.x). This way, the traffic reaches the correct destination.

Finally, update your Transit Gateway route tables to prioritize the unique 192.168.x.x addresses. This setup avoids confusion caused by overlapping ranges and guarantees clear, predictable routing.

Remember: setting this up properly requires careful planning of your route tables both within your VPCs and on the Transit Gateway. Accurate configuration makes sure all traffic moves smoothly and without conflict.

For detailed instructions, you can refer here: https://docs.aws.amazon.com/whitepapers/latest/building-scalable-secure-multi-vpc-network-infrastructure/private-nat-gateway.html