Network and Resource Planning

To use an enterprise router and a NAT gateway to allow VPCs in the same region to share an EIP to access the Internet, you need:

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

Network Planning

Figure 1 shows the network planning for enabling VPCs in the same region to share an EIP to access the Internet.

Figure 1 Network planning for enabling VPCs in the same region to share an EIP to access the Internet

**Table 1** Network traffic flows
Path	Description
Request traffic: from VPC 1 to Internet	In the route table of VPC 1, there is a route with the next hop set to the enterprise router to forward traffic from VPC 1 to the enterprise router. In the route table of the enterprise router, there is a static route with the next hop set to the VPC 4 attachment to forward traffic from the enterprise router to VPC 4. In the route table of VPC 4, there is a route with the next hop set to the NAT gateway to forward traffic from VPC 4 to the NAT gateway. The NAT gateway forwards the traffic to a destination on the Internet through the EIP configured in an SNAT rule.
Response traffic: from Internet to VPC 1	The destination on the Internet forwards the traffic to the NAT gateway through the EIP configured in the SNAT rule. The NAT gateway forwards the traffic to VPC 4 based on the SNAT rule. In the route table of VPC 4, there is a route with the next hop set to the enterprise router to forward traffic from VPC 4 to the enterprise router. In the route table of the enterprise router, there is a propagated route with the next hop set to the VPC 1 attachment to forward traffic from the enterprise router to VPC 1.

**Table 2** Description of network planning for enabling VPCs in the same region to share an EIP to access the Internet
Resource	Description
VPCs	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. The routes in the default route table are described as follows: Local: a system route for communications between subnets in a VPC. Enterprise Router: a custom route for routing traffic from a VPC subnet to the enterprise router. In order to keep route configuration simple, VPC 1, VPC 2 and VPC 3 each have such a route with destination set to 0.0.0.0/0. For route details, see Table 3. The route table of VPC 4 has a route with NAT gateway as the next hop and 0.0.0.0/0 as the destination. To prevent conflicts with this route, set the destinations to CIDR blocks of the other three VPCs for routes with enterprise router as the next hop in this route table. For route details, see Table 4. NAT gateway: a route configured by the system for routing traffic from the VPC subnet to the NAT gateway.
NAT gateway	Create a public NAT gateway in VPC 4, and add an SNAT rule with an EIP associated.
Enterprise router	After Default Route Table Association and Default Route Table Propagation are enabled and VPC attachments are created, Enterprise Router will automatically: Associate VPC attachments with the default route table of the enterprise router. Propagate VPC attachments to the default route table of the enterprise router. The route table automatically learns the VPC CIDR blocks as the destination of routes. For details, see Table 5. In the enterprise router route table, add a static route with the destination set to 0.0.0.0/0 for routing traffic for accessing the Internet through VPC 4.
ECSs	The three ECSs are in different VPCs. If the ECSs are in different security groups, add rules to the security groups to allow access to each other.

**Table 3** Route table for VPC 1, VPC 2, and VPC 3
Destination	Next Hop	Route Type
0.0.0.0/0	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 enable Auto Add Routes. After the attachment is created, manually add routes.
You need to add a route to VPC route tables with destination set to a CIDR block on the Internet and next hop set to enterprise router.
To reduce the number of routes, you can 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. However, in this case, ECSs in VPCs cannot be bound with EIPs. 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** VPC 4 route table
Destination	Next Hop	Route Type
VPC 1 CIDR block: 10.1.0.0/16	Enterprise router	Static route (custom)
VPC 2 CIDR block: 10.2.0.0/16	Enterprise router	Static route (custom)
VPC 3 CIDR block: 10.3.0.0/16	Enterprise router	Static route (custom)
0.0.0.0/0	NAT gateway	Static route (custom)

Do not enable Auto Add Routes when creating attachments. Manually add routes to VPC route tables after the attachments are created.
Do not bind an EIP to an ECS in VPCs. If you do that, policy-based routes with destination set to 0.0.0.0/0 are added to ECS route tables. The priority of the routes is higher than that of the route with destination to the NAT gateway. As a result, the traffic will be forwarded to the EIP of the ECS instead of the NAT gateway.

**Table 5** Enterprise router route table
Destination	Next Hop	Route Type
VPC 1 CIDR block: 10.1.0.0/16	VPC 1 attachment: er-attach-business-01	Propagated
VPC 2 CIDR block: 10.2.0.0/16	VPC 2 attachment: er-attach-business-02	Propagated
VPC 3 CIDR block: 10.3.0.0/16	VPC 3 attachment: er-attach-business-03	Propagated
0.0.0.0/0	VPC 4 attachment: er-attach-nat	Static route

Resource Planning

An enterprise router, a NAT gateway, an EIP, four VPCs, and three ECSs are in the same region but can be in different AZs.

The following resource details are only examples. You can modify them if needed.

One enterprise router. See details in Table 6.

**Table 6** 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	Enabled	Enabled	Default route table	Default route table	er-attach-business-01
						er-attach-business-02
						er-attach-business-03
						er-attach-nat

One EIP. Set the EIP type and bandwidth size as required. In this practice, the EIP is 123.60.73.78.

One public NAT gateway. See details in Table 7.

**Table 7** Public NAT gateway details
Public NAT Gateway Name	VPC	Subnet	SNAT Rule Scenario	SNAT Rule CIDR Block
nat-demo	vpc-nat	subnet-nat	VPC	Custom: 0.0.0.0/0

Four VPCs that do not overlap with each other. See details in Table 8.

**Table 8** VPC details
VPC	VPC CIDR Block	Subnet	Subnet CIDR Block	Association Route Table
VPC 1: vpc-business-01	10.1.0.0/16	subnet-business-01	10.1.0.0/24	Default route table
VPC 2: vpc-business-02	10.2.0.0/16	subnet-business-02	10.2.0.0/24	Default route table
VPC 3: vpc-business-03	10.3.0.0/16	subnet-business-03	10.3.0.0/24	Default route table
VPC 4: vpc-nat	192.168.0.0/16	subnet-nat	192.168.0.0/24	Default route table

Three ECSs, respectively, in three VPCs. See details in Table 9.

**Table 9** ECS details
ECS Name	Image	VPC	Subnet	Security Group	Private IP Address
ECS 1: ecs-business-01	Public image: CentOS 7.5 64-bit	vpc-business-01	subnet-business-01	sg-demo (general-purpose web server)	10.1.0.134
ECS 2: ecs-business-02		vpc-business-02	subnet-business-02		10.2.0.215
ECS 3: ecs-business-03		vpc-business-03	subnet-business-03		10.3.0.14