Main Route Table And Subnet Specific Route Table

Difference Between Main Route Table (Main RT) and Subnet-Specific Route Table (Custom RT) in AWS VPC

In AWS VPC, route tables (RTs) control how traffic is routed within the VPC and to external networks. There are two types of route tables:

1. Main Route Table (Default Route Table)

   • Created automatically when a VPC is created.
   • Controls routing for all subnets that do not have an explicit (custom) route table associated.
   • Can be modified but cannot be deleted.

2. Subnet-Specific (Custom) Route Table

   • Manually created by users.
   • Explicitly associated with one or more subnets to override the main route table.
   • Allows fine-grained control over routing for specific subnets.

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Key Differences

Feature	Main Route Table	Subnet-Specific Route Table
Creation	Automatically created with VPC.	Manually created by the user.
Association	Applies to subnets not explicitly linked to a custom RT.	Must be explicitly assigned to a subnet.
Modification	Can be edited but cannot be deleted.	Can be edited, replaced, or deleted.
Default Routes	Initially has only a local route (10.0.0.0/16 → local).	Starts empty (only local route if not modified).
Use Case	Best for subnets with default routing (e.g., private subnets).	Used for custom routing (e.g., public subnets with IGW, VPN, or peering).

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Example Scenario with Diagram

VPC Setup:

• VPC CIDR: 10.0.0.0/16
• Subnets:
  • Public Subnet: 10.0.1.0/24 (needs internet access via IGW)
  • Private Subnet: 10.0.2.0/24 (no internet access)

Route Tables:

1. Main Route Table (Default)

   • Routes:
     • 10.0.0.0/16 → local (default)
   • Associated with Private Subnet (since no custom RT is assigned).

2. Custom Route Table (Public-RT)

   • Routes:
     • 10.0.0.0/16 → local
     • 0.0.0.0/0 → igw-123 (Internet Gateway)
   • Explicitly associated with Public Subnet.

+---------------------VPC (10.0.0.0/16)---------------------+
|                                                           |
|  +------------------+        +------------------+        |
|  |   Public Subnet  |        |  Private Subnet  |        |
|  | (10.0.1.0/24)    |        | (10.0.2.0/24)    |        |
|  |                  |        |                  |        |
|  | EC2 Instance     |        | EC2 Instance     |        |
|  +--------▲---------+        +--------▲---------+        |
|           |                            |                 |
|  +--------▼---------+        +--------▼---------+        |
|  | Custom RT (Public)|        | Main RT (Default)|        |
|  | Routes:           |        | Routes:          |        |
|  | 10.0.0.0/16 → local|       | 10.0.0.0/16 → local|     |
|  | 0.0.0.0/0 → igw-123|       +------------------+        |
|  +------------------+                                    |
+----------------------------------------------------------+

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When to Use Which?

✅ Use Main Route Table:

• For subnets that don’t need special routing (e.g., private DB subnets).

✅ Use Custom Route Table:

• For public subnets (needs 0.0.0.0/0 → IGW).
• For VPN/Peering/NAT Gateways (requires custom routes).
• For isolated subnets (blocking internet access).

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How to Check & Modify in AWS Console?

1. Go to VPC Dashboard → Route Tables.
2. The Main RT is marked as “Yes” under “Main” column.
3. To assign a Custom RT to a subnet:
   • Select the RT → Subnet Associations → Edit → Choose Subnet.

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Key Takeaway

• Main RT = Default routing (applies if no custom RT is assigned).
• Subnet-Specific RT = Custom routing (overrides Main RT for selected subnets).