When Transit Routers (TRs) in multiple regions are connected to an Express Connect Router (ECR), you can set the Multi-Exit Discriminator (MED) value for a specific TR to Default on the ECR. This lets you select a specific path and prevent traffic from taking suboptimal routes between multiple TRs.
Basic route selection rules
The Multi-Exit Discriminator (MED) is an important route attribute in the Border Gateway Protocol (BGP). A smaller MED value indicates a higher priority. The route with the smaller MED value is preferred.
Alibaba Cloud defines three MED values: 1000 for intra-region connections between an ECR and a TR, 2000 for a TR that is manually set as the default path on an ECR, and 3000 for inter-region connections where the ECR and TR are not directly connected.
Route selection rules for outbound traffic
Outbound traffic is routed based on the routes of an Enterprise Edition transit router (TR).
When a TR learns routes to the same destination CIDR block from ECR instances and other TR instances, the system compares the value of each property in descending order of priority based on the TR route priority. The route with the higher-priority property value is selected to forward traffic. If multiple routes have the same value for a property, the system compares the value of the next property.
The route selection priorities are listed in the following table:
The route attribute priorities are, from highest to lowest: P1 > P2 > P3 > P4.
Route attribute priority | Route attribute | Attribute value description |
P1 | AS_Path | The system compares the AS_Path of routes to the same destination CIDR block. A shorter AS_Path is preferred. If the AS_Path lengths are the same, the local preference attribute is compared. |
P2 | Local preference | A next hop to an intra-region connection has a higher priority than a next hop to an inter-region connection.
|
P3 | MED | If the VPC is in a region that does not have a connection to an ECR and a TR, the MED values are compared. The route with the smaller MED value is preferred. You can set the MED value of the destination TR to Default in the ECR to change the path selection for outbound traffic. For more information, see Procedure. If routes to the same destination CIDR block have the same MED value, the lexicographical order attribute is compared. |
P4 | Byte order | The system compares the region IDs of the routes to the same destination CIDR block. The region IDs are sorted alphabetically. The route from the region ID that comes first alphabetically has a higher priority. For example, a route from the China (Beijing) (cn-beijing) region has a higher priority than a route from the China (Hangzhou) (cn-hangzhou) region. |
Route selection rules for inbound traffic
Inbound traffic is routed based on the routes of an ECR.
When an ECR learns routes to the same destination CIDR block from TR instances and other ECR instances, the system compares the attribute values of each route. The comparison follows the route priority from highest to lowest. The route with the higher-priority attribute is used to forward traffic. If multiple routes have the same value for an attribute, the system compares the next attribute.
The route selection priorities are listed in the following table:
The route attribute priorities are, from highest to lowest: P1 > P2 > P3 > P4.
Route attribute priority | Route attribute | Attribute value description |
P1 | MED | An ECR learns routes to the same destination CIDR block from TR instances. The MED value is 1000 for routes from a TR in the same region. The MED value is 3000 for routes from a TR in a different region. If all learned routes to the same destination CIDR block are from inter-region TRs, their MED values are all 3000. The local preference attribute is then compared. |
P2 | Local preference | If the region of the virtual border router (VBR) has connections to an ECR and a TR, inbound traffic preferentially enters the cloud through the ECR and TR connections in the same region as the VBR. If the VBR's region does not have connections to an ECR and a TR, the default egress tag attribute is compared. |
P3 | Default egress | If the VBR's region does not have connections to an ECR and a TR, the system tags the TR route received from the connection that is set as the default egress. This ensures that the tagged route is preferred when it enters the ECR. If the TR route received by the ECR egress connection is not tagged, the lexicographical order attribute is compared. |
P4 | Byte order | The system compares the region IDs of the routes to the same destination CIDR block. The region IDs are sorted alphabetically. The route from the region ID that comes first alphabetically has a higher priority. For example, a route from the China (Beijing) (cn-beijing) region has a higher priority than a route from the China (Hangzhou) (cn-hangzhou) region. |
Scenarios
The following three scenarios are described in increasing order of complexity. These scenarios explain the route selection principles between TRs and ECRs and how to change traffic paths. All scenarios assume that the AS_Path lengths are the same.
Scenario 1: Multiple TRs, but only one is connected to the ECR
As shown in the figure, a company's data center (IDC) is connected to a virtual border router (VBR) in the China (Hangzhou) region. The company has services deployed in VPCs in the China (Hangzhou) and China (Shanghai) regions.
The company uses a VBR, an ECR, and TRs to connect the on-premises IDC to the cloud VPCs. An inter-region connection is established between TR1 in Hangzhou and TR2 in Shanghai. The ECR is attached to TR1 and the VBR, both of which are in the Hangzhou region.
1. Route propagation from the IDC to the VPCs
A: The IDC propagates routes to the VBR.
B: The VBR propagates routes to the ECR.
C: The ECR propagates routes to the TR.
D: Routes are propagated between TRs. TR1 propagates the routes from the ECR to TR2.
2. Route propagation from the VPCs to the IDC
A: The VPCs propagate routes to the TRs. (Route learning is enabled on the TRs.) For more information, see Enable route learning.
B: Routes are propagated between TRs and to the ECR. TR1 propagates the routes from TR2 to the ECR.
C: The ECR propagates routes to the VBR.
D: The VBR propagates routes to the IDC.
3. Traffic paths
Because there is only one path between the VBR and VPC1, and one path between the VBR and VPC2, no route selection is needed. The traffic paths are shown in the figure.
Scenario 2: Multiple TRs are connected to the ECR
As shown in the figure, a company has an IDC in the China (Hangzhou) region and services deployed in VPCs in the China (Hangzhou) and China (Shanghai) regions.
The company uses a VBR, an ECR, and TRs to connect the on-premises IDC to the cloud VPCs. An inter-region connection is established between the TR in Hangzhou and the TR in Shanghai. The ECR is attached to TR1 in Hangzhou, TR2 in Shanghai, and the VBR in Hangzhou. The TRs and the ECR are fully interconnected.
1. Route propagation from the IDC to the VPCs
A: The IDC propagates routes to the VBR.
B: The VBR propagates routes to the ECR.
C: The ECR propagates routes to TR1 and TR2. The ECR first sets the MED attribute in the routes and then sends the routes to the two TRs.
When the source VBR and the destination TR of a route are in the same region, the MED value is set to 1000. When they are in different regions, the MED value is set to 3000. Therefore:
The VBR and TR1 are in the same region. The ECR sets the MED value of the route to 1000 and then propagates it to TR1.
The VBR and TR2 are in different regions. The ECR sets the MED value of the route to 3000 and then propagates it to TR2.
D: Routes are propagated between TRs. After propagation, the active routes on each TR are as follows:
TR1 is directly connected to the ECR. Based on the intra-region priority rule, the route from the ECR takes precedence. When outbound traffic from VPC1 accesses the VBR, TR1 selects the ECR as the next hop.
TR2 is also directly connected to the ECR. Based on the intra-region priority rule, the route from the ECR takes precedence. When outbound traffic from VPC2 accesses the VBR, TR2 selects the ECR as the next hop.
2. Route propagation from the VPCs to the IDC
A: Routes are propagated between TRs.
B: TR1 and TR2 propagate routes to the ECR. The TRs first set the MED attribute in the routes and then send the routes to the ECR.
When the source VPC and the current TR of a route are in the same region, the MED value is set to 1000. When they are in different regions, the MED value is set to 3000.
For routes originating from VPC1:
VPC1 and TR1 are in the same region. TR1 sets the MED value of the route to 1000 and propagates it to the ECR.
VPC1 and TR2 are in different regions. TR2 sets the MED value of the route to 3000 and propagates it to the ECR.
After the ECR receives the routes, the route with the lower MED value has a higher priority. The route from TR1 takes precedence. When inbound traffic accesses VPC1, the ECR selects TR1 as the next hop.
For routes originating from VPC2: The logic is the same as for routes from VPC1. The ECR gives precedence to the route from TR2. When inbound traffic accesses VPC2, the ECR selects TR2 as the next hop.
C: The ECR propagates routes to the VBR.
D: The VBR propagates routes to the IDC.
3. Traffic paths
Based on the preceding route analysis:
Outbound traffic: On the TRs, the routes from the ECR take precedence based on the intra-region priority rule because both TRs are directly connected to the ECR. Therefore, the TRs select the ECR as the next hop. Traffic is sent outbound directly from the local TR to the ECR.
Inbound traffic: On the ECR, the route from the TR in the same region as the source VPC has a higher priority. Therefore, traffic preferentially enters the cloud through the TR in the VPC's region.
Scenario 3: Multiple TRs are interconnected, and one TR is not connected to the ECR
As shown in the figure, a company has connected three IDCs to VBRs in the China (Beijing), China (Shanghai), and China (Hangzhou) regions. The company also has services deployed in VPCs in the China (Beijing), China (Shanghai), and China (Hangzhou) regions.
The company has connected the three VBRs to an ECR and connected TR1 and TR2 to the ECR. Inter-region connections are established among all three TRs.
1. Route propagation from the IDCs to the VPCs
A: The IDCs propagate routes to the VBRs.
B: The VBRs propagate routes to the ECR.
C: The ECR propagates routes to TR1 and TR2. The ECR first sets the MED attribute in the routes and then sends the routes to the TRs.
When the source VBR and the destination TR of a route are in the same region, the MED value is set to 1000. When they are in different regions, the MED value is set to 3000.
Take the route originating from VBR1 as an example:
VBR1 and TR1 are in the same region. The ECR sets the MED value of the route to 1000 and then propagates it to TR1.
VBR1 and TR2 are in different regions. The ECR sets the MED value of the route to 3000 and then propagates it to TR2.
D: Routes are propagated between TRs. After propagation, the active routes on each TR are as follows:
TR1 and TR2 are both directly connected to the ECR. The situation is the same as in Scenario 2. Based on the intra-region priority rule, the routes from the ECR take precedence.
TR3 is not directly connected to the ECR. The situation is as follows:
When receiving routes originating from VBR1:
Route from TR1: The MED value is 1000.
Route from TR2: The MED value is 3000.
The route with an MED of 1000 has a higher priority. The route from TR1 takes precedence. When outbound traffic from VPC3 accesses VBR1, TR3 selects TR1 as the next hop.
When receiving routes originating from VBR2, the logic is similar to that for VBR1. The route from TR2 takes precedence. When outbound traffic from VPC3 accesses VBR2, TR3 selects TR2 as the next hop.
When receiving routes originating from VBR3:
Route from TR1: The source VBR3 is in Hangzhou and TR1 is in Beijing. This is an inter-region connection, so the MED value is 3000.
Route from TR2: The source VBR3 is in Hangzhou and TR2 is in Shanghai. This is an inter-region connection, so the MED value is 3000.
The MED values are the same. Priority is determined by lexicographical order. `beijing` comes before `hangzhou` alphabetically, so the route from TR1 takes precedence. When outbound traffic from VPC3 accesses VBR3, TR3 selects TR1 as the next hop.
Based on the preceding routing rules, the outbound traffic paths from each VPC are as follows:
Outbound traffic path from VPC1 | Outbound traffic path from VPC2 | Outbound traffic path from VPC3 |
 |  |  |
2. Route propagation from the VPCs to the IDCs
A: The VPCs propagate routes to the TRs.
B: Routes are propagated between TRs and to the ECR. The TRs first set the MED attribute in the routes and then send the routes to the ECR.
When the source VPC and the current TR of a route are in the same region, the MED value is set to 1000. When they are in different regions, the MED value is set to 3000.
For routes originating from VPC1:
VPC1 and TR1 are in the same region. TR1 sets the MED value of the route to 1000 and propagates it to the ECR.
VPC1 and TR2 are in different regions. TR2 sets the MED value of the route to 3000 and propagates it to the ECR.
After the ECR receives the routes, the route with the lower MED value has a higher priority. The route from TR1 takes precedence. When inbound traffic accesses VPC1, the ECR selects TR1 as the next hop.
For routes originating from VPC2: The logic is the same as for routes from VPC1. The ECR gives precedence to the route from TR2. When inbound traffic accesses VPC2, the ECR selects TR2 as the next hop.
For routes originating from VPC3:
VPC3 and TR1 are in different regions. TR1 sets the MED value of the route to 3000 and propagates it to the ECR.
VPC3 and TR2 are in different regions. TR2 sets the MED value of the route to 3000 and propagates it to the ECR.
The ECR receives routes with the same MED value. The intra-region priority logic is applied:
The TR in the same region as VBR1 is TR1, which is directly connected to the ECR. Therefore, when inbound traffic from VBR1 accesses VPC3, the ECR selects TR1 as the next hop.
The TR in the same region as VBR2 is TR2, which is directly connected to the ECR. Therefore, when inbound traffic from VBR2 accesses VPC3, the ECR selects TR2 as the next hop.
The TR in the same region as VBR3 is TR3, which is not directly connected to the ECR. Lexicographical comparison is used:
`beijing` comes before `hangzhou` alphabetically, so the route from TR1 takes precedence. When inbound traffic from VBR3 accesses VPC3, the ECR selects TR1 as the next hop.
C: The ECR propagates routes to the VBRs.
D: The VBRs propagate routes to the IDCs.
Based on the preceding routing rules, the inbound traffic paths from the IDCs to the three VPCs are as follows:
Inbound traffic path to VPC1 | Inbound traffic path to VPC2 | Inbound traffic path to VPC3 |
 |  |  |
3. Changing traffic paths
You can set TR2 as the default path on the ECR to change the traffic path for communication between VPC3 and IDC3:
Change the outbound traffic path:
After you set TR2 as the default path on the ECR, the MED attribute of all routes sent from the ECR to TR2 is set to 2000 based on the MED propagation policy. After TR2 sends these routes to TR3, TR3 gives precedence to the routes from TR2. This changes the outbound traffic egress for VPC3 from TR1 to TR2.
Change the inbound traffic path:
After you set TR2 as the default path on the ECR, inbound traffic preferentially selects the default TR2 as the next hop based on the default egress rule. As a result, when inbound traffic from VBR3 accesses VPC3, the ECR selects TR2 as the next hop.
Traffic path between VPC3 and IDC3 before changing the MED | Traffic path between VPC3 and IDC3 after changing the MED |
 |  |
The following table summarizes the traffic paths for communication between the three VPCs and the IDCs:
MED modification status | Traffic path between VPC1 and IDC | Traffic path between VPC2 and IDC | Traffic path between VPC3 and IDC |
Before changing the MED |  |  |  |
After changing the MED | The traffic path remains unchanged | The traffic path remains unchanged |  |
Limits
If an ECR is associated with a single Cloud Enterprise Network (CEN), you can set only one TR in that CEN as the default path by setting its MED value. No other TRs in that CEN can be set as the default path.
If an ECR is associated with multiple CENs, you can set one TR in each CEN as the default path. No other TRs in the same CEN can be set as the default path.
Prerequisites
You have created an Express Connect Router (ECR). For more information, see Create an ECR.
You have used Cloud Enterprise Network (CEN) to enable inter-region communication between VPCs. For more information, see Enable inter-region communication between VPCs using CEN.
You have created an Express Connect circuit and a virtual border router (VBR) for each data center, and the ECR is attached to the VBRs. For more information, see Express Connect circuits, VBRs, and Add a VBR.
Procedure
On the ECR, you can set the MED value of a TR in a specific region to Default. This can resolve traffic detour issues that may occur in inter-region communication scenarios where multiple TRs are connected to an ECR, but the ECR and a TR are not directly connected:
Go to the Basic Information page of the destination ECR. Find the destination TR and click Set As Default in the Actions column.
