Route tables configuration in AWS - Step-by-Step Execution

Process Flow - Route tables configuration

Create VPC

↓

Create Subnets

↓

Create Route Table

↓

Add Routes to Route Table

↓

Associate Route Table with Subnet

↓

Traffic follows routes for subnet

This flow shows how to create a route table, add routes, and associate it with a subnet so traffic knows where to go.

Execution Sample

AWS

resource "aws_vpc" "main" {
  cidr_block = "10.0.0.0/16"
}

resource "aws_subnet" "subnet1" {
  vpc_id = aws_vpc.main.id
  cidr_block = "10.0.1.0/24"
}

resource "aws_internet_gateway" "igw" {
  vpc_id = aws_vpc.main.id
}

resource "aws_route_table" "rt" {
  vpc_id = aws_vpc.main.id
}

resource "aws_route" "route1" {
  route_table_id = aws_route_table.rt.id
  destination_cidr_block = "0.0.0.0/0"
  gateway_id = aws_internet_gateway.igw.id
}

resource "aws_route_table_association" "a" {
  subnet_id = aws_subnet.subnet1.id
  route_table_id = aws_route_table.rt.id
}

This code creates a VPC, a route table, adds a route to the internet gateway, and associates the route table with a subnet.

Process Table

Step	Action	Resource Created/Modified	State Change	Result
1	Create VPC	aws_vpc.main	VPC with CIDR 10.0.0.0/16 created	VPC ready for subnets
2	Create Route Table	aws_route_table.rt	Route table linked to VPC created	Route table ready for routes
3	Add Route	aws_route.route1	Route to 0.0.0.0/0 via IGW added	Route table can send traffic to internet
4	Associate Route Table	aws_route_table_association.a	Route table associated with subnet1	Subnet1 uses this route table
5	Traffic Flow	N/A	Subnet1 traffic follows route table routes	Subnet1 can reach internet via IGW
6	End	N/A	All resources created and linked	Configuration complete

💡 All steps completed; route table configured and associated with subnet

Status Tracker

Resource	Initial	After Creation	After Route Added	After Association	Final
VPC	None	10.0.0.0/16 created	No change	No change	10.0.0.0/16 active
Route Table	None	Created linked to VPC	Route to 0.0.0.0/0 added	Associated with subnet1	Active with routes and association
Route	None	None	Route to internet gateway added	No change	Active route to IGW
Subnet Association	None	None	None	Route table associated	Subnet1 uses route table

Key Moments - 3 Insights

Why do we need to associate a route table with a subnet?

Because without association, the subnet does not know which routes to use. See execution_table step 4 where association links the route table to subnet1.

What happens if we add a route but do not associate the route table with any subnet?

The route exists but no subnet uses it, so traffic from subnets will not follow that route. This is shown by the absence of association in step 4.

Can a route table have multiple routes?

Yes, route tables can have many routes directing traffic to different destinations. Here we added one route in step 3, but more can be added similarly.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the route to the internet gateway added?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Check the 'Action' and 'Result' columns in execution_table row for step 3.

According to variable_tracker, what is the state of the route table after association?

A Created but no routes

B Associated with subnet but no routes

C Has routes and associated with subnet

D No association or routes

💡 Hint

Look at the 'Route Table' row under 'After Association' in variable_tracker.

If we skip step 4 (association), what will be the result for subnet1 traffic?

A Subnet1 traffic will not use the route table routes

B Subnet1 traffic will follow the route table routes

C Subnet1 will have internet access anyway

D Subnet1 will be deleted

💡 Hint

Refer to key_moments question about association importance and execution_table step 4.

Concept Snapshot

Route tables control where subnet traffic goes.
Create a route table in your VPC.
Add routes to direct traffic (e.g., to internet gateway).
Associate the route table with subnets.
Subnets use associated route tables to send traffic.
Without association, routes are ignored.

Full Transcript

This visual execution shows how to configure route tables in AWS. First, a VPC is created with a CIDR block. Then a route table is created and linked to the VPC. Next, a route is added to the route table directing traffic to the internet gateway. After that, the route table is associated with a subnet so that subnet's traffic follows the routes. Finally, traffic from the subnet can reach the internet via the route table. Key points include the necessity of associating route tables with subnets and that routes only take effect when associated. The execution table and variable tracker show each step and resource state changes clearly.

Practice

1. What is the main purpose of a route table in AWS networking?

easy

A. To manage user permissions

B. To store user data securely

C. To direct network traffic between subnets and gateways

D. To monitor server health

Solution

1. Step 1: Understand the role of route tables

   Route tables control how network traffic moves inside a cloud network by defining paths.

2. Step 2: Identify what route tables connect

   They connect subnets to gateways or other networks, enabling communication.

3. Final Answer:

   To direct network traffic between subnets and gateways -> Option C

4. Quick Check:

   Route tables = traffic direction [OK]

Hint: Route tables guide traffic flow inside the cloud [OK]

Common Mistakes:

• Confusing route tables with security groups
• Thinking route tables store data
• Mixing route tables with monitoring tools

2. Which of the following is the correct way to associate a route table with a subnet in AWS CLI?

easy

A. aws ec2 create-route-table --subnet-id subnet-12345 --route-table-id rtb-67890

B. aws ec2 associate-route-table --subnet-id subnet-12345 --route-table-id rtb-67890

C. aws ec2 attach-route-table --subnet subnet-12345 --table rtb-67890

D. aws ec2 link-route-table --subnet subnet-12345 --route-table rtb-67890

Solution

1. Step 1: Identify the correct AWS CLI command for association

   The command to associate a route table with a subnet is 'associate-route-table'.

2. Step 2: Check the correct syntax and parameters

   The correct syntax uses '--subnet-id' and '--route-table-id' flags with IDs.

3. Final Answer:

   aws ec2 associate-route-table --subnet-id subnet-12345 --route-table-id rtb-67890 -> Option B

4. Quick Check:

   Associate route table = associate-route-table command [OK]

Hint: Use 'associate-route-table' with subnet and route table IDs [OK]

Common Mistakes:

• Using 'create-route-table' instead of 'associate-route-table'
• Wrong parameter names like '--subnet' instead of '--subnet-id'
• Using non-existent commands like 'attach-route-table'

3. Given a route table with the following routes:
Destination: 0.0.0.0/0, Target: igw-12345
Destination: 10.0.1.0/24, Target: local
What happens when an instance in subnet 10.0.1.0/24 tries to reach 8.8.8.8?

medium

A. Traffic is sent to the internet gateway (igw-12345)

B. Traffic is blocked because no route exists

C. Traffic is sent to the local subnet only

D. Traffic is sent to a NAT gateway automatically

Solution

1. Step 1: Analyze the route for 0.0.0.0/0

   This route sends all traffic not matching other routes to the internet gateway (igw-12345).

2. Step 2: Determine route for 8.8.8.8

   Since 8.8.8.8 is outside the local subnet, it matches the 0.0.0.0/0 route and goes to the internet gateway.

3. Final Answer:

   Traffic is sent to the internet gateway (igw-12345) -> Option A

4. Quick Check:

   Default route sends traffic to internet gateway [OK]

Hint: Default 0.0.0.0/0 route sends traffic outside subnet [OK]

Common Mistakes:

• Assuming traffic is blocked without explicit deny
• Confusing local route with internet access
• Thinking NAT gateway is used without configuration

4. You created a route table and associated it with a subnet, but instances in that subnet cannot access the internet. What is the most likely mistake?

medium

A. The route table lacks a route to an internet gateway

B. The subnet is not associated with any route table

C. The route table has a route to a NAT gateway instead of an internet gateway

D. The instances have no security group attached

Solution

1. Step 1: Check route table routes for internet access

   Internet access requires a route to an internet gateway (igw) for 0.0.0.0/0.

2. Step 2: Identify missing or incorrect routes

   If the route to the internet gateway is missing, instances cannot reach the internet despite association.

3. Final Answer:

   The route table lacks a route to an internet gateway -> Option A

4. Quick Check:

   Internet needs 0.0.0.0/0 route to igw [OK]

Hint: Check for 0.0.0.0/0 route to internet gateway [OK]

Common Mistakes:

• Assuming subnet association alone grants internet access
• Confusing NAT gateway with internet gateway routes
• Ignoring security group rules as cause

5. You have two subnets: Subnet A (10.0.1.0/24) and Subnet B (10.0.2.0/24). You want instances in Subnet A to access the internet via a NAT gateway in Subnet B, but Subnet B should not have direct internet access. How should you configure the route tables?

hard

A. Associate Subnet A's route table with 0.0.0.0/0 to the NAT gateway; Subnet B's route table with no route to internet gateway

B. Associate Subnet A's route table with 0.0.0.0/0 to the internet gateway; Subnet B's route table with 0.0.0.0/0 to the NAT gateway

C. Associate both subnets' route tables with 0.0.0.0/0 to the internet gateway

D. Associate Subnet A's route table with a route 0.0.0.0/0 to the NAT gateway; Subnet B's route table with 0.0.0.0/0 to the internet gateway

Solution

1. Step 1: Understand NAT gateway purpose

   NAT gateway allows instances in private subnet (Subnet A) to access internet outbound.

2. Step 2: Configure Subnet B's route table (NAT subnet)

   Subnet B must have 0.0.0.0/0 to internet gateway so NAT can reach internet. Direct access for instances in B can be restricted via security groups.

3. Step 3: Configure Subnet A's route table

   Subnet A has 0.0.0.0/0 to NAT gateway.

4. Final Answer:

   Associate Subnet A's route table with a route 0.0.0.0/0 to the NAT gateway; Subnet B's route table with 0.0.0.0/0 to the internet gateway -> Option D

5. Quick Check:

   Private to NAT; NAT subnet to igw [OK]

Hint: Private subnet (A) routes to NAT; NAT subnet (B) routes to igw [OK]

Common Mistakes:

• Omitting igw route in NAT subnet (B), breaking NAT functionality
• Routing private subnet (A) directly to igw
• Confusing NAT gateway and internet gateway roles