Introduction to AWS Transit Gateway: Simplifying Network Connectivity
Published: 2023-09-07 00:01:59

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title: Introduction to AWS Transit Gateway: Simplifying Network Connectivity
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### Introduction to AWS Transit Gateway: Simplifying Network Connectivity

The network architecture in the cloud can quickly become complex as the number of VPCs, accounts, regions, on-premises data centers, and edge locations grows. Transit Gateways allow customers to simplify the network layout and connectivity between all these environments.

## What is AWS Transit Gateway?

AWS Transit Gateway is a service that acts as a hub to connect VPCs and on-premises networks. It acts as a central routing engine that eliminates the need for each VPC to have individual connections between them.

With Transit Gateway, you only need to create connections from the VPCs, VPNs, and Direct Connect links to the Transit Gateway. Transit Gateway will then dynamically route traffic between all the connected networks.

## Benefits of Using Transit Gateway

Some of the main benefits of using Transit Gateway include:

- **Simplified network topology** - No need for mesh network between VPCs. Just connect each VPC to the Transit Gateway.
- **Scalability** - Easily scale up to tens of thousands of VPC and remote office connections.
- **Reduced operational complexity** - Centralized connectivity configuration and no need to manage routing tables between VPCs.
- **Shared network transit** - Allows different accounts and VPCs to use the same Transit Gateway.
- **On-premises connectivity** - Connect seamlessly to on-premises data centers.

## Transit Gateway Use Cases

Transit Gateway is a versatile service that can cater to a variety of use cases:

- Connect VPCs across multiple accounts and AWS Regions.
- Create a hub-and-spoke model for segmented networks.
- Share centralized internet connectivity across accounts.
- Migrate from a mesh or hub-and-spoke model to a Transit Gateway.
- Connect remote offices and data centers to AWS.

## Getting Started with Transit Gateway

To start using Transit Gateway, you need to perform the following steps:

### 1. Create a Transit Gateway in a specific region

To get started with Transit Gateway, the first step is to create the Transit Gateway resource in your desired AWS region.

When creating the Transit Gateway, you need to specify a name tag so it can be easily identified. You also have the option to enable DNS support if you need resolution between your connected networks.

Some key considerations when creating the Transit Gateway:

- Transit Gateways are regional resources. So you need to decide which region makes the most sense as the connectivity hub for your use case.
- By default, a new Transit Gateway will be created in the default VPC for the region. You can choose to create it in a custom VPC if required.
- Select the appropriate size for your Transit Gateway based on the expected network traffic volume. Sizing can be adjusted later if needed.
- You can enable sharing with other accounts upon creation or do it later. Account sharing allows connections from other accounts.
- Logging can be enabled to track connection activity and events. The logs will be sent to CloudWatch Logs.

Once the Transit Gateway is created, you will get an ID for it that is needed to attach VPCs and other networks. It takes some time for the Transit Gateway to be ready for use after creation.

So those are some of the key options to consider when creating your Transit Gateway in the region of your choice. The console wizard will guide you through all the necessary configuration.

### 2. Attach VPCs by creating Transit Gateway attachments

Once the Transit Gateway is created, the next step is to start attaching VPCs. Each VPC that needs to connect to the Transit Gateway needs to have an attachment created.

Some key points when creating VPC attachments:

- You can attach VPCs from the same account as the Transit Gateway or from other accounts if account sharing is enabled.
- For each VPC attachment, you need to provide the ID of the Transit Gateway, the ID of the VPC, and the subnets to associate.
- An attachment propagation setting determines whether routes get automatically propagated to the VPC route table. You can enable or disable propagation.
- Option to enable DNS support for private IP addresses in the VPC to be accessible across networks.
- You can control access to the VPC by adding a transit gateway route table and using resource attachments.
- Creating an attachment adds an entry in the VPC route table with the Transit Gateway as the target.
- Attachment creation takes time to complete. The VPC can start sending traffic to the Transit Gateway once the state changes to available.
- You can create multiple attachments from the same VPC for redundancy and scaling.

The Transit Gateway provides connectivity between the VPCs as soon as the attachments are created and routes propagated. So you can build out connectivity to more VPCs incrementally.

### 3. For on-premises connectivity, create VPN or Direct Connect attachments

The Transit Gateway allows you to connect your on-premises networks and data centers using VPN or Direct Connect links.

For VPN connectivity, you need to create an AWS Site-to-Site VPN connection from your customer gateway router to the Transit Gateway. The customer gateway can be a physical device or a software appliance.

To create the VPN attachment:

- Provide the Transit Gateway ID, customer gateway ID, VPN connection ID.
- Configure the inside and outside IP addresses for the VPN tunnel.
- Specify the AWS side ASN for BGP routing.
- Enable route propagation to exchange routes between the Transit Gateway and on-premises network.

For Direct Connect connectivity, you need to link your Direct Connect connection or LAG to the Transit Gateway.

To create the Direct Connect attachment:

- Specify the Direct Connect connection ID and the Transit Gateway ID.
- Provide the inside and outside IP addresses.
- Enable BGP for propagating routes.
- Specify the ASNs for the AWS and customer side.
- Enable route propagation.

The attachment creation process will take some time to complete. Once available, your on-premises network will be able to connect to the VPCs and networks attached to the Transit Gateway.

You can create multiple VPN or Direct Connect attachments for high availability and failover between your data center and Transit Gateway.

### 4. Configure route tables to define traffic flow between connections

Here are some more details on configuring route tables with AWS Transit Gateway to define traffic flow between the connected networks:

Transit Gateway uses route tables to determine how traffic should flow between the VPCs, VPNs, and Direct Connect attachments.

Some key points on configuring route tables:

- By default, there is a default route table that allows full communication between all attachments and VPCs.
- You can create additional, custom route tables that can selectively allow or deny traffic between resources.
- Route tables can be associated with VPC or VPN/Direct Connect attachments to control which networks they can communicate with.
- Each route table can have multiple route table associations and propagations.
- Associations determine which attachments can route traffic using the route table. Propagations automatically add routes to the associated attachments.
- Routes can be manually added, for example, to route traffic for a particular VPC subnet to an internet gateway.
- You can create complex network segmentation policies by leveraging multiple route tables.

For example, you can create tiers like Public, Private, Restricted and assign VPC subnets to them via route tables.

- Route priorities determine which route takes effect if there are multiple routes to a destination.

By leveraging custom route tables, you can dial in fined-grained control over how traffic flows between your connected networks using Transit Gateway.

### 5. Share the Transit Gateway with other accounts

Transit Gateways can be shared with other AWS accounts to allow inter-account connectivity. Here are some key points on sharing:

- When you create a Transit Gateway, you can enable sharing with other accounts. This allows accounts you authorize to attach their VPCs.
- To enable sharing, you need to provide the account ID or organization ARN with which the Transit Gateway will be shared.
- You can share the Transit Gateway only within the same AWS organization if you have enabled Resource Access Manager.
- The owner account has full control over the Transit Gateway. Shared accounts have limited privileges.
- Shared accounts can view and work only with their own VPC attachments and route tables.
- To simplify management, shared accounts can be provided access via IAM to work with attachments and routes.
- Sharing is transitive. If Account A shares the TGW with Account B, and B shares with C, then C can also use the TGW.
- For security, enable VPC route table propagation sparingly for shared accounts.
- Use RAM resources to allow sharing Transit Gateways across regions.

By sharing Transit Gateways, you can significantly simplify connectivity and reduce provisioning time across different accounts in your organization. But balance the convenience with appropriate access controls.

AWS provides an easy-to-use wizard in the console to guide you through the configuration process.

## Conclusion

AWS Transit Gateway simplifies cloud network architectures by acting as a hub to interconnect your VPCs, VPNs, and data centers. It eliminates complex mesh topologies and provides easy scalability, centralized management, and secure network segmentation. As your cloud footprint grows, Transit Gateway is key to maintaining a simple, efficient, and secure network topology.