Using route tables to direct traffic appropriately (for example, automatic propagation, BGP)

What is the purpose of a route table in AWS, and how does it contribute to directing traffic within a VPC?

The purpose of a route table in AWS is to define rules, known as routes, that determine where network traffic from subnets or the VPC itself should be directed. Each subnet in a VPC must be associated with a route table, which tells the network how to handle traffic coming from and going to different IP addresses. Route tables help in setting up public and private subnets by controlling the flow of traffic to and from the internet or between the subnets.

Can you explain how to automatically propagate routes to a route table in an AWS VPC?

Automatic route propagation is done by enabling a Virtual Private Gateway (VGW) or a Direct Connect Gateway (DXGW) associated with a VPN connection or AWS Direct Connect to automatically update the VPC route tables. When you enable route propagation, the gateway propagates routes from the BGP advertisements, typically from on-premises networks, to your VPC route tables. You enable it by editing the route table and turning on the route propagation setting for the chosen VGW or DXGW.

Describe BGP and its role in managing traffic routes within AWS Direct Connect.

BGP, or Border Gateway Protocol, is the standard routing protocol used to exchange routing information over the internet and between different networks. Within AWS Direct Connect, BGP is used to exchange routing information between the AWS network and a customer’s on-premises network. It ensures that data transferred over Direct Connect uses the most efficient routes, and it allows for dynamic updates if network changes occur.

What is the difference between static and dynamic routes in AWS route tables, and when would you use each type?

Static routes are fixed paths that a network administrator manually sets up in the route table to direct traffic to a specific destination. They do not change unless manually updated. Dynamic routes, on the other hand, are automatically adjusted by the routing protocol in response to changes in the network, such as BGP. Static routes are typically used in simple or controlled environments with predictable traffic, while dynamic routes are preferred in complex, evolving environments for more flexibility and automation.

What metrics are used by BGP to determine the best path for data to travel across the internet or AWS network?

BGP uses attributes called path attributes, the most important of which are the following:

• AS_PATH: The sequence of AS (Autonomous System) numbers traffic will pass through, preferring shorter paths.
• NEXT_HOP: Determines the next router to which traffic is sent.
• LOCAL_PREF: Indicates the preference for a route within an AS; routes with a higher local preference are preferred.
• MULTI_EXIT_DISC (MED): Suggests to external AS the preferred entry point into an AS.

BGP uses these attributes to make informed decisions on choosing the best path for routing traffic.

How does AWS allow you to influence the BGP route selection process when using AWS Direct Connect?

AWS allows you to influence the BGP route selection process by using BGP attributes such as AS_PATH prepending, LOCAL_PREF, and MED. When configuring your BGP on AWS Direct Connect, you can set these attributes to manipulate the route priority and help BGP choose a particular path over another.

What is an IP prefix list in AWS, and how can it be utilized in conjunction with route tables?

An IP prefix list in AWS is a set of IP address ranges that can be used to filter traffic by matching IP address ranges in route tables or network ACLs. Prefix lists can be used to specify allowed or denied CIDR blocks, thus simplifying the management of access control and routing policies, particularly when dealing with multiple CIDR blocks.

Can you explain the concept of “most specific route” in AWS route tables and why it is important?

The “most specific route” in AWS route tables refers to the route with the longest subnet mask, or the smallest address range, that matches the destination IP address. This is important because AWS uses the longest prefix match rule when deciding which route to use for traffic. The most specific route is preferred as it is assumed to be the best path to reach the specific destination.

How does AWS ensure redundancy and failover for network traffic using route tables?

AWS ensures redundancy and failover by allowing multiple routes to be specified in the route tables for the same destination with different targets. AWS Route 53 can also be used in conjunction with health checks to route traffic away from unhealthy endpoints. Additionally, AWS Transit Gateway supports Equal-Cost Multi-Path routing (ECMP) for VPN connections, enabling you to use multiple paths to your on-premises network for failover and load balancing.

What are the implications of modifying a route table associated with a subnet that is currently in use in AWS?

Modifying a route table associated with a subnet currently in use can impact the flow of traffic to and from instances in the subnet. Changes can potentially lead to traffic being routed incorrectly, causing disruptions or security issues. It is important to thoroughly understand the implications and to plan changes carefully, considering all the dependencies and potential impacts.

When setting up BGP peering for AWS Direct Connect, what are the necessary steps for ensuring secure communication?

For ensuring secure BGP communication, you need to:

• Exchange public BGP router IPs between your network and AWS.
• Share BGP Autonomous System Numbers (ASN).
• Establish a BGP session on both ends.
• Use MD5 authentication for the BGP session.
• Optionally, filter traffic using prefix lists or route-maps.
• Ensure appropriate routing policies are in place to prevent potential routing issues.

Can you explain how route summarization in BGP affects AWS networking infrastructure and why it might be beneficial?

Route summarization in BGP involves consolidating a group of IP prefixes into a single, broader IP prefix. This reduces the number of routes that must be advertised, which simplifies the BGP table and can improve network performance and scalability. In an AWS context, summarization can minimize route propagation to the VPC route tables, leading to more manageable and efficient routing for AWS resources.