Enable multi-region VBR ECMP routing

Limitations

Multi-region VBR ECMP routing is supported only by route tables of Enterprise Edition transit routers.

Route selection rules

Default route selection rules

When an Enterprise Edition transit router route table learns multiple non-conflicting routes to the same destination from different VBR instances, it compares route attributes in descending order of priority. The route with the highest-priority attribute value becomes the primary route; others become standby routes. If routes have the same value for an attribute, the system compares the next attribute. The following table describes the supported route attributes and their priorities.

Note

The route attribute priorities are, in descending order: P1 > P2 > P3 > P4.

Attribute priority	Route attribute	Description	Value priority
P1	Route type of the VBR route	If all candidate routes originate from VBR instances, the system compares their route types. BGP route: A route learned through the Border Gateway Protocol (BGP). Static route: A manually configured route.	BGP route > Static route If multiple routes are BGP routes, the route with the shorter AS path has higher priority.
P2	Region of the VBR route	The system compares the VBR route's region with the region of the transit router's route table. Local region: The VBR route is in the same region as the route table. Other regions: The VBR route is in a different region from the route table.	Local region > Other regions A route from the same region as the route table has higher priority than a route learned from another region.
P3	Routing policy	If a routing policy is configured on the route table of the Enterprise Edition transit router and multiple routes match it, the system compares their policy values. For more information about policy values, see Routing policy (legacy, for TR only).	The system selects the route with the higher priority based on the policy values.
P4	Region ID of the route	The system compares the region IDs of the routes.	Region IDs are sorted alphabetically. The route from the region whose ID comes first alphabetically has higher priority. For example, a route from China (Hangzhou) (cn-hangzhou) has higher priority than a route from China (Shenzhen) (cn-shenzhen).

Route selection rules with ECMP

After you enable multi-region VBR ECMP routing, the system compares only the values of the following three route attributes: Route type of the VBR route, Region of the VBR route, and Routing policy. If multiple routes have identical values for these three attributes, the system does not compare the Region ID of the route. Instead, the routes form an ECMP group and forward traffic simultaneously.

Warning

If multiple VBR routes in an ECMP group are configured as static routes, the system cannot detect if one of them becomes unavailable. Traffic may continue to be sent to the unavailable route, which can disrupt your services.
With multi-region VBR ECMP routing enabled, if a route table learns routes from transit routers in different regions, it distributes traffic evenly among them. Ensure that the maximum bandwidth of the inter-region connections can handle the traffic load under ECMP.

Example

An enterprise connects its data center to CEN using two Express Connect connections (VBR1 and VBR2) to communicate with VPCs in the China (Shenzhen) region. The transit router in the China (Shenzhen) region learns the destination CIDR blocks 10.0.0.0/24 and 10.0.1.0/24 from the data center through two inter-region connections. No routing policy is configured on the route tables of the transit routers in the China (Shenzhen), China (Hangzhou), or China (Shanghai) regions.

Before multi-region VBR ECMP routing is enabled, the route table of the transit router in the China (Shenzhen) region selects a primary route for the 10.0.0.0/24 and 10.0.1.0/24 CIDR blocks by comparing route attributes.

Note

The following example describes the route selection for the 10.0.0.0/24 destination CIDR block. The process for 10.0.1.0/24 is identical.

Attribute priority	Route attribute	Details
P1	Route type of the VBR route	The routes to 10.0.0.0/24 learned through inter-region connection 1 and inter-region connection 2 are both BGP routes with the same AS path length. Because the attribute values are identical, the system compares the next attribute.
P2	Region of the VBR route	The route learned through inter-region connection 1 is from the China (Hangzhou) region. This differs from the region of the current route table, China (Shenzhen). The route learned through inter-region connection 2 is from the China (Shanghai) region. This also differs from the region of the current route table. The routes have the same priority for this attribute. The system compares the next attribute.
P3	Routing policy	No routing policy is configured for the transit routers in the China (Hangzhou) and China (Shanghai) regions. The routes have the same value for this attribute. The system compares the next attribute.
P4	Region ID of the route	The route learned through inter-region connection 1 is from the China (Hangzhou) region, with the region ID cn-hangzhou. The route learned through inter-region connection 2 is from the China (Shanghai) region, with the region ID cn-shanghai. The system finds that `cn-hangzhou` comes before `cn-shanghai` alphabetically. Therefore, the system selects the route from the China (Hangzhou) region as the primary route.

After multi-region VBR ECMP routing is enabled, the route table of the transit router in the China (Shenzhen) region selects a primary route for 10.0.0.0/24 by comparing the following attributes.

Attribute priority	Route attribute	Details
P1	Route type of the VBR route	The routes to 10.0.0.0/24 learned through inter-region connection 1 and inter-region connection 2 are both BGP routes with the same AS path length. Because the attribute values are identical, the system compares the next attribute.
P2	Region of the VBR route	The route learned through inter-region connection 1 is from the China (Hangzhou) region. This differs from the region of the current route table, China (Shenzhen). The route learned through inter-region connection 2 is from the China (Shanghai) region. This also differs from the region of the current route table. The routes have the same priority for this attribute. The system compares the next attribute.
P3	Routing policy	No routing policy is configured for the transit routers in the China (Hangzhou) and China (Shanghai) regions. The attribute values are identical. Because there are no more attributes to compare, the process ends.

After the comparison, the system determines that the routes to 10.0.0.0/24 from both inter-region connections have identical values for the preceding three attributes. Therefore, the two routes form an ECMP group and forward traffic simultaneously.

Impacts on traffic forwarding

After you enable multi-region VBR ECMP routing, routes to the same destination CIDR block from different VBR instances form an ECMP group if they have identical values for route type, route region, and routing policy (as shown in Figure 2). This affects traffic latency and the maximum bandwidth required for inter-region connections. Evaluate these impacts before enabling the feature.
After you disable multi-region VBR ECMP routing, the system reverts to selecting one primary route by comparing Route region ID values when routes have identical VBR route type, VBR route region, and Route priority policy attributes. The other routes become standby routes (as shown in Figure 1). This can affect traffic latency and the maximum bandwidth of inter-region connections. Assess the impact before disabling this feature.

Warning

Before you enable multi-region VBR ECMP routing, make sure that you understand the potential impacts. For more information, see Impacts on traffic forwarding.

Enable during route table creation

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 Basic Information > Transit Router 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 pane, click Create Route Table. In the Create Route Table dialog box, configure the parameters and click OK.

Parameter	Description
Transit Router	Select the target transit router. The ID of the selected transit router is displayed automatically.
Name	Enter a name for the custom route table.
Description	Enter a description for the custom route table.
Tag	Add a tag to the custom route table. Tag key: cannot be an empty string. The key can be up to 64 characters in length and cannot start with `aliyun` or `acs:`. It cannot contain `http://` or `https://`. Tag value: can be an empty string. The value can be up to 128 characters in length and cannot start with `aliyun` or `acs:`. It cannot contain `http://` or `https://`. You can add multiple tags to a custom route table. For more information about tags, see Tags.
Multi-region ECMP Routing	Enables multi-region ECMP routing for routes learned from virtual border routers (VBRs). This feature is disabled by default. If an Enterprise Edition transit router learns routes with the same destination CIDR block from multiple VBRs, and all other route attributes are identical, the system by default forwards traffic to the VBR in the alphabetically first region. If you enable this feature, the transit router treats these as equal-cost routes and uses equal-cost multipath (ECMP) to distribute traffic among them. For more information, see Multi-region VBR equal-cost routing. To enable this feature, click the switch. In the dialog box that appears, carefully read the risk notice and click OK.

Enable on an existing route table

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 Basic Information > Transit Router 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 pane, find the target route table and click its ID.
In the Basic Settings section, find Multi-region ECMP Routing and turn on the switch.
In the Do you want to enable multi-region ECMP routing? dialog box, carefully read the risk notice and click OK.

Disable multi-region VBR ECMP routing

Warning

Before you disable multi-region VBR ECMP routing, make sure that you understand the potential impacts. For more information, see Impacts on traffic forwarding.

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 Basic Information > Transit Router 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 pane, find the target route table and click its ID.
In the Basic Settings section, find Multi-region ECMP Routing and turn off the switch.
In the Are you sure that you want to disable multi-region ECMP routing? dialog box, carefully read the risk notice and click OK.