Why Did Communication Fail Between VPCs That Were Connected by a VPC Peering Connection?

Symptom

After a VPC peering connection is created, the local and peer VPCs cannot communicate with each other.

Troubleshooting

The issues here are described in order of how likely they are to occur.

**Table 1** Possible causes and solutions
No.	Possible Cause	Solution
1	Overlapping CIDR blocks of local and peer VPCs All their subnet CIDR blocks overlap. Some of their subnet CIDR blocks overlap.	Refer to Overlapping CIDR Blocks of Local and Peer VPCs.
2	Incorrect route configuration for the local and peer VPCs No routes are added. Incorrect routes are added.	Refer to Incorrect Route Configuration for Local and Peer VPCs.
3	Incorrect network configuration The security group rules of the ECSs that need to communicate deny inbound traffic from each other. The firewall of the ECS NIC blocks traffic. The network ACL rules of the subnets connected by the VPC peering connection deny inbound traffic. Check the policy-based routing configuration of an ECS with multiple NICs.	Refer to Incorrect Network Configuration.
4	ECS network failure	Refer to ECS Network Failure.

Overlapping CIDR Blocks of Local and Peer VPCs

If the CIDR blocks of VPCs connected by a VPC peering connection overlap, the connection may not take effect due to route conflicts.

**Table 2** Overlapping CIDR blocks of local and peer VPCs
Scenario	Description	Solution
VPCs with overlapping CIDR blocks also include subnets that overlap.	As shown in Figure 1, the CIDR blocks of VPC-A and VPC-B overlap, and all their subnets overlap. Overlapping CIDR blocks of VPC-A and VPC-B: 10.0.0.0/16 Overlapping CIDR blocks of Subnet-A01 in VPC-A and Subnet-B01 in VPC-B: 10.0.0.0/24 Overlapping CIDR blocks of Subnet-A02 in VPC-A and Subnet-B02 in VPC-B: 10.0.1.0/24	VPC-A and VPC-B cannot be connected using a VPC peering connection. Replan the network.
Two VPCs have overlapping CIDR blocks but some of their subnets do not overlap.	As shown in Figure 2, the CIDR blocks of VPC-A and VPC-B overlap, and some of their subnets overlap. Overlapping CIDR blocks of VPC-A and VPC-B: 10.0.0.0/16 Overlapping CIDR blocks of Subnet-A01 in VPC-A and Subnet-B01 in VPC-B: 10.0.0.0/24 CIDR blocks of Subnet-A02 in VPC-A and Subnet-B02 in VPC-B do not overlap.	A VPC peering connection cannot connect the entire VPCs, VPC-A and VPC-B. A connection can connect their subnets (Subnet-A02 and Subnet-B02) that do not overlap. For details, see Figure 3.

Figure 1 Networking diagram (IPv4)

Figure 2 Networking diagram (IPv4)

If CIDR blocks of VPCs overlap and some of their subnets overlap, you can create a VPC peering connection between their subnets with non-overlapping CIDR blocks. Figure 3 shows the networking diagram of connecting Subnet-A02 and Subnet-B02. Table 3 describes the routes required.

Figure 3 Networking diagram (IPv4)

**Table 3** Routes required for the VPC peering connection between Subnet-A02 and Subnet-B02
Route Table	Destination	Next Hop	Description
VPC-A route table	10.0.2.0/24	Peering-AB	Add a route with the CIDR block of Subnet-B02 as the destination and Peering-AB as the next hop.
VPC-B route table	10.0.1.0/24	Peering-AB	Add a route with the CIDR block of Subnet-A02 as the destination and Peering-AB as the next hop.

Incorrect Route Configuration for Local and Peer VPCs

Check the routes in the route tables of the local and peer VPCs by referring to Viewing Routes Configured for a VPC Peering Connection. Table 4 lists the items that you need to check.

**Table 4** Route check items
Item	Solution
Check whether routes are added to the route tables of the local and peer VPCs.	If routes are not added, add routes by referring to: Creating a VPC Peering Connection with Another VPC in Your Account
Check the destinations of routes added to the route tables of the local and peer VPCs. In the route table of the local VPC, check whether the route destination is the CIDR block, subnet CIDR block, or related private IP address of the peer VPC. In the route table of the peer VPC, check whether the route destination is the CIDR block, subnet CIDR block, or related private IP address of the local VPC.	If the route destination is incorrect, change it by referring to Modifying Routes Configured for a VPC Peering Connection.

Incorrect Network Configuration

Check whether the security group rules of the ECSs that need to communicate with each other allow inbound traffic from each other.
- If the ECSs are associated with the same security group, you do not need to check their rules.
- If the ECSs are associated with different security groups, add an inbound rule to allow access from each other by referring to Security Group Configuration Examples.
Check whether the firewall of the ECS NIC blocks traffic.
If the firewall blocks traffic, configure the firewall to allow inbound traffic.
Check whether network ACL rules of the subnets connected by the VPC peering connection deny inbound traffic.
If the network ACL rules deny inbound traffic, configure the rules to allow the traffic.
If an ECS has more than one NIC, check whether correct policy-based routing has been configured for the ECS and packets with different source IP addresses match their own routes from each NIC.
If an ECS has two NICs (eth0 and eth1):
- IP address of eth0: 192.168.1.10; Subnet gateway: 192.168.1.1
- IP address of eth1: 192.168.2.10; Subnet gateway: 192.168.2.1
Command format:
- ping -l IP address of eth0 Subnet gateway address of eth0
- ping -l IP address of eth1 Subnet gateway address of eth1
Run the following commands:
- ping -I 192.168.1.10 192.168.1.1
- ping -I 192.168.2.10 192.168.2.1
If the network communication is normal, the routes of the NICs are correctly configured.

ECS Network Failure

Log in to the ECS.
Check whether the ECS NIC has an IP address assigned.
- Linux ECS: Use the ifconfig or ip address command to view the IP address of the NIC.
- Windows ECS: In the search box, enter cmd and press Enter. In the displayed command prompt, run the ipconfig command.
If the ECS NIC has no IP address assigned, see
Check whether the subnet gateway of the ECS can be pinged.
1. In the ECS list, click the ECS name.
  The ECS details page is displayed.
2. On the ECS details page, click the hyperlink of VPC.
  The Virtual Private Cloud page is displayed.
3. In the VPC list, locate the target VPC and click the number in the Subnets column.
  The Subnets page is displayed.
4. In the subnet list, click the subnet name.
  The subnet details page is displayed.
5. Click the IP Addresses tab and view the gateway address of the subnet.
6. Check whether the gateway communication is normal:
  ping Subnet gateway address
  
  Example command: ping 172.17.0.1