The route propagation feature allows a Transit Router to automatically learn routes from network instances, reducing manual configuration errors and improving configuration efficiency.
Limitations
Route propagation is supported only by route tables of an Enterprise Edition transit router. To check the edition of a Transit Router, see View the edition of a Transit Router.
Example
As shown in the figure, two VPCs are created in the China (Hangzhou) region and connected to the same Transit Router.
With route propagation enabled, the Transit Router learns the system routes from both VPCs and adds them to its default route table.
If the connections of the two VPCs are associated with the default route table of the Transit Router, traffic from the VPCs is forwarded to the corresponding next hop based on the routes automatically learned by the Transit Router.
Route propagation from network instances
A route table of an Enterprise Edition transit router can learn routes from multiple network instance connections, and a single network instance connection can propagate its routes to multiple route tables.
After you enable route propagation from a network instance connection to a route table of an Enterprise Edition transit router, the network instance propagates the following routes to the route table by default:
Network instance connection | Corresponding network instance | Automatically propagated routes |
VPC connection | VPC | The system routes of the VPC.
|
VBR connection | virtual border router (VBR) |
To find these routes, see Add and manage routes. |
ECR connection | Express Connect Router (ECR) | Routes whose next hop is a VBR. |
VPN connection | IPsec-VPN connection | Routes from the destination and BGP route tables of the IPsec-VPN connection. |
Inter-region connection | Transit Router |
|
Impact of route propagation on your business
When you enable the route propagation feature of Cloud Enterprise Network (CEN), the Transit Router automatically learns system routes from network instances (such as VPCs) and propagates them to achieve connectivity. In most cases, enabling route propagation for CEN cross-region connections or VPC connections does not disrupt your existing VPC routes or live business traffic.
When route propagation is safe
The operation is generally safe under the following conditions:
The CIDR blocks of all connected VPCs are completely different, with no overlapping address ranges.
No conflicting static routes exist across the associated Transit Router route tables.
Under these conditions, route synchronization does not affect existing traffic flows.
Potential risks
If route conflicts exist — for example, two network instances advertise routes to the same destination CIDR block — route propagation may trigger route selection changes or cause brief traffic fluctuations. Before enabling route propagation, verify that there are no overlapping or conflicting routes across all connected network instances.
Recommendations
Perform route propagation operations during off-peak business hours to minimize potential impact on live traffic.
Plan your routing strategy in advance: review all CIDR blocks and existing route entries in the involved Transit Router route tables before making changes.
For large-scale deployments involving multiple VPCs or cross-region connections, validate routing in a non-production environment before applying changes to production.
Enable route propagation
Method 1: When creating a network instance connection
When you create a network instance connection, select Propagate system routes to transit router route table in the Advanced Settings section. This enables route propagation, allowing the Transit Router's default route table to learn routes from the network instance.
For more information, see Create a VPC connection, Create an ECR connection, Create a VBR connection, Create a VPN connection, and Create an inter-region connection.
Method 2: After creating a network instance connection
You can also enable route propagation in the following scenarios:
You did not select "Propagate System Routes to Default Route Table of Transit Router" when creating the network instance connection.
You want a custom route table in the Transit Router to learn routes from a network instance.
Follow these steps:
Log on to the CEN console.
On the CEN Instance page, click the ID of the CEN instance that you want to manage.
Go to the tab and click the ID of the transit router that you want to manage.
On the details page of the transit router, click the Route Table tab.
In the left-side section, click the ID of the route table that you want to manage. On the details page of the route table, click the Route Propagation tab.
On the Route Propagation tab, click Enable Route Propagation. In the Enable Route Propagation dialog box, configure the following parameters and click OK.
Parameter
Description
Route Table
The system selects the current route table by default.
Transit Router ID
The system selects the current Transit Router instance by default.
Attachment
Select the network instance connection from which you want the route table to learn routes.
After route propagation is enabled, the network instance automatically propagates routes to the route table. You can view these routes on the Route Entry tab.
Disable route propagation
When you disable route propagation, the routes propagated from the network instance to the route table of the Enterprise Edition transit router are automatically withdrawn.
Log on to the CEN console.
On the CEN Instance page, click the ID of the CEN instance that you want to manage.
Go to the tab and click the ID of the transit router that you want to manage.
On the details page of the transit router, click the Route Table tab.
In the left-side section, click the ID of the route table that you want to manage. On the details page of the route table, click the Route Propagation tab.
On the Route Propagation tab, find the target network instance connection and click Delete in the Actions column.
In the Disable Route Propagation dialog box, confirm the information and click OK.
FAQ
What should I do if I cannot access ECS instances or Bastionhost after configuring route propagation?
Cause
The route table associated with the target ECS instance may be missing a route entry to the source IP address (such as the IP address of Bastionhost).
Solution
Check your CEN and Transit Router configuration to confirm that the source network instance (the VPC where Bastionhost resides) has a route propagation relationship established with the Transit Router route table.
Verify that the routes from the source network instance have been correctly propagated to the Transit Router route table associated with the target ECS instance.
On the Transit Router route table details page, confirm that the propagated routes include the CIDR block or the specific IP address of the source network instance.
Establishing a route propagation relationship alone does not guarantee traffic reachability. You must also ensure one of the following:
The route table of the target VPC is associated with the Transit Router route table that contains the propagated routes.
The target VPC's route table contains the corresponding return routes with the Transit Router as the next hop.
If traffic is still blocked after completing the above steps, check the security group rules and network ACL configurations of the target ECS instance to ensure that inbound traffic from the source IP address is permitted.
Related APIs
EnableTransitRouterRouteTablePropagation: Enables route propagation.
ListTransitRouterRouteTablePropagations: Queries the route propagation settings of a transit router's route table.
DisableTransitRouterRouteTablePropagation: Disables route propagation.