How Enterprise Routers Work

You can attach your network connections to an enterprise router to quickly construct diversified networks and meet various service requirements. Figure 1 shows the process of using an enterprise router, including creating an enterprise router, adding attachments to the enterprise router, and configure routes.

Currently, enterprise routers support the following attachments:

VPC attachment: Attach VPCs from the same region as that of an enterprise router to the router.
Virtual gateway attachment: Attach Direct Connect virtual gateways from the same region as that of an enterprise router to the router.
VPN gateway attachment: Attach VPN gateways from the same region as that of an enterprise router to the router.

Figure 1 Processing of using an enterprise router

Figure 2 shows how an enterprise router works. Table 2 describes the traffic flows in detail if an enterprise router is used for networking.

Figure 2 How an enterprise router works

**Table 1** Network traffic flows
No.	Route	Description
1	Request from VPC 1 to Direct Connect virtual gateway	After receiving requests from VPC 1 to the virtual gateway, enterprise router 1 searches the default route table for the route to the virtual gateway and forwards the requests through this route.
1	Response from Direct Connect virtual gateway to VPC 1	After receiving responses from the virtual gateway to VPC 1, enterprise router 1 searches the default route table for the route to VPC 1 and forwards the responses through this route.
2	Request from VPC 2 to Direct Connect virtual gateway	Enterprise router 1 cannot forward requests from VPC 2 to the virtual gateway because the custom route table of enterprise router 1 that is associated with VPC 2 does not contain the route to this virtual gateway.

**Table 2** Working principles of an enterprise router
No.	Action	Description
1	Add attachments to the enterprise router.	Attach network instances to enterprise router 1 in region A. Network instances from the same region VPC attachments: VPC 1, VPC 2, and VPC 3 Virtual gateway attachment: Virtual gateway VPN gateway attachment: VPN gateway
2	Associate the attachments with the route tables of the enterprise router. Each attachment can only be associated with one route table.	Associate VPC 1 with the default route table of enterprise router 1 and create a propagation to propagate the routes learned from VPC 1 attachment to the default route table and custom route table of enterprise router 1. Associate VPC 2 with the custom route table of enterprise router 1 and create a propagation to propagate the routes learned from VPC 2 to the custom route table. Associate VPC 3 with the custom route table of enterprise router 1, and add static routes for VPC 3 to this custom route table. Associate the Direct Connect virtual gateway with the default route table of enterprise router 1 and create a propagation to propagate the routes learned from the virtual gateway attachment to the default route table. Associate the VPN gateway with the default route table of enterprise router 1 and create a propagation to propagate the routes learned from the VPN gateway attachment to the default route table.
3	Create propagation for the attachments to propagate the routes to the enterprise router's route tables. You can create multiple propagation records for the same attachment.

Attachments

If you want to attach a network instance to an enterprise router, you need to add an attachment of a specific type to the enterprise router. The attachment type varies according to the network instance, as listed in Table 3.

**Table 3** Attachments
Attachment Type	Network Instance
VPC attachment	VPC
Virtual gateway attachment	Virtual gateway of Direct Connect
VPN gateway attachment	VPN gateway

Route Tables

Route tables are used by enterprise routers to forward packets. Route tables contain associations, propagations, and routes. Route tables are classified into custom and default route tables, as detailed in Table 4.

**Table 4** Route tables
Route Table	Description
Custom route table	You can create multiple custom route tables on an enterprise router and use different routes for flexible communication and isolation between network instances.
Default route table	If you enable Default Route Table Association and Default Route Table Propagation, the system then automatically associates and propagates new attachments with the default route table. You can specify a custom route table as the default route table. If you do not specify any route table as the default route table, the system automatically creates a default route table.

Associations

Each attachment can be associated with one route table for:

Packet forwarding: Packets from the attachment are forwarded through the routes specified in the associated route table.
Route propagation: The routes in the associated route tables are automatically propagated to the route table of the attachment.

Not all attachments can propagate routes. For details, see Table 5.

**Table 5** Associations
Attachment	Route Learning
VPC	Not supported
Virtual gateway	Supported
VPN gateway	Supported

Route Propagation

You can create a propagation for each attachment to propagate routes to one or more route tables on an enterprise router.

For VPC attachments, their CIDR blocks are propagated to the enterprise router. For other attachments, all routes are propagated to the enterprise router. For details, see Table 6.

**Table 6** Propagation
Attachment	Propagated Routes to Enterprise Router
VPC	VPC CIDR blocks
Virtual gateway	All routes
VPN gateway	All routes

Routes

Routes are used to forward packets. A route contains information such as the destination, next hop, and route type. Table 7 describes the routes of different types.

**Table 7** Routes
Route Type	Description	Attachment
Propagated routes	Propagated routes are automatically learned through propagation and cannot be modified or deleted.	VPC Virtual gateway VPN gateway
Static routes	Static routes are manually created and can be modified or deleted.	VPC