Lesson 088 · AWS Learning Path

AWS 088: VPC design across Availability Zones

Anil Jalela · Published Sep 21, 2025 · 9 min read

The real problem

A team recognizes the name VPC design across Availability Zones but has not connected the feature to a real requirement, identity boundary, network or data path, failure mode, price dimension, and cleanup owner. A plausible configuration could still fail the workload.

Final outcome

The learner will produce a requirement-led artifact for VPC design across Availability Zones, inspect the matching AWS control plane in the Management Console, run a matching CloudShell or AWS CLI query, interpret the output, diagnose one failure, defend one architecture choice, and prove cleanup or approved retained state.

The practical outcome is not a command transcript. It must show what was expected, what happened, what the result proves, what it does not prove, and which evidence would change the decision.

Learning objectives

By the end of this lesson, the learner can:

• explain vpc is regional; subnets are zonal;
• explain regional services still use zonal attachments;
• explain symmetry reduces failure surprises;
• explain route to local zonal services;
• explain data resilience is service-specific;
• connect control-plane state to the real data, network, identity, or application behavior;
• identify cost and cleanup ownership before any optional mutation;
• troubleshoot from evidence without opening broad access or adding broad permissions.

Relationship model

Requirement
   |
   v
Identity and policy -> AWS configuration -> network or data path -> workload behavior
        |                    |                      |                    |
        +--------------------+----------------------+--------------------+
                                      |
                                      v
                         monitoring, cost, recovery, cleanup

Use this model to separate an AWS object that exists from a result that actually works. Every arrow is a verification boundary.

Prerequisites, permissions, Region, and safety

• Learning baseline: This sequence assumes practical Linux knowledge but no prior cloud-computing or AWS knowledge. Cloud, networking, security, data, automation, and architecture concepts must come from completed earlier lessons. If a prerequisite checkpoint is incomplete, return to its linked lesson before continuing.
• Confirm a non-root caller with aws sts get-caller-identity and keep the account number private.
• Use ap-south-1 unless this lesson explicitly names a second Region.
• Confirm the intended profile and Region with aws configure list before interpreting an empty result.
• Use read-only List, Get, and Describe permissions for the named services. Design exercises run locally and require no resource-creation permission.
• This is a no-create lesson. Console and CLI work is read-only, and every design artifact is created locally.
• Never publish account IDs, public addresses, ARNs containing private account data, session IDs, presigned URLs, object data, credentials, or KMS material.
• Do not use root, world-open SSH or RDP, disabled TLS verification, unowned resources, or irreversible retention controls in a training exercise.

Core model

How it works

A standard three-AZ pattern repeats public ingress, private application, and protected data subnet groups. Regional load balancing distributes to healthy zonal targets. Stateless application nodes can be replaced; state uses a service with a defined replication and recovery model.

Read the result in layers:

1. Scope: account, Region, VPC, bucket, AZ, endpoint, principal, object version, or resource ARN.
2. Control plane: the requested configuration exists and reached an expected state.
3. Behavior: the request, connection, health check, replication, restore, or application result meets the requirement.
4. Operations: monitoring, failure owner, cost, retention, rollback, and cleanup are known.

Control-plane success is necessary but not sufficient. A resource can be available while policy, routing, DNS, health, data, or application behavior remains wrong.

Architecture decision table

Professional questions normally contain several valid services. State the requirement that selects one option, why the nearest alternative fails it, and what changed requirement would reverse the choice.

AWS Management Console guided practice

Before opening a service page, write the expected account, Region, starting state, and evidence. Do not choose Create, Save, Purchase, Lock, or Delete unless the lesson explicitly authorizes the live track.

1. Open VPC Subnets and group the list by Availability Zone and tier name.
2. For each AZ, verify the expected public, application, data, endpoint, and inspection subnets and available IP capacity.
3. Trace each private subnet to its same-AZ NAT or endpoint and then inspect load-balancer targets and database subnet groups.

For each step, capture the field name and value in text. A screenshot may support the record but does not replace the explanation. Console labels can evolve, so use the service search and current documentation if a navigation label differs.

CloudShell and AWS CLI practice

CloudShell is the default browser-based command environment taught in AWS 028. AWS 029 and AWS 030 cover local CLI installation and authentication. This lesson therefore does not assume that an unconfigured local shell is ready.

Start every session with:

export AWS_DEFAULT_REGION="ap-south-1"
aws sts get-caller-identity --query Arn --output text
aws configure list

Redact the account portion of the ARN before sharing. Then perform the topic query:

Group subnets by AZ and tier, then look for missing zonal dependencies.

aws ec2 describe-subnets --filters Name=vpc-id,Values=vpc-0123456789abcdef0 --query 'Subnets[].{AZ:AvailabilityZone,Subnet:SubnetId,CIDR:CidrBlock,Free:AvailableIpAddressCount,Name:Tags[?Key==`Name`]|[0].Value}' --output table

Expected interpretation:

A subnet list proves placement, not resilience. Trace load-balancer nodes, targets, NAT, endpoints, routes, quotas, and data replication in every selected AZ.

Replace every replace-with-... sample value before running its command, and use only an explicitly owned resource. Explain each option first. These queries are read-only; a successful response does not authorize a later create or delete operation.

Practical work

Submit p05-network-blueprint.md. Use 10.50.0.0/16 and the four approved /24 subnets across two account-visible AZ IDs. Include public ALB, private app tier, same-AZ production NAT alternatives, S3 gateway endpoint, optional interface endpoints, IPv6 branch, DNS, route tables, NACL and SG boundaries, IP capacity, failure paths, price assumptions, and the reduced-cost AWS 106 implementation.

The evidence package must contain:

• the problem and final requirement in the learner's own words;
• caller type and Region with private identifiers redacted;
• exact planned values, ownership, and cost class;
• one Console observation and matching CLI or API evidence;
• one behavior result or supplied data-plane record;
• one denied, failed, or counterexample result and evidence-led diagnosis;
• one architecture choice plus the rejected alternative;
• cleanup proof or explicit retained-state owner, expiry, and next lesson.

Verification standard

Use expected state before observed state. Record timestamps in UTC and preserve the original failure before changing anything. A passing submission answers all four questions:

1. What exact requirement was tested?
2. Which evidence proves the AWS configuration?
3. Which evidence proves the workload behavior?
4. What remains unproven or requires later monitoring?

If AWS returns no rows, verify account, Region, permission, filters, pagination, resource type, and deletion state before concluding that nothing exists.

Common failures and troubleshooting

Do not troubleshoot by attaching administrator access, opening administration ports to the internet, disabling encryption, retrying uncontrolled creation, deleting unknown resources, or weakening retention.

Cost, cleanup, and retained state

No AWS resource is created. Close CloudShell and remove or redact downloaded evidence.

Cleanup evidence requires terminal state and an after-inventory. Search related ENIs, public IPv4 addresses, EBS volumes and snapshots, load balancers, target groups, Auto Scaling instances, endpoints, logs, S3 versions and delete markers, backup recovery points, and global IAM roles when they apply. Billing data can lag, so schedule a later review.

Architecture and certification decisions

• Certification coverage: SAA-C03; SOA-C03; SAP-C02; DOP-C02.
• Exam mapping: SAA D1-D3.
• Explain service scope, failure boundary, consistency, recovery, security, operations, and price rather than matching a keyword.
• Treat availability and durability, encryption and authorization, routing and filtering, health and lifecycle, backup and replication, and discount and capacity as separate concepts.
• Do not reproduce protected certification questions.

Knowledge check

1. Does one large subnet cover three AZs?

Expected direction: No. A subnet belongs to one AZ.

1. Does deploying two EC2 instances in one AZ provide Multi-AZ availability?

Expected direction: No. Both share the same AZ failure domain.

1. Why use same-AZ NAT routes?

Expected direction: They avoid dependence on another AZ and reduce cross-AZ traffic charges.

1. Do identical database subnets replicate database data?

Expected direction: No. The database service or application must provide replication.

Completion gate and assessment

Pass at 80 out of 100 with no critical safety failure. A missing practical artifact, unexplained output, unsafe access, destructive action outside the owned scope, unplanned billed resource, or false cleanup claim requires remediation and a changed retest.

Official sources

• VPC subnet design
• Reliability design principles