Lesson 056 · AWS Learning Path

AWS 056: Route tables and routing decisions

Anil Jalela · Published Aug 20, 2025 · 7 min read

The problem

A team uses Route tables and routing decisions as a label or Console setting without proving the identity, scope, behavior, failure boundary, cost, or operational result. The configuration appears complete but the real requirement remains untested.

Final outcome

The learner will inspect, explain, test, and document Route tables and routing decisions. The submission must connect the Console and CLI view to the same AWS control, state what the evidence proves, diagnose one likely failure, and make a requirement-based architecture decision.

Learning outcomes

You will be able to:

1. explain a route is destination plus target;
2. explain a subnet uses one route table;
3. explain longest prefix wins;
4. explain ipv4 and ipv6 routes are separate;
5. explain tie rules depend on source;

1. distinguish successful configuration from successful workload behavior;
2. preserve redacted evidence and complete the stated cleanup or retention decision.

Mental model

requirement
   -> identity and permission
   -> account, Region, and resource scope
   -> configuration or request
   -> observable state
   -> workload result
   -> failure evidence
   -> cleanup or controlled retention

Never begin with a create button or a copied command. Start with the result that must be proven and the boundary that must remain protected.

Core facts

How to reason about it

Troubleshoot from one packet’s source and destination. Find the source subnet route table, select the most specific route, validate the target, then repeat for the return packet. Only after routing is correct should you test security groups, network ACLs, firewalls, DNS, and the application port.

Use the following decision table as a starting point, then change the answer when the scenario changes.

Prerequisites, permissions, Region, and cost

Run this lesson as the approved non-root course identity. Begin with aws sts get-caller-identity, confirm the private account record, and set the fixed project Region before any regional query. IAM resources are global within an account, while STS endpoints and the services reached by an identity can be regional.

Use only the read or change actions required for this lesson. An AccessDenied result is evidence to analyse, not permission to switch to root or attach AdministratorAccess. Record the action, resource, principal type, request context, and smallest justified correction.

The listed cost tier is T0 - no resource creation. Free Tier eligibility and credits are account-specific. Before a mutating lab, identify every resource that can charge, estimate its duration, start a timer, and write the reverse cleanup order. A budget reports cost after billing data arrives and is not a real-time stop control.

AWS Management Console method

1. Open VPC Route tables and select the table associated with the source subnet.
2. Open Routes and compare destination, target, status, propagated flag, and prefix length.
3. For a test destination, select the longest matching prefix and then inspect the return subnet route table.

For every step record the service page, selected account and Region, exact object, visible state, and why that state matters. A Console label or green status is not enough unless it is tied to the final workload outcome.

AWS CLI or API evidence

Read destinations, targets, origin, and state together.

aws ec2 describe-route-tables --route-table-ids rtb-0123456789abcdef0 --query 'RouteTables[0].Routes[].{Destination:CidrBlock,IPv6:DestinationIpv6CidrBlock,PrefixList:DestinationPrefixListId,Gateway:GatewayId,NAT:NatGatewayId,Peering:VpcPeeringConnectionId,Transit:TransitGatewayId,Origin:Origin,State:State}' --output table

Choose by longest prefix, not by table position. A route marked blackhole is selected normally but cannot forward traffic.

Before running the command, replace every placeholder, explain each option, and decide whether the operation is read-only or mutating. Capture the exit code immediately. Redact account IDs, ARNs, public addresses, request identifiers, and personal data before sharing.

The CLI and Console are clients of AWS APIs. Matching state across them increases confidence, but neither substitutes for data-plane or application verification.

Practical work

Create p04-route-plan.md with two explicit route tables. nw-p04-public-rt has the automatic 10.42.0.0/16 local route plus planned 0.0.0.0/0 to nw-p04-igw; associate the two public subnets. nw-p04-private-rt has only the local route at this stage and associates the two private subnets. For five destinations, apply longest-prefix match and trace the return path. Do not create routes yet.

The evidence package must include:

• non-root principal type, account verified privately, and Region;
• exact intended and observed state;
• one Console observation and the matching CLI or API result;
• one successful result and one denied, failed, or counterexample result;
• what each result does not prove;
• cost state and elapsed lab time;
• cleanup evidence or a named retained-state owner and next lesson.

Verification standard

Use three levels of proof:

1. Control plane: the object or policy exists with the intended configuration.
2. Data plane or behavior: the request, packet, session, storage path, or application does what the requirement states.
3. Operations: monitoring, failure diagnosis, cost, ownership, and cleanup are known.

A control-plane response can precede final readiness. Use waiters or state polling where supported, then test the actual behavior. If a request times out, do not assume it failed. Inspect state and use documented idempotency before retrying a mutation.

Troubleshooting method

Keep the original symptom and timestamp. State one hypothesis, make one reversible change, repeat the original test, and record rollback. Never open a management port to the world, expose credentials, disable TLS verification, format an unknown disk, or add broad permissions as a generic fix.

Architecture and certification decisions

Professional-level questions provide competing valid features. Identify the requirement that decides between them: human or workload identity, same-account or cross-account access, regional or zonal scope, stateful or stateless filtering, durable or ephemeral data, latency, RTO/RPO, cost, or operational ownership.

Explain why the selected option fits and why each plausible alternative fails one stated requirement. Do not rely on feature memorization or reproduce protected certification questions.

Knowledge check

1. Which route handles 10.20.5.8: 10.0.0.0/8 or 10.20.0.0/16?

Expected direction: The /16 route because it has the longer matching prefix.

1. Does 0.0.0.0/0 handle IPv6?

Expected direction: No. IPv6 needs an IPv6 destination such as ::/0.

1. Which table applies when a subnet has no explicit association?

Expected direction: The VPC main route table.

1. Will a blackhole /24 fall back to a working /16?

Expected direction: No. The more-specific route still wins and its failed target drops traffic.

Cost, cleanup, and retained state

Retain only the state explicitly required by the next P04 lesson and record it in the private resource ledger.

Cleanup evidence includes the final state query, not only a successful delete response. Search related resources, other Regions used by the lab, retained storage, public IPv4 addresses, logging destinations, and service-managed dependencies. Schedule a later billing review because cost data can lag.

Completion gate

Pass when the practical artifact explains the real problem, matches Console and CLI evidence, answers every knowledge check, diagnoses one failure without broadening access, records cost, and proves cleanup or approved retention. The learner must defend one decision orally and name the requirement that would change it.

Official sources

• VPC route tables
• Route priority