q and a scenarios on vpc

1. VPC Architecture and High Availability

VPC design for a highly available web application across multiple Availability Zones

• Create multiple public and private subnets across at least two Availability Zones (AZs) for fault tolerance.

• Attach an Internet Gateway (IGW) to the VPC for internet connectivity.

• Deploy NAT Gateways in each public subnet to allow private subnets outbound internet access without exposing instances directly.

• Use Elastic Load Balancer (ELB) across AZs for distributing traffic.

• Deploy resources like EC2 and RDS in private subnets spanning multiple AZs for resiliency.

Designing subnets and routing when some resources must have no public internet access

• Place those resources in private subnets without routes to the Internet Gateway.

• Route private subnet internet-bound traffic through a NAT Gateway if outbound internet access is needed (e.g., for updates).

• Apply strict security group and NACL rules to block inbound internet traffic.

2. Security and Access Control

Ensure databases are only accessible from specific internal servers

• Configure DB security group to allow inbound access only from the security group or private IP addresses of allowed internal servers.

• Use NACLs to restrict subnet-level inbound and outbound internet traffic.

• Ensure DB subnets route only internally (no 0.0.0.0/0 route to IGW or NAT Gateway).

Restrict SSH/RDP access to only certain IP addresses

• Configure EC2 instance security groups to allow inbound SSH (port 22) or RDP (port 3389) connections only from specific trusted IP ranges.

• Avoid using open rules like 0.0.0.0/0 for these protocols.

Investigate and mitigate if a private-subnet EC2 instance can access the internet directly

• Verify the instance does not have a Public or Elastic IP.

• Check the private subnet’s route table for any direct route (0.0.0.0/0) to the Internet Gateway — remove if present.

• Confirm routing sends internet-bound traffic via NAT Gateway instead.

• Review security groups and NACLs for unintended egress permissions.

3. Connectivity Troubleshooting

Troubleshoot an EC2 instance in a public subnet that cannot reach the internet despite having a public IP

• Check that the subnet’s route table has a route sending 0.0.0.0/0 traffic to the Internet Gateway.

• Validate Security Group allows outbound traffic.

• Verify Network ACLs allow outbound internet traffic.

• Confirm the instance has an assigned public IP.

• Test OS firewall or routing inside the instance.

Diagnose failing internal connectivity between two subnets

• Ensure both subnets share the same VPC and have the “local” route in their route tables.

• Verify Security Groups allow traffic between the subnets on required ports.

• Check subnet NACLs permit relevant traffic inbound and outbound.

• Conduct ping/traceroute tests between instances.

Verify VPC peering configuration for communication between two VPCs

• Confirm the peering connection is in "active" state.

• Validate route tables in both VPCs have routes for the peer CIDRs pointing to the peering connection.

• Check for overlapping CIDR blocks (not permitted).

• Ensure security groups and NACLs allow cross-VPC traffic.

4. Integrating with On-Premises or External Networks

Secure, encrypted communication between data center and AWS VPC

• Use AWS Site-to-Site VPN or Direct Connect with VPN for encryption.

• Set up a Virtual Private Gateway in the VPC.

• Configure a Customer Gateway device on-premises.

• Create and establish the VPN connection.

• Update route tables and firewall rules accordingly.

Migrating resources from one VPC to another with minimal downtime

• Use AMIs and snapshots to migrate EC2 and database resources.

• Establish VPC peering or Transit Gateway between old and new VPC for smooth transition and data sync.

• Challenges include IP conflicts, reconfiguring security, temporary downtime during DNS or endpoint switch-over.

5. Accessing AWS Services from Private Subnets

Allow private subnet instances to access S3 and DynamoDB with no direct internet access

• Create VPC Gateway Endpoints for S3 and DynamoDB.

• This enables private, scalable access over AWS backbone without exposing resources or needing NAT Gateway traffic, improving security and cost-efficiency.

Options for a private instance to reach an external API without a public IP

• Route outbound traffic through a NAT Gateway in a public subnet.

• Alternatively, use AWS PrivateLink if the external API supports a PrivateLink endpoint.

6. Monitoring and Compliance

Monitor and log all traffic between subnets and instances within your VPC

• Enable VPC Flow Logs at VPC, subnet, or ENI level.

• Integrate logs with CloudWatch or S3 for analysis and alerting.

Compliance controls (e.g., GDPR, HIPAA) that influence VPC design

• Implement strict segmentation using subnets to isolate sensitive workloads.

• Encrypt data at rest (EBS, RDS) and in transit (TLS).

• Use fine-grained IAM policies and monitor access logs.

• Leverage VPC Endpoints to limit internet exposure.

• Maintain audit logs using CloudTrail, VPC Flow Logs for regulatory audits.

7. VPC Peering, Transitive Routing, and Multi-Region Design

Enable inter-VPC communication between two VPCs in different AWS accounts and limitations

• Establish a VPC peering connection and accept it in both accounts.

• Update route tables to enable routing across peered VPCs.

• Limitations: no overlapping CIDRs, no transitive routing, security rules must allow cross-VPC traffic.

Is transitive routing possible with standard VPC peering? Alternatives if not

• Standard VPC peering does NOT support transitive routing. Traffic cannot route through a third VPC.

• Use AWS Transit Gateway for many-to-many VPC connections, multi-account support, and cross-region peering.

8. Best Practices and Operations

Improve VPC performance and resiliency

• Distribute resources across multiple AZs.

• Use multiple NAT Gateways to avoid single points of failure.

• Optimize security group/NACL rules to reduce latency.

• Monitor network traffic with VPC Flow Logs and CloudWatch.

• Automate recovery and failover using CloudFormation and autoscaling.

Addressing resource limits (subnets, NAT gateways) as VPC grows

• Request AWS service quota increases via support.

• Refactor architecture, possibly splitting workloads across multiple VPCs.

• Use AWS Transit Gateway for scalable hub-and-spoke network topology.

• Carefully plan CIDR block sizes and reuse IP ranges judiciously.

If you'd like expanded diagrams, a step-by-step design example, or conversion of specific Q&As into a stepper or other advanced GitBook blocks, tell me which section to focus on.