How to choose between CoIP and Direct VPC routing modes on AWS Outposts rack

This blog post is written by Sumit Menaria, Senior Hybrid Solutions Architect AWS WWSO Core Services.

AWS Outposts Rack is a fully-managed service that extends AWS infrastructure, services, APIs, and tools to customer premises. By providing local access to AWS managed infrastructure and services, Outposts rack enables customers to build and run applications on premises using the same programming interfaces as in AWS Regions, while using local compute and storage resources for low latency, local data processing, and data residency needs.

There are various data sources on premises that you might want to connect from your Outpost. These sources can include field devices, on-premises databases, mainframes, storage arrays, or end users. Each Outpost supports a single Local Gateway (LGW) construct, which enables connectivity from your Outpost subnets to an on-premises network. Note that this post is specific to Outposts racks and a different method of local communication is used for AWS Outposts servers.

Two different options for facilitating communication between your Outpost based resources and on-premises network: Direct VPC routing and customer-owned IP pool. Both of these are mutually exclusive options, and routing works differently based on your choice of the mode. The two modes are the attributes of the LGW route table that your Outpost subnets’ VPC is associated with, which specifies the communication mode for the Outpost subnets.

Direct VPC routing mode

Direct VPC routing uses the private IP address of the instances in the VPC CIDR block to facilitate communication with your on-premises network. These addresses are advertised to your on-premises network with Border Gateway Protocol (BGP). Advertisement via BGP is only for the private IP addresses that belong to the subnets on your Outpost and have a route pointing to the LGW via the subnet’s route table. This type of routing is the default mode for Outposts Rack. In this mode, the LGW doesn’t perform Network Address Translation (NAT) for instances. Furthermore, you don’t have to assign an Elastic IP address to your Amazon Elastic Compute Cloud (Amazon EC2) instance from a (CoIP) to enable communication with your on-premises resources.

In this diagram, when the instance Y wants to communicate with an on-premises server, it traverses the LGW and can talk to the on-premises server using its source address (10.0.1.11) in the Subnet CIDR range (10.0.1.0/24) that is advertised over BGP from the LGW to the Customer Network Device. Similarly when the on-premises server wants to initiate communication with the Outpost based EC2 instance, it uses the instance’s private IP address (10.0.1.11) as the destination IP address to set up the connection.

CoIP mode

Utilizing CoIP mode means that you must provide a separate IP address range from your on-premises IP space for AWS to create an address pool, known as a CoIP. With CoIP, when an Outpost based resources, such as EC2 instances, Application Load Balancer (ALB), or Amazon Relational Database Service (Amazon RDS) instances, need to communicate to your on-premises network, the Local Gateway will perform 1:1 NAT from the resource’s private IP address from the Outpost subnet range to an IP address from the CoIP pool. The subnet-to-CoIP address mapping is done by assigning an Elastic IP (EIP) from the CoIP address range allocated for resources such as EC2 instances. To enable the communication with the CoIP pool from the on-premises network, and then the LGW advertises the CoIP pool through BGP over its peering with the Customer Network Device.

In this diagram, when the instance Y wants to talk to an on-premises server, the traffic traverses the LGW and the source IP address (10.0.1.11) of the instance gets translated to an IP address (192.168.0.11) in the CoIP range that is associated with the instance. Similarly, when the on-premises server initiates the communication, the request will be sent with the CoIP address (192.168.0.11) of the instance as the destination IP address. This will be changed to the instance’s private IP address (10.0.1.11) via NAT at the LGW. The CoIP pool (192.168.0.0/26) is advertised via BGP to the Customer Network Device to provide the route to the on-premises environment for reaching the Outpost based resources.

When to choose CoIP routing mode

CoIP is particularly useful when you want to isolate your Outpost based workloads from the on-premises infrastructure and only need specific resources on the Outpost to be able to communicate to the on-premises infrastructure. This is useful in situations where large enterprise networks have hundreds of IP pools allocated and there is a high chance of overlap between IP addresses allocated to Outpost based VPCs/Subnets and those allocated to on-premises infrastructure. Furthermore, CoIP can act as another layer of security, as you may choose to allocate the CoIP addresses to only the resources which must communicate with the on-premises network. Then you can allocate for the rest of them using the subnet private IP address range for communication within the Outpost or Region based resources.

This means that you don’t need to have the number of IPs in the CoIP pool be equal to the number of resources on your Outpost. For example, you may choose to configure a /26 CoIP range and a /22 pool for subnets to meet your workload requirements.

CoIP mode can also be useful when using an external ALB on Outpost and you want to make it routable through the local internet connectivity. By using a smaller internet routable CoIP address range assignment for your external ALB, you can route traffic to the ALB on the Outpost without needing to traverse through the internet gateway (IGW) in the parent Region.

When to choose Direct VPC routing mode

You can choose Direct VPC routing if you don’t want the operational overhead of managing the additional IP pools for NAT between your Outpost based resources and on-premises network. There are also few applications which may not work well if there is an NAT of IPs between the two endpoints communicating with each other. Some examples you may see are Active Directory communication with on-premises based servers, or iSCSI mount of your instances as an additional storage to on-premises Storage Area Network (SAN). These applications may not work or may need additional tuning if they encounter NATed IP addresses between an Outpost based client on an EC2 instance and on-premises based server for a two-way communication.

When Direct VPC routing mode is used, multiple VPCs can be associated to an Outpost LGW route table, and the Outpost subnets with the LGW as the route target, are automatically advertised to the on-premises network through BGP. Therefore, you must make sure that appropriate IP planning is in place to avoid any overlap of the Outpost VPC/Subnet IP range with the on-premises IP range, as they are directly advertised from LGW toward the Customer Network Device. Having overlapping IP subnets in your network can lead to undesired effects on your application connectivity and you must pay special attention when allocating IP pools for your on-premises and Outpost based VPC address space. You can use Amazon VPC IP Address Manager (IPAM) to plan the IP space of your VPCs and CoIP pools, as well as add on-premises based IP Pools using manual allocation.

Conclusion

You can select either Direct VPC routing or CoIP mode for routing through an Outpost Local Gateway. Since this selection affects the routing for all of the subnets on your Outpost associated with the LGW route table, it should be selected based on your workload requirements and existing IP infrastructure planning. You can also change the LGW route table mode at a later stage. However, that involves network disruption and the creation of a new LGW route table. To learn more about Outposts Racks routing, visit the LGW Route table documentation.