By default, when an Enterprise Edition transit router learns the same destination CIDR block from virtual border routers (VBRs) in different regions, it selects a primary route based on a priority chain ending with alphabetical region ID ordering. Multi-region equal-cost multi-path (ECMP) routing removes the region ID tiebreaker: routes that match on all other attributes share traffic as equal-cost routes.
Enable multi-region ECMP routing when you need active/active load balancing across Express Connect circuits in different regions. Leave it disabled for active/passive failover, where one circuit carries traffic and others stand by.
Supported scope
Only route tables of Enterprise Edition transit routers support multi-region ECMP routing for VBRs.
How it works
Default route selection
When a transit router learns the same destination CIDR block from multiple VBRs, it compares route attributes in priority order. The route with the highest-priority attribute value becomes the primary route; all others become standby routes that carry no traffic unless the primary route fails.
Attribute priorities in descending order: P1 > P2 > P3 > P4.
| Priority | Attribute | Values (highest to lowest) |
|---|---|---|
| P1 | VBR route type | BGP route > static route. Among BGP routes, the shortest AS path wins. |
| P2 | VBR route region | Local region > other regions. A VBR route in the same region as the route table takes priority over routes from other regions. |
| P3 | Routing policy value | The route with the highest routing policy value priority wins. For more information, see Routing policy overview. |
| P4 | Route region ID | Region IDs sorted alphabetically — the first ID alphabetically wins. For example, cn-hangzhou ranks higher than cn-shenzhen. |
New routes learned by the transit router cannot overlap with routes already in the route table.
Route selection with multi-region ECMP enabled
When multi-region ECMP is enabled, the transit router compares only P1 (VBR route type), P2 (VBR route region), and P3 (routing policy value). If all three match, the comparison stops — P4 (route region ID) is skipped — and those routes become equal-cost routes that share traffic.
When does ECMP take effect? All three of the following must be true:
All routes have the same VBR route type (for example, all BGP with the same AS path length).
All routes have the same VBR route region relative to the route table (all local, or all from other regions).
All routes have the same routing policy value, or no routing policy is configured.
Static routes have a detection gap: if one of multiple static equal-cost VBR routes fails, the transit router cannot detect the failure and continues forwarding traffic as before. This may affect your services. Use BGP routes where possible. After you enable multi-region ECMP, traffic is distributed evenly across transit routers in different regions that advertise ECMP routes. Make sure you allocate enough bandwidth for inter-region connections between the local and peer transit routers.
Example
An enterprise uses two Express Connect circuits connected to VBR1 and VBR2 to connect its data center to CEN, which allows the data center to communicate with VPCs in the China (Shenzhen) region. The transit router in China (Shenzhen) learns CIDR blocks 10.0.0.0/24 and 10.0.1.0/24 from the data center via Inter-region Connection 1 and Inter-region Connection 2. No routing policy is configured for the route tables of the transit routers in the China (Shenzhen), China (Hangzhou), or China (Shanghai) region.
Figure 1. Figure 1
Before ECMP is enabled — the transit router compares attributes for the routes pointing to 10.0.0.0/24:
| Priority | Attribute | Result |
|---|---|---|
| P1 | VBR route type | Both routes are BGP with the same AS path length. Tie — compare next attribute. |
| P2 | VBR route region | Both routes come from other regions (Hangzhou and Shanghai are both different from Shenzhen). Tie — compare next attribute. |
| P3 | Routing policy value | No routing policy configured. Tie — compare next attribute. |
| P4 | Route region ID | cn-hangzhou ranks before cn-shanghai alphabetically. The route via Inter-region Connection 1 (China (Hangzhou)) is selected as the primary route. |
The route from China (Shanghai) becomes a standby route.
Figure 2. Figure 2
After ECMP is enabled — the transit router compares the same attributes:
| Priority | Attribute | Result |
|---|---|---|
| P1 | VBR route type | Tie (both BGP, same AS path length). Compare next attribute. |
| P2 | VBR route region | Tie (both from other regions). Compare next attribute. |
| P3 | Routing policy value | Tie (no policy configured). No more attributes to compare — ECMP applies. |
Both routes for 10.0.0.0/24 become equal-cost routes. Traffic is distributed across Inter-region Connection 1 and Inter-region Connection 2.
Potential impacts
| Scenario | Effect |
|---|---|
| ECMP enabled | Routes with the same P1/P2/P3 values share traffic. Network latency and maximum bandwidth of inter-region connections may be affected. |
| ECMP disabled | The transit router falls back to comparing P4 (route region ID). One route becomes primary; others become standby and only carry traffic if the primary fails. Network latency and maximum bandwidth of inter-region connections may be affected. |
Enable multi-region ECMP routing
Review Potential impacts before enabling multi-region ECMP routing.
Enable ECMP when creating a route table
Log on to the CEN console.
On the Instances page, click the ID of the CEN instance you want to manage.
Go to Basic Information > Transit Router and click the ID of the transit router.
On the transit router details page, click the Route Table tab.
In the left-side section, click Create Route Table. In the Create Route Table dialog box, configure the parameters and click OK.
Parameter Description Transit Router Select the transit router for which you want to create a route table. The transit router ID is displayed automatically. Route Table Name Enter a name for the route table. Description Enter a description. Tag Add tags to the route table. A tag consists of a Tag Key and a Tag Value. Tag Key: up to 64 characters; cannot be blank or start with acs:oraliyun; cannot containhttp://orhttps://. Tag Value: up to 128 characters; can be blank; cannot start withacs:oraliyun; cannot containhttp://orhttps://. For more information, see Manage tags.Multi-region ECMP Routing Toggle on to enable multi-region ECMP routing. Disabled by default. When enabled, routes from multiple VBRs that match on all attributes except region ID are treated as equal-cost routes and share traffic. When disabled, the transit router selects the primary route by region ID in alphabetical order. WarningIf you use static VBR routes, read the warning above before enabling this option.
Enable ECMP on an existing route table
Log on to the CEN console.
On the Instances page, click the ID of the CEN instance you want to manage.
Go to Basic Information > Transit Router and click the ID of the transit router.
On the transit router details page, click the Route Table tab.
In the left-side navigation pane, click the ID of the route table.
In the Basic Information section, turn on Multi-region ECMP Routing.
In the Do you want to enable multi-region ECMP routing message, read the potential risks and click OK.
Disable multi-region ECMP routing
Review Potential impacts before disabling multi-region ECMP routing.
Log on to the CEN console.
On the Instances page, click the ID of the CEN instance you want to manage.
Go to Basic Information > Transit Router and click the ID of the transit router.
On the transit router details page, click the Route Table tab.
In the left-side navigation pane, click the ID of the route table.
In the Basic Information section, turn off Multi-region ECMP Routing.
In the Are you sure that you want to disable multi-region ECMP routing message, read the potential risks and click OK.