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Difficulty: EasyVPC Connectivity and Routing Troubleshooting

A developer is troubleshooting an Amazon EC2 instance that has a public IPv4 address but cannot access the internet to download updates. The security groups and network ACLs are configured to allow all outbound traffic. What is the most likely cause of this connectivity failure?

  1. A
    The network ACL is stateful and is automatically blocking the outbound HTTP responses from reaching the instance.
  2. B
    The route table associated with the subnet has a route for 0.0.0.0/0 pointing to a Virtual Private Gateway.
  3. The route table associated with the subnet does not contain a route for 0.0.0.0/0 pointing to the VPC's Internet Gateway.Answer
  4. D
    The route table associated with the subnet is missing a route for the local VPC CIDR block pointing to the Internet Gateway.

Answer

The route table associated with the subnet does not contain a route for 0.0.0.0/0 pointing to the VPC's Internet Gateway.
For an EC2 instance with a public IP address to access the internet, the subnet in which it is launched must be configured as a public subnet. This requires a route table entry that sends all internet-bound traffic (0.0.0.0/0) to the VPC's attached Internet Gateway.

Step-by-Step Solution

1
Analyze the requirements for public internet connectivity for an EC2 instance.
The instance needs a public IP address, and its subnet's route table must route internet traffic (0.0.0.0/0) to an Internet Gateway (IGW).
Security groups and network ACLs are already configured to allow outbound traffic, so the issue points to a routing layer configuration.
2
Evaluate the route table configurations.
A missing route for 0.0.0.0/0 pointing to the IGW is the most common reason why an instance with a public IP in a subnet cannot reach the internet.
Without this route, the VPC router does not know where to send traffic destined for external internet addresses.

Key Concept

VPC Route Tables and Internet Gateway Routing
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Topics

VPC Connectivity and Routing Troubleshooting

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