Step 1: Plan Networks and Resources

To use an enterprise router to connect VPCs in the same region, you need to:

Network Planning: Plan CIDR blocks of VPCs and subnets, and route tables of VPCs and the enterprise router.
Resource Planning: Plan the quantity, names, and parameters of cloud resources, including VPCs, ECSs, and the enterprise router.

Network Planning

Figure 1 and Table 2 show the network planning and its description for communications among VPCs in the same region.

Figure 1 Network planning for communications among VPCs in the same region

**Table 1** Network traffic flows
Scenario	Description
Request from VPC 1 to VPC 3	The route table of VPC 1 has a route with next hop set to the enterprise router to forward traffic from VPC 1 to the enterprise router. The route table of the enterprise router has a propagated route with next hop set to the VPC 3 attachment to forward traffic from the enterprise router to VPC 3.
Response from VPC 3 to VPC 1	The route table of VPC 3 has a route with next hop set to the enterprise router to forward traffic from VPC 3 to the enterprise router. The route table of the enterprise router has a propagated route with next hop set to the VPC 1 attachment to forward traffic from the enterprise router to VPC 1.

**Table 2** Network planning for communications among VPCs in the same region
Resource	Description
VPC	The CIDR blocks of the VPCs to be connected cannot overlap with each other. In this example, the CIDR blocks of the VPCs are propagated to the enterprise router route table as the destination in routes. The CIDR blocks cannot be modified and overlapping CIDR blocks may cause route conflicts. If your existing VPCs have overlapping CIDR blocks, do not use propagated routes. Instead, you need to manually add static routes to the route table of the enterprise router. The destination can be VPC subnet CIDR blocks or smaller ones. Each VPC has a default route table. Routes in the default route table can be: Local: a system route for communications between subnets in a VPC. Enterprise router: custom routes with 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 as the destinations for routing traffic from a VPC subnet to the enterprise router. See Table 3 for route details.
Enterprise router	After Default Route Table Association and Default Route Table Propagation are enabled and a VPC attachment is created, the system will automatically: Associate VPC attachments with the default route table of the enterprise router. Propagate VPC attachments with the default route table of the enterprise router. The route table automatically learns the VPC CIDR block as the destination of routes. For details, see Table 4.
ECS	The four ECSs are in different VPCs. If the ECSs are associated with different security groups, add rules to their security groups to allow access to each other.

**Table 3** VPC route table
Destination	Next Hop	Route Type
10.0.0.0/8	Enterprise router	Static route: Custom
172.16.0.0/12	Enterprise Router	Static route: Custom
192.168.0.0/16	Enterprise Router	Static route: Custom

If you enable Auto Add Routes when creating a VPC attachment, you do not need to manually add static routes to the VPC route table. Instead, the system automatically adds routes (with this enterprise router as the next hop and 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 as the destinations) to all route tables of the VPC.
If an existing route in the VPC route tables has a destination to 10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16, the routes will fail to be added. In this case, do not to enable Auto Add Routes. After the attachment is created, manually add routes.
Do not set the destination of a route (with an enterprise router as the next hop) to 0.0.0.0/0 in the VPC route table. If an ECS in the VPC has an EIP bound, the VPC route table will have a policy-based route with 0.0.0.0/0 as the destination, which has a higher priority than the route with the enterprise router as the next hop. In this case, traffic is forwarded to the EIP and cannot reach the enterprise router.

**Table 4** Enterprise router route table
Destination	Next Hop	Route Type
VPC 1 CIDR block: 192.168.0.0/16	VPC 1 attachment: er-attach-01	Propagated route
VPC 2 CIDR block: 172.16.0.0/16	VPC 2 attachment: er-attach-02	Propagated route
VPC 3 CIDR block: 10.1.0.0/16	VPC 3 attachment: er-attach-03	Propagated route
VPC 4 CIDR block: 10.2.0.0/16	VPC 4 attachment: er-attach-04	Propagated route

Resource Planning

The enterprise router, VPCs, and ECSs must be in the same region. You can select any AZ within the region.

The following resource details are only examples. You can modify them as required.

One enterprise router.

**Table 5** Enterprise router details
Enterprise Router Name	ASN	Default Route Table Association	Default Route Table Propagation	Association Route Table	Propagation Route Table	Attachment
er-test-01	64512	Enable	Enable	Default route table	Default route table	er-attach-01
						er-attach-02
						er-attach-03
						er-attach-04

Four VPCs, each with a unique CIDR block.

**Table 6** VPC details
VPC Name	VPC CIDR Block	Subnet Name	Subnet CIDR Block	Association Route Table
vpc-demo-01	192.168.0.0/16	subnet-demo-01	192.168.1.0/24	Default route table
vpc-demo-02	172.16.0.0/16	subnet-demo-02	172.16.1.0/24	Default route table
vpc-demo-03	10.1.0.0/16	subnet-demo-03	10.1.1.0/24	Default route table
vpc-demo-04	10.2.0.0/16	subnet-demo-04	10.2.1.0/24	Default route table

Four ECSs in four different VPCs.

**Table 7** ECS details
ECS	Image	VPC	Subnet	Security Group	Private IP Address
ecs-demo-01	Public image: EulerOS 2.5 6	vpc-demo-01	subnet-demo-01	sg-demo (general-purpose web server)	192.168.1.12
ecs-demo-02		vpc-demo-02	subnet-demo-02		172.16.1.189
ecs-demo-03		vpc-demo-03	subnet-demo-03		10.1.1.105
ecs-demo-04		vpc-demo-04	subnet-demo-04		10.2.1.83