:Troubleshoot route conflict errors in VPC route tables or CEN

Problem

A VPC route table or CEN instance displays a route conflict error:

• A VPC route table displays Conflict on the System Route tab.

• A CEN instance displays Route Conflict on the Network Routes tab.

Causes

Common causes:

• The CIDR blocks of two or more VPCs overlap.

• A VPC is attached to a CEN instance while also using a VPC peering connection, and the peer route overlaps with a CEN route.

• The route quota of the CEN instance is exhausted.

Solution 1: Resolve overlapping VPC CIDR blocks

Example scenario

VPC1 and VPC2 in China (Hangzhou) are attached to the same CEN instance with overlapping CIDR blocks. The VPC route tables show Conflict on the System Route tab, and the CEN instance shows Route Conflict on the Network Routes tab.

Network instance	CIDR block
VPC1	Primary CIDR block: 172.16.0.0/16 CIDR block of vSwitch1: 172.16.0.0/24 CIDR block of vSwitch2: 172.16.1.0/29 CIDR block of vSwitch3: 172.16.102.40/29 CIDR block of vSwitch4: 172.16.100.0/24
VPC2	Primary CIDR block: 172.16.0.0/16 CIDR block of vSwitch1: 172.16.0.0/24 CIDR block of vSwitch2: 172.16.1.0/29 CIDR block of vSwitch3: 172.16.10.0/24 CIDR block of vSwitch4: 172.16.2.0/24

Step 1: Identify the overlapping CIDR blocks

Identify overlapping CIDR blocks in the VPC or CEN console:

• VPC console: Log on to the VPC console, click the ID of the VPC, and then check the System Route tab.

• CEN console: Log on to the CEN console, find the CEN instance, and then navigate to the Basic Information > Transit Router tab. Click the ID of the transit router to which the VPC is connected, and then check the Network Routes tab.

Step 2: Fix the overlapping CIDR blocks

Choose one approach:

• Delete the overlapping vSwitch. What do I do if the CIDR blocks of vSwitches overlap with each other?

• Remove the VPC with the overlapping CIDR block from CEN. In the CEN console, delete the network instance connection. Delete a network instance connection.

  Overlapping CIDR blocks cannot communicate with other VPCs. CEN prioritizes routes from other attached VPCs.

• Use a NAT gateway. Allow VPCs with overlapping CIDR blocks to access each other by using VPC NAT gateways.

Solution 2: Resolve peering connection and CEN route conflicts

Example scenario

VPC1, VPC2, and VPC3 are in the same region. VPC1 and VPC2 are connected through a peering connection. VPC1 and VPC3 are connected through CEN. The peering connection is configured first. After the CEN connection is added, VPC3 displays the Route Conflict error.

Step 1: Identify the overlapping CIDR blocks

Identify overlapping CIDR blocks in the VPC or CEN console:

• VPC console: Log on to the VPC console and navigate to the System Route tab of VPC3 to view the overlapping CIDR block.

  

• CEN console: Log on to the CEN console and click the ID of the CEN instance. On the Basic Information > Transit Router tab, click the ID of the transit router to which VPC3 is connected. On the Network Routes tab, view the overlapping CIDR block.

  

Step 2: Fix the overlapping CIDR blocks

Set the peering connection route destination to a smaller CIDR block (for example, 172.16.0.0/25 or 172.16.0.0/20) that does not overlap with VPC3.

Solution 3: Resolve CEN route quota exhaustion

Step 1: Check for quota exhaustion events

1. Log on to the CloudMonitor console.

2. In the left-side navigation pane, choose Event Center > System Event.

3. Filter system events for CEN and check whether a QuotaExceeded:Route event exists.

Step 2: Set up proactive monitoring

Create a threshold-triggered alert rule for the route quota. Monitor route usage.

Applicable scope

• VPC

• CEN