Help Center> Virtual Private Cloud> Best Practices> VPC Peering Configurations> Connecting Specific Subnets from Different VPCs
Updated on 2023-08-30 GMT+08:00
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Connecting Specific Subnets from Different VPCs

Scenarios

You can configure VPC peering connections and set the destination of the routes added to VPC route tables to a subnet CIDR block of a peer VPC. In this way, specific subnets from different VPCs can communicate with each other. Table 1 shows example scenarios.
Table 1 Scenario description

Scenario

Scenario Description

IP Address Version

Networking Configuration

Related Reference

Two VPCs peered to two subnets in a central VPC

You have a central VPC that requires access to the multiple other VPCs. The other VPCs need to be isolated from each other.

  • The central VPC has separate sets of resources in different subnets.
  • The other VPCs require access to some of the resources, but not all of them.

IPv4

Two VPCs Peered to Two Subnets in a Central VPC (IPv4)

IPv6/IPv4

Two VPCs Peered to Two Subnets in a Central VPC (IPv6/IPv4)

One central VPC peered to specific subnets in two VPCs

You have a central VPC that requires access to two other VPCs. The other VPCs need to be isolated from each other.

  • The central VPC has public resources deployed and the other VPCs require access to all resources in the central VPC.
  • Other VPCs have multiple subnets and only one in each VPC is used for accessing resources in the central VPC.

IPv4

One Central VPC Peered to Specific Subnets in Two VPCs (IPv4)

One central VPC peered to overlapping subnets from two VPCs

This scenario is similar to the preceding one. If two VPCs with overlapping subnets need to peer with the central VPC, traffic may fail to be forwarded to the required destination. To prevent this, plan the network according to this example.

IPv4

One Central VPC Peered to Overlapping Subnets from Two VPCs (IPv4)

Notes and Constraints

  • If the ECSs in VPCs connected by a VPC peering connections are in different security groups, you need to add rules to the security groups to allow access to each other. For details, Enabling ECSs in Different Security Groups to Communicate with Each Other Through an Internal Network.

    In all examples in this section, the ECSs in local and peer VPCs are in the same security group. No additional security group rule is required.

  • Each route table of a VPC can have a maximum of 200 routes. If you want to establish VPC peering connections between multiple VPCs, consider this restriction when planning networking.
  • In a VPC route table, the route priority is as follows:
    • Local route: A route that is automatically added by the system for communication within a VPC. It has a higher priority than a custom route.
    • Custom route: A route added by a user. It uses the longest prefix match rule to find a destination for packet forwarding.

Two VPCs Peered to Two Subnets in a Central VPC (IPv4)

The central VPC-A has two subnets, Subnet-A01 and Subnet-A02. The subnets are associated with different route tables. You need to create Peering-AB between Subnet-A01 and VPC-B, and Peering-AC between Subnet-A02 and VPC-C. VPC-B and VPC-C have the same CIDR block. However, there will be no route conflicts because the two subnets in VPC-A are associated with different route tables.
  • For details about resource planning, see Table 2.
  • For details about VPC peering relationships, see Table 3.
Figure 1 Networking diagram (IPv4)
Table 2 Resource planning details (IPv4)

VPC Name

VPC CIDR Block

Subnet Name

Subnet CIDR Block

VPC Route Table

ECS Name

Security Group

Private IP Address

VPC-A

172.16.0.0/16

Subnet-A01

172.16.0.0/24

rtb-VPC-A01

ECS-A01

sg-web: general-purpose web server

172.16.0.111

Subnet-A02

172.16.1.0/24

rtb-VPC-A02

ECS-A02

172.16.1.91

VPC-B

10.0.0.0/16

Subnet-B01

10.0.0.0/24

rtb-VPC-B

ECS-B01

10.0.0.139

VPC-C

10.0.0.0/16

Subnet-C01

10.0.0.0/24

rtb-VPC-C

ECS-C01

10.0.0.71

VPC-A has two route tables. Route table rtb-VPC-A01 is associated with Subnet-A01, and route table rtb-VPC-A02 is associated with Subnet-A02. The two subnets can communicate with each other.

Table 3 Peering relationships (IPv4)

Peering Relationship

Peering Connection Name

Local VPC

Peer VPC

Subnet-A01 of VPC-A is peered to VPC-B.

Peering-AB

VPC-A

VPC-B

Subnet-A02 of VPC-A is peered to VPC-C.

Peering-AC

VPC-A

VPC-C
After the VPC peering connections are created, add the following routes to the route tables of the local and peer VPCs:
Table 4 VPC route table details (IPv4)

Route Table

Destination

Next Hop

Route Type

Description

rtb-VPC-A01

172.16.0.0/24

Local

System

Local routes are automatically added for communications within a VPC.

172.16.1.0/24

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-A02

172.16.0.0/24

Local

System

Local routes are automatically added for communications within a VPC.

172.16.1.0/24

Local

System

10.0.0.0/16 (VPC-C)

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

10.0.0.0/24

Local

System

Local routes are automatically added for communications within a VPC.

172.16.0.0/24 (Subnet-A01)

Peering-AB

Custom

Add a route with the CIDR block of Subnet-A01 as the destination and Peering-AB as the next hop.

rtb-VPC-C

10.0.0.0/24

Local

System

Local routes are automatically added for communications within a VPC.

172.16.1.0/24 (Subnet-A02)

Peering-AC

Custom

Add a route with the CIDR block of Subnet-A02 as the destination and Peering-AC as the next hop.

Two VPCs Peered to Two Subnets in a Central VPC (IPv6/IPv4)

The central VPC-A has two subnets, Subnet-A01 and Subnet-A02. The subnets are associated with different route tables. You need to create Peering-AB between Subnet-A01 and VPC-B for IPv6 communication, and Peering-AC between Subnet-A02 and VPC-C for IPv4 communication. VPC-B and VPC-C have the same CIDR block. However, there will be no route conflicts because the two subnets in VPC-A are associated with different route tables.
  • For details about resource planning, see Table 5.
  • For details about VPC peering relationships, see Table 6.
Figure 2 Networking diagram (IPv6/IPv4)
Table 5 Resource planning details (IPv6/IPv4)

VPC Name

VPC CIDR Block

Subnet Name

Subnet CIDR Block

VPC Route Table

ECS Name

Security Group

Private IP Address

VPC-A

172.16.0.0/16

Subnet-A01
  • IPv4: 172.16.0.0/24
  • IPv6: 2407:c080:802:c34::/64

rtb-VPC-A01

ECS-A01

sg-web: general-purpose web server
  • IPv4: 172.16.0.111
  • IPv6: 2407:c080:802:c34:a925:f12e:cfa0:8edb

Subnet-A02

172.16.1.0/24

rtb-VPC-A02

ECS-A02

172.16.1.91

VPC-B

10.0.0.0/16

Subnet-B01
  • IPv4: 10.0.0.0/24
  • IPv6: 2407:c080:802:c35::/64

rtb-VPC-B

ECS-B01
  • IPv4: 10.0.0.139
  • IPv6: 2407:c080:802:c35:493:33f4:4531:5162

VPC-C

10.0.0.0/16

Subnet-C01

10.0.0.0/24

rtb-VPC-C

ECS-C01

10.0.0.71

VPC-A has two route tables. Route table rtb-VPC-A01 is associated with Subnet-A01, and route table rtb-VPC-A02 is associated with Subnet-A02. The two subnets can communicate with each other.

Table 6 Peering relationships (IPv6/IPv4)

Peering Relationship

Peering Connection Name

Local VPC

Peer VPC

Subnet-A01 of VPC-A is peered to VPC-B. (IPv6)

Peering-AB

VPC-A

VPC-B

Subnet-A02 of VPC-A is peered to VPC-C. (IPv4)

Peering-AC

VPC-A

VPC-C
After the VPC peering connections are created, add the following routes to the route tables of the local and peer VPCs:
Table 7 VPC route table details (IPv6/IPv4)

Route Table

Destination

Next Hop

Route Type

Description

rtb-VPC-A01

172.16.0.0/24

Local

System

Local routes are automatically added for communications within a VPC.

2407:c080:802:c34::/64

Local

System

172.16.1.0/24

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 for IPv4 communication.

2407:c080:802:c35::/64 (Subnet-B01)

Peering-AB

Custom

Add a route with the IPv6 CIDR block of Subnet-B01 as the destination and Peering-AB as the next hop for IPv6 communication.

rtb-VPC-A02

172.16.0.0/24

Local

System

Local routes are automatically added for communications within a VPC.

2407:c080:802:c34::/64

Local

System

172.16.1.0/24

Local

System

10.0.0.0/16 (VPC-C)

Peering-AC

Custom

Add a route with the CIDR block of VPC-C as the destination and Peering-AC as the next hop for IPv4 communication.

rtb-VPC-B

10.0.0.0/24

Local

System

Local routes are automatically added for communications within a VPC.

2407:c080:802:c35::/64

Local

System

172.16.0.0/24 (Subnet-A01)

Peering-AB

Custom

Add a route with the CIDR block of Subnet-A01 as the destination and Peering-AB as the next hop for IPv4 communication.

2407:c080:802:c34::/64 (Subnet-A01)

Peering-AB

Custom

Add a route with the IPv6 CIDR block of Subnet-A01 as the destination and Peering-AB as the next hop for IPv6 communication.

rtb-VPC-C

10.0.0.0/24

Local

System

Local routes are automatically added for communications within a VPC.

172.16.1.0/24 (Subnet-A02)

Peering-AC

Custom

Add a route with the CIDR block of Subnet-A02 as the destination and Peering-AC as the next hop for IPv4 communication.

One Central VPC Peered to Specific Subnets in Two VPCs (IPv4)

You need to create Peering-AB between central VPC-A and Subnet-B01 in VPC-B, and Peering-AC between central VPC-A and Subnet-C02 in VPC-C. VPC-B and VPC-C have the same CIDR block, but the CIDR blocks of Subnet-B01 and Subnet-C02 do not overlap. Therefore, there will be no route conflicts.
  • For details about resource planning, see Table 8.
  • For details about VPC peering relationships, see Table 9.
Figure 3 Networking diagram (IPv4)
Table 8 Resource planning details (IPv4)

VPC Name

VPC CIDR Block

Subnet Name

Subnet CIDR Block

VPC Route Table

ECS Name

Security Group

Private IP Address

VPC-A

172.16.0.0/16

Subnet-A01

172.16.0.0/24

rtb-VPC-A

ECS-A01

sg-web: general-purpose web server

172.16.0.111

VPC-B

10.0.0.0/16

Subnet-B01

10.0.0.0/24

rtb-VPC-B

ECS-B01

10.0.0.139

Subnet-B02

10.0.1.0/24

rtb-VPC-B

ECS-B02

10.0.1.167

VPC-C

10.0.0.0/16

Subnet-C01

10.0.0.0/24

rtb-VPC-C

ECS-C01

10.0.0.71

Subnet-C02

10.0.1.0/24

rtb-VPC-C

ECS-C02

10.0.1.116
Table 9 Peering relationships (IPv4)

Peering Relationship

Peering Connection Name

Local VPC

Peer VPC

VPC-A is peered to Subnet-B01 of VPC-B.

Peering-AB

VPC-A

VPC-B

VPC-A is peered to Subnet-C02 of VPC-C.

Peering-AC

VPC-A

VPC-C
After the VPC peering connections are created, add the following routes to the route tables of the local and peer VPCs:
Table 10 VPC route table details (IPv4)

Route Table

Destination

Next Hop

Route Type

Description

rtb-VPC-A

172.16.0.0/24

Local

System

Local routes are automatically added for communications within a VPC.

10.0.0.0/24 (Subnet-B01)

Peering-AB

Custom

Add a route with the CIDR block of Subnet-B01 as the destination and Peering-AB as the next hop.

10.0.1.0/24 (Subnet-C02)

Peering-AC

Custom

Add a route with the CIDR block of Subnet-C02 as the destination and Peering-AC 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

172.16.0.0/16 (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.

rtb-VPC-C

10.0.0.0/24

Local

System

Local routes are automatically added for communications within a VPC.

10.0.1.0/24

Local

System

172.16.0.0/16 (VPC-A)

Peering-AC

Custom

Add a route with the CIDR block of VPC-A as the destination and Peering-AC as the next hop.

One Central VPC Peered to Overlapping Subnets from Two VPCs (IPv4)

If you want to create VPC peering connections between a VPC and multiple overlapping subnets from different VPCs, ensure that the destinations of the routes added for the peering connections do not conflict and traffic can be correctly forwarded.

In this example, you need to create Peering-AB between central VPC-A and Subnet-B02 in VPC-B, and Peering-AC between central VPC-A and Subnet-C02 in VPC-C. Subnet-B02 and Subnet-C02 have the same CIDR block, and ECS-B02 and ECS-C02 have the same private IP address (10.0.1.167/32).
  • For details about resource planning, see Table 11.
  • For details about VPC peering relationships, see Table 12.
Figure 4 Networking diagram (IPv4)
Table 11 Resource planning details (IPv4)

VPC Name

VPC CIDR Block

Subnet Name

Subnet CIDR Block

VPC Route Table

ECS Name

Security Group

Private IP Address

VPC-A

172.16.0.0/16

Subnet-A01

172.16.0.0/24

rtb-VPC-A

ECS-A01

sg-web: general-purpose web server

172.16.0.111

VPC-B

10.0.0.0/16

Subnet-B01

10.0.0.0/24

rtb-VPC-B

ECS-B01

10.0.0.139

Subnet-B02

10.0.1.0/24

rtb-VPC-B

ECS-B02

10.0.1.167

VPC-C

10.0.0.0/16

Subnet-C01

10.0.0.0/24

rtb-VPC-C

ECS-C01

10.0.0.71

Subnet-C02

10.0.1.0/24

rtb-VPC-C

ECS-C02

10.0.1.167
Table 12 Peering relationships (IPv4)

Peering Relationship

Peering Connection Name

Local VPC

Peer VPC

VPC-A is peered to Subnet-B02 of VPC-B.

Peering-AB

VPC-A

VPC-B

VPC-A is peered to Subnet-C02 of VPC-C.

Peering-AC

VPC-A

VPC-C
If you add routes to the route tables of the local and peer VPCs according to Table 13, the response traffic cannot be correctly forwarded. The details are as follows:
  1. ECS-B02 in Subnet-B02 of VPC-B sends request traffic to VPC-A through the route with Peering-AB as the next hop in the rtb-VPC-B route table.
  2. VPC-A receives the request traffic from ECS-B02 and expects to send the response traffic to ECS-B02. The rtb-VPC-A route table has the route with 10.0.1.167/32 as the destination, but its next hop is Peering-AC. The response traffic is incorrectly sent to VPC-C.
  3. ECS-C02 in Subnet-C02 of VPC-C has the same private IP address (10.0.1.167/32) as ECS-B02. The response traffic is incorrectly sent to ECS-C02, and ECS-B02 cannot receive the response traffic.
Table 13 VPC route table details (IPv4)

Route Table

Destination

Next Hop

Route Type

Description

rtb-VPC-A

172.16.0.0/24

Local

System

Local routes are automatically added for communications within a VPC.

10.0.1.0/24 (Subnet-C02)

Peering-AC

Custom

Add a route with the CIDR block of Subnet-C02 as the destination and Peering-AC 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

172.16.0.0/16 (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.

rtb-VPC-C

10.0.0.0/24

Local

System

Local routes are automatically added for communications within a VPC.

10.0.1.0/24

Local

System

172.16.0.0/16 (VPC-A)

Peering-AC

Custom

Add a route with the CIDR block of VPC-A as the destination and Peering-AC as the next hop.
If there are overlapping subnets, configure routes as follows to prevent traffic from being incorrectly forwarded:
  • Suggestion 1: In the rtb-VPC-A route table, add a route with Peering-AB as the next hop and the private IP address of ECS-B02 (10.0.1.167/32) as the destination. The route with 10.0.1.167/32 as the destination is preferentially matched based on the longest prefix match rule to ensure that VPC-A sends the response traffic to ECS-B02. For more configurations, see Connecting Specific ECSs from Different VPCs.
    Table 14 VPC route table details

    Route Table

    Destination

    Next Hop

    Route Type

    Description

    rtb-VPC-A

    172.16.0.0/24

    Local

    System

    Local routes are automatically added for communications within a VPC.

    10.0.1.167/32 (ECS-B02)

    Peering-AB

    Custom

    Add a route with the private IP address of ECS-B02 as the destination and Peering-AB as the next hop.

    10.0.1.0/24 (Subnet-C02)

    Peering-AC

    Custom

    Add a route with the CIDR block of Subnet-C02 as the destination and Peering-AC as the next hop.
  • Suggestion 2: In the rtb-VPC-A route table, change the destination of the route with Peering-AC as the next hop from Subnet-C02 to Subnet-C01. Add a route with Peering-AB as the next hop and Subnet-B02 as the destination to ensure that VPC-A can send the response traffic to Subnet-B02 in VPC-B.
    Table 15 VPC route table details

    Route Table

    Destination

    Next Hop

    Route Type

    Description

    rtb-VPC-A

    172.16.0.0/24

    Local

    System

    Local routes are automatically added for communications within a VPC.

    10.0.1.0/24 (Subnet-B02)

    Peering-AB

    Custom

    Add a route with the CIDR block of Subnet-B02 as the destination and Peering-AB as the next hop.

    10.0.0.0/24 (Subnet-C01)

    Peering-AC

    Custom

    Add a route with the CIDR block of Subnet-C01 as the destination and Peering-AC as the next hop.