AWS VPC Peering: Unable to Ping Instance - Troubleshooting Guide

Troubleshooting VPC Peering Ping Issue

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Question

You have 2 VPCs, VPC A and VPC B.

Both the VPCs have been peered.

You have configured the route tables in VPC A so that traffic can flow from VPCA to VPCB.

You try to ping an instance in VPCB from VPCA, but are unable to do so.

You have confirmed that the NACLs and Security Groups have been configured properly.

What could be the reason for this issue?

Answers

Explanations

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A. B. C. D.

The most likely reason why you cannot ping an instance in VPCB from VPCA despite having peered the VPCs and configured the route tables in VPC A is that the Network Access Control Lists (NACLs) in VPC B are blocking the traffic.

When VPCs are peered, the routing tables are automatically updated to allow traffic to flow between the VPCs. However, NACLs are stateless, which means that they do not allow traffic based on the established state of a connection. As a result, you must ensure that both the inbound and outbound rules in the NACLs for both VPCs allow the necessary traffic.

Security groups, on the other hand, are stateful, which means that they allow traffic based on the established state of a connection. Therefore, it is less likely that the issue is related to security groups.

Overlapping CIDR blocks (answer A) is also a possible reason for the issue. If the CIDR blocks of the VPCs overlap, it can cause routing conflicts and prevent traffic from flowing between the VPCs.

Lastly, if the route tables in VPCB have not been configured properly (answer D), it could also cause the issue.

However, given that you have confirmed that the NACLs and Security Groups have been configured properly, it is more likely that the issue is related to NACLs blocking the traffic. Therefore, it is recommended to check the NACLs of VPCB to ensure that they allow the necessary traffic.