An Express Connect Router (ECR) lets you control which routes are advertised to your on-premises network by specifying allowed route prefixes. When route prefixes are configured, the ECR advertises only the aggregated prefixes instead of every specific route in the route tables of associated virtual private clouds (VPCs) or transit routers (TRs).
Use cases
Route prefix aggregation is useful in three common scenarios:
Security management: A firewall at the data center egress filters inbound traffic. Advertising many specific routes from Alibaba Cloud increases the number of firewall rules. Aggregating specific routes into general routes reduces rule complexity.
Multi-cloud connectivity: A third-party cloud provider connected via an Express Connect circuit may have a BGP route acceptance limit. Aggregating specific routes lets you stay within that limit without requesting a quota increase from the provider.
Hybrid cloud with dense subnets: A VPC divided into many subnets—for example, 24 vSwitches within 192.168.1.0/24—causes the virtual border router (VBR) to advertise 24 separate CIDR blocks to the data center. Aggregating these into a single general route reduces route management overhead.
Prerequisites
Before you begin, ensure that you have:
An ECR created with at least one VPC or transit router associated. See Create and manage an ECR
Billing for outbound data transfer enabled. See Enable billing for outbound data transfer
Limits
Each ECR supports up to 20 route prefixes. To request a quota increase, see Manage Express Connect quotas.
Behavior notes
Both IPv4 and IPv6 route prefixes are supported.
When no route prefixes are configured, or after all prefixes are cleared, the ECR automatically resumes advertising specific routes to the data center.
If the ECR is advertising only one route prefix and you modify it, Alibaba Cloud temporarily resumes advertising specific routes to maintain service continuity. Once the modified prefix is active, specific routes are withdrawn again. Pay attention to the impacts on your peer networks after the specific routes are advertised.
Choose a mode
Route prefix configuration supports two modes. Select the mode based on what you want to happen to routes outside the configured prefix range.
| Mode | Effect on routes within the range | Effect on routes outside the range | When to use |
|---|---|---|---|
| Matching mode | Withdraws specific routes; advertises the configured prefixes | Specific routes are not advertised | Advertise only the configured prefixes, nothing else |
| Incremental mode | Withdraws specific routes that fall within the configured range; advertises the configured prefixes | Continues to advertise specific routes as before | Aggregate a subset of routes while keeping other specific routes advertised |
Switching between modes:
Matching to incremental: Specific routes outside the configured prefix range are re-advertised. Configured prefixes remain active.
Incremental to matching: Specific routes outside the configured prefix range are withdrawn. Configured prefixes remain active.
Add or update route prefixes
If you are associating a VPC or transit router with an ECR for the first time, configure the Allowed Route Prefixes parameter during the association step. See the Associate a VPC with an ECR and Associate a TR with an ECR sections of "Create and manage an ECR."
To add or update route prefixes after the association is established:
Log on to the Express Connect console.
In the left-side navigation pane, click Express Connect Router (ECR). Find the ECR you want to manage and click its ID.
On the ECR details page, go to the tab for the resource you want to update:
VPC tab: Find the VPC and click the
icon in the Dynamic CIDR Block Propagation column.TR tab: Find the transit router and click the
icon in the Dynamic CIDR Block Propagation column.
In the Update Prefix List dialog box, select Matching Mode or Incremental Mode, enter the allowed route prefixes, select the protocol, and click OK.
Delete route prefixes
Deleting route prefixes causes the ECR to withdraw all configured prefixes and automatically resume advertising specific routes to the data center.
Log on to the Express Connect console.
In the left-side navigation pane, click Express Connect Router (ECR). Find the ECR you want to manage and click its ID.
On the ECR details page, go to the tab for the resource you want to update:
VPC tab: Find the VPC and click the
icon in the Dynamic CIDR Block Propagation column.TR tab: Find the transit router and click the
icon in the Dynamic CIDR Block Propagation column.
In the Update Prefix List dialog box, delete the allowed route prefixes, select the protocol, and click OK.