AWS Certified Advanced Networking - Specialty: Exam Question on Creating Custom Route Tables in VPC

Understanding Custom Route Tables for AWS VPC Subnets

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Question

Your VPC currently has 2 subnets in AWS.

The VPC has a CIDR block of 10.0.0.0/16 and the subnets have a CIDR block of 10.0.1.0/24 and 10.0.2.0/24 respectively.

You decide on creating a custom route table for each subnet.

When you initially create a custom route table , which of the following routes will be present in the route table automatically,

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Explanations

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A. B. C. D.

Answer - A.

By default when the new route table is created , it will have a default route for the CIDR block of the VPC.The AWS documentation mentions the following.

Every route table contains a local route for communication within the VPC over IPv4

If your VPC has more than one IPv4 CIDR block, your route tables contain a local route for each IPv4 CIDR block.

If you've associated an IPv6 CIDR block with your VPC, your route tables contain a local route for the IPv6 CIDR block.

You cannot modify or delete these routes.

For more information on Route tables, please visit the below link:

http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Route_Tables.html

When you create a custom route table for a subnet in a VPC, the route table is initially empty, which means there will be no default routes in the route table.

A default route (also known as 0.0.0.0/0) is a special route that tells the VPC how to handle traffic for which there is no specific route defined. It acts as a catch-all route for all traffic that doesn't match any other routes in the table.

In this case, there is no need for a default route since both subnets have specific routes defined. The route table for the first subnet will have a route for 10.0.1.0/24 as the destination, and the route table for the second subnet will have a route for 10.0.2.0/24 as the destination.

It's worth noting that the VPC itself will have a default route that sends all traffic destined for outside the VPC to the Internet Gateway attached to the VPC. However, this default route is not specific to any subnet, so it won't appear in the custom route tables you create for individual subnets.