When we need to connect one VPC to other VPCs that we have created inside our environment, we can use VPC peering connections. A VPC peering connection is a networking connection between two VPCs that enables you to route traffic between them privately and have the ability to connect instances in private subnets within separate VPCs to communicate directly. AWS provides the ability to create VPC peering connections between our own VPCs or with a VPC in another AWS account. The VPC peering connections can be established within or across different AWS Regions. AWS uses existing, scalable network technologies with no single point of failure to enable VPC peering. When enabling VPC peering between regions, AWS will take care of the VPN connection between the locations and maintain redundant pathways for the traffic. Since the traffic between regions will pass over the internet, standard inter-region charges apply when using VPC peering between regions. No charges to traffic within a region are applied:

When setting up a VPC peering connection, we need to be aware of the limitations of VPC peering:

• It is not possible to peer VPC with overlapping IPv4 or IPv6 CIDR blocks
• Transitive peering is not supported – for example, peering A to B and B to C does not establish a link between A and C
• Unicast reverse path forwarding in VPC peering connections is not supported
• Tagging a VPC peering connection is only applied in the account or region in which you create them

Additionally, inter-region VPC peering connections have the following limitations:

• The peer VPC's security groups cannot be referenced in security groups that are created in the other VPC
• DNS resolution of hostnames that have both public and private IPs will only resolve public IPs when queried from the peered VPC
• Communication over IPv6 is not supported
• Communication over ClassicLink for EC2-Classic instances is not supported
• Jumbo frames are not supported across the inter-region VPC peering connection