VPC Peering Connection Usage Examples

A VPC peering connection is a networking connection between two VPCs in the same region and enables them to communicate. Table 1 lists different scenarios of using VPC peering connections.

**Table 1** VPC peering connection usage examples
Location	CIDR Block	Description	Usage Example
VPCs in the same region	VPC CIDR blocks do not overlap. Subnet CIDR blocks of VPCs do not overlap.	You can create VPC peering connections to connect entire CIDR blocks of VPCs. Then, all resources in the VPCs can communicate with each other.	Peering Two or More VPCs Peering One Central VPC with Multiple VPCs
VPCs in the same region	VPC CIDR blocks overlap. Some subnet CIDR blocks overlap.	You can create VPC peering connections to connect specific subnets or ECSs from different VPCs. To connect specific subnets from two VPCs, the subnet CIDR blocks cannot overlap. To connect specific ECSs from two VPCs, each ECS must have a unique private IP address.	Peering Two VPCs with Overlapping CIDR Blocks
VPCs in the same region	VPC CIDR blocks overlap. Some subnet CIDR blocks overlap.		Peering ECSs in a Central VPC with ECSs in Two Other VPCs
VPCs in the same region	VPC CIDR blocks overlap. All subnet CIDR blocks overlap.	VPC peering connections are not usable.	Invalid VPC Peering Connections

Peering Two or More VPCs

Two VPCs peered together: Figure 1 shows the networking diagram of a VPC peering connection that connects VPC-A and VPC-B.

Figure 1 Networking diagram (IPv4)

**Table 2** Peering relationships (IPv4)
Peering Relationship	Peering Connection Name	Local VPC	Peer VPC
VPC-A is peered with VPC-B.	Peering-AB	VPC-A	VPC-B

**Table 3** VPC route tables (IPv4)
Route Table	Destination	Next Hop	Route Type	Description
rtb-VPC-A	10.0.0.0/16	Peering-AB	Custom	Add a route with the CIDR block of VPC-B as the destination and Peering-AB as the next hop.
rtb-VPC-B	172.16.0.0/16	Peering-AB	Custom	Add a route with the CIDR block of VPC-A as the destination and Peering-AB as the next hop.

Multiple VPCs peered together: Figure 2 shows the networking diagram of VPC peering connections that connect VPC-A, VPC-B, and VPC-C.

Figure 2 Networking diagram (IPv4)

**Table 4** Peering relationships (IPv4)
Peering Relationship	Peering Connection Name	Local VPC	Peer VPC
VPC-A is peered with VPC-B.	Peering-AB	VPC-A	VPC-B
VPC-A is peered with VPC-C.	Peering-AC	VPC-A	VPC-C
VPC-B is peered with VPC-C.	Peering-BC	VPC-B	VPC-C

**Table 5** VPC route tables (IPv4)
Route Table	Destination	Next Hop	Route Type	Description
rtb-VPC-A	10.0.0.0/16	Peering-AB	Custom	Add a route with the CIDR block of VPC-B as the destination and Peering-AB as the next hop.
rtb-VPC-A	192.168.0.0/16	Peering-AC	Custom	Add a route with the CIDR block of VPC-C as the destination and Peering-AC as the next hop.
rtb-VPC-B	172.16.0.0/16	Peering-AB	Custom	Add a route with the CIDR block of VPC-A as the destination and Peering-AB as the next hop.
rtb-VPC-B	192.168.0.0/16	Peering-BC	Custom	Add a route with the CIDR block of VPC-C as the destination and Peering-BC as the next hop.
rtb-VPC-C	172.16.0.0/16	Peering-AC	Custom	Add a route with the CIDR block of VPC-A as the destination and Peering-AC as the next hop.
rtb-VPC-C	10.0.0.0/16	Peering-BC	Custom	Add a route with the CIDR block of VPC-B as the destination and Peering-BC as the next hop.

Peering One Central VPC with Multiple VPCs

Figure 3 shows the networking diagram of VPC peering connections that connect VPC-B, VPC-C, VPC-D, VPC-E, VPC-F, VPC-G, and central VPC-A.

Figure 3 Networking diagram (IPv4)

**Table 6** Peering relationships (IPv4)
Peering Relationship	Peering Connection Name	Local VPC	Peer VPC
VPC-A is peered with VPC-B.	Peering-AB	VPC-A	VPC-B
VPC-A is peered with VPC-C.	Peering-AC	VPC-A	VPC-C
VPC-A is peered with VPC-D.	Peering-AD	VPC-A	VPC-D
VPC-A is peered with VPC-E.	Peering-AE	VPC-A	VPC-E
VPC-A is peered with VPC-F.	Peering-AF	VPC-A	VPC-F
VPC-A is peered with VPC-G.	Peering-AG	VPC-A	VPC-G

**Table 7** VPC route table details (IPv4)
Route Table	Destination	Next Hop	Route Type	Description
rtb-VPC-A	10.0.0.0/16	Peering-AB	Custom	Add a route with the CIDR block of VPC-B as the destination and Peering-AB as the next hop.
	192.168.0.0/16	Peering-AC	Custom	Add a route with the CIDR block of VPC-C as the destination and Peering-AC as the next hop.
	10.2.0.0/16	Peering-AD	Custom	Add a route with the CIDR block of VPC-D as the destination and Peering-AD as the next hop.
	10.3.0.0/16	Peering-AE	Custom	Add a route with the CIDR block of VPC-E as the destination and Peering-AE as the next hop.
	172.17.0.0/16	Peering-AF	Custom	Add a route with the CIDR block of VPC-F as the destination and Peering-AF as the next hop.
	10.4.0.0/16	Peering-AG	Custom	Add a route with the CIDR block of VPC-G as the destination and Peering-AG as the next hop.
rtb-VPC-B	172.16.0.0/16	Peering-AB	Custom	Add a route with the CIDR block of VPC-A as the destination and Peering-AB as the next hop.
rtb-VPC-C	172.16.0.0/16	Peering-AC	Custom	Add a route with the CIDR block of VPC-A as the destination and Peering-AC as the next hop.
rtb-VPC-D	172.16.0.0/16	Peering-AD	Custom	Add a route with the CIDR block of VPC-A as the destination and Peering-AD as the next hop.
rtb-VPC-E	172.16.0.0/16	Peering-AE	Custom	Add a route with the CIDR block of VPC-A as the destination and Peering-AE as the next hop.
rtb-VPC-F	172.16.0.0/16	Peering-AF	Custom	Add a route with the CIDR block of VPC-A as the destination and Peering-AF as the next hop.
rtb-VPC-G	172.16.0.0/16	Peering-AG	Custom	Add a route with the CIDR block of VPC-A as the destination and Peering-AG as the next hop.

Peering Two VPCs with Overlapping CIDR Blocks

As shown in Figure 4, VPC-A and VPC-B have overlapping CIDR blocks, and their Subnet-A01 and Subnet-B01 also have overlapping CIDR blocks. In this case, a VPC peering connection can connect their Subnet-A02 and Subnet-B02 that do not overlap with each other.

Figure 4 Networking diagram (IPv4)

**Table 8** Peering relationships (IPv4)
Peering Relationship	Peering Connection Name	Local VPC	Peer VPC
VPC-A is peered with VPC-B.	Peering-AB	VPC-A	VPC-B

**Table 9** VPC route table details (IPv4)
Route Table	Destination	Next Hop	Route Type	Description
rtb-VPC-A	10.0.2.0/24	Peering-AB	Custom	Add a route with the CIDR block of Subnet-B02 as the destination and Peering-AB as the next hop.
rtb-VPC-B	10.0.1.0/24	Peering-AB	Custom	Add a route with the CIDR block of Subnet-A02 as the destination and Peering-AB as the next hop.

Peering ECSs in a Central VPC with ECSs in Two Other VPCs

As shown in Figure 5, VPC-B and VPC-C have overlapping CIDR blocks, and their Subnet-B01 and Subnet-C01 have overlapping CIDR blocks. You can only create a VPC peering connection between ECSs.

Use VPC peering connection Peering-AB to connect ECSs in Subnet-B01 and Subnet-A01.
Use VPC peering connection Peering-AC to connect ECSs in Subnet-C01 and Subnet-A01.

Figure 5 Networking diagram (IPv4)

**Table 10** Peering relationships (IPv4)
Peering Relationship	Peering Connection Name	Local VPC	Peer VPC
ECS-A01-1 in VPC-A is peered with ECS-B01 in VPC-B.	Peering-AB	VPC-A	VPC-B
ECS-A01-2 in VPC-A is peered with ECS-C01 in VPC-C.	Peering-AC	VPC-A	VPC-C

**Table 11** VPC route table details (IPv4)
Route Table	Destination	Next Hop	Route Type	Description
rtb-VPC-A	10.0.0.139/32	Peering-AB	Custom	Add a route with the private IP address of ECS-B01 as the destination and Peering-AB as the next hop.
rtb-VPC-A	10.0.0.71/32	Peering-AC	Custom	Add a route with the private IP address of ECS-C01 as the destination and Peering-AC as the next hop.
rtb-VPC-B	172.16.0.111/32	Peering-AB	Custom	Add a route with the private IP address of ECS-A01-1 as the destination and Peering-AB as the next hop.
rtb-VPC-C	172.16.0.218/32	Peering-AC	Custom	Add a route with the private IP address of ECS-A01-2 as the destination and Peering-AC as the next hop.

Invalid VPC Peering Connections

If VPCs with the same CIDR block also include subnets that overlap, VPC peering connections are not usable. VPC-A and VPC-B have the same CIDR block and their subnets have the same CIDR block. If a VPC peering connection is created between VPC-A and VPC-B, traffic cannot be routed between them because there are routes with the same destination.

In the rtb-VPC-A route table, the custom route for routing traffic from VPC-A to VPC-B and the local route have overlapping destinations. The local route has a higher priority and traffic will be forwarded within VPC-A and cannot reach VPC-B.

Figure 6 Networking diagram (IPv4)

**Table 12** VPC route table details
Route Table	Destination	Next Hop	Route Type	Description
rtb-VPC-A	10.0.0.0/24	Local	System	Local routes are automatically added for communications within a VPC.
	10.0.1.0/24	Local	System
	10.0.0.0/16 (VPC-B)	Peering-AB	Custom	Add a route with the CIDR block of VPC-B as the destination and Peering-AB as the next hop.
rtb-VPC-B	10.0.0.0/24	Local	System	Local routes are automatically added for communications within a VPC.
	10.0.1.0/24	Local	System
	10.0.0.0/16 (VPC-A)	Peering-AB	Custom	Add a route with the CIDR block of VPC-A as the destination and Peering-AB as the next hop.