Cross-Region Kubernetes: The Ultimate Guide to Pod Communication

Bridging the Divide: How to Seamlessly Connect Pods Across Geographical Boundaries

As organizations scale their Kubernetes deployments globally, a common challenge emerges: how do you enable efficient and secure pod-to-pod communication between clusters spanning different regions, VPCs, and even cloud providers? Whether you're implementing disaster recovery, reducing latency for global users, or building a hybrid cloud infrastructure, mastering cross-cluster connectivity is essential.

In this comprehensive guide, we'll explore the conceptual foundations of connecting distributed Kubernetes ecosystems, with special focus on AWS EKS implementations across multiple regions.

The Challenge of Cross-Region Kubernetes

Before diving into solutions, let's understand what makes cross-region pod communication challenging:

1. Network Isolation: Kubernetes clusters in different VPCs or regions are network-isolated by default

2. Service Discovery: Pods in one cluster can't naturally discover services in another

3. DNS Resolution: Kubernetes DNS services are cluster-scoped

4. Security Boundaries: Cross-region traffic must maintain security standards

5. Latency Considerations: Inter-region communication adds latency

6. Cost Management: Cross-region data transfer incurs costs

Foundational Networking: Creating the Infrastructure Layer

Before Kubernetes services can communicate across clusters, we need to establish the underlying network connectivity. There are two primary approaches when working with AWS EKS:

Transit Gateway: The Enterprise Hub-and-Spoke Approach

AWS Transit Gateway provides a scalable, managed solution for connecting multiple VPCs and on-premises networks, acting as a network transit hub.

Conceptual Overview:

Think of Transit Gateway as a cloud router that connects all your VPCs. Each EKS cluster resides in a VPC, and Transit Gateways in different regions connect to each other via peering connections. This creates a backbone network that allows pods to communicate across regions.

Key Components:

• Transit Gateway in each region

• Transit Gateway Peering between regions

• VPC Attachments connecting each cluster's VPC to its regional Transit Gateway

• Route Tables directing traffic between VPCs

Advantages:

• Highly scalable for multiple VPCs and regions

• Centralized network management

• Supports transitive routing (A can talk to C through B)

• AWS-managed redundancy and high availability

Disadvantages:

• More complex setup

• Higher cost than direct VPC peering

• Additional hop in the network path

VPC Peering: The Straightforward Approach

VPC Peering provides a simpler alternative for connecting two VPCs directly, establishing a networking connection between them.

Conceptual Overview:

VPC Peering creates a direct highway between two VPCs, allowing resources in either VPC to communicate with each other as if they were in the same network. For EKS clusters, this means pods can directly reach pods or services in the peered VPC using private IP addresses.

Key Components:

• VPC Peering Connection between regional VPCs

• Route Tables in each VPC pointing to the peer CIDR blocks

• Security Groups allowing traffic between VPCs

Advantages:

• Simpler to set up and manage

• Direct connection between VPCs

• Lower latency than Transit Gateway

• No additional cost beyond standard data transfer rates

Disadvantages:

• No transitive routing (requires mesh connections for multiple VPCs)

• Limited to 125 peering connections per VPC

• No centralized management for multiple connections

Service Connectivity: How Pods Actually Communicate

Once the network infrastructure is in place, we need to address how Kubernetes services discover and communicate with each other across clusters.

Basic Pod-to-Pod Communication

With either Transit Gateway or VPC Peering established, pods can technically communicate directly using IP addresses. However, this approach has significant limitations:

• IP Address Management: Cluster CIDR ranges must not overlap

• No Service Discovery: Requires hardcoding IP addresses

• No Load Balancing: Traffic doesn't get distributed across service endpoints

• No Health Checking: Unhealthy pods still receive traffic

• Manual Updates: Changes require configuration updates

This approach works for simple testing but isn't viable for production environments.

Internal Load Balancers + DNS

A step up from direct IP communication is exposing services via internal load balancers and using DNS for discovery:

How It Works:

1. Expose services in each cluster using Kubernetes Service type LoadBalancer with internal-only accessibility

2. Configure AWS Route 53 Private Hosted Zones shared across VPCs

3. Register the load balancer endpoints in Route 53

4. Services in one cluster connect to services in another cluster using DNS names

Advantages:

• Uses Kubernetes native Service construct

• Provides load balancing across pods

• Abstracts away direct IP dependencies

• Leverages DNS for service discovery

Limitations:

• Requires additional AWS resources (NLBs/ALBs)

• Manual DNS management or additional tooling like ExternalDNS

• No automatic failover between clusters

• Additional hop through the load balancer

Kubernetes MultiCluster Services API

The Kubernetes MultiCluster Services (MCS) API provides a more native approach to connecting services across clusters:

Conceptual Overview:

The MCS API extends Kubernetes' native service discovery and load balancing capabilities across cluster boundaries. It introduces two key resources:

1. ServiceExport: Marks a Service in one cluster to be exposed to other clusters

2. ServiceImport: Represents a service from another cluster

How It Works:

1. A service is created in Cluster A

2. The service is exported using ServiceExport in Cluster A

3. A ServiceImport is created in Cluster B

4. Pods in Cluster B can now discover and connect to the service in Cluster A using the imported service name

Key Components:

• MultiCluster Service Controller: Manages ServiceExport and ServiceImport resources

• DNS Controller: Configures DNS resolution for imported services

• Endpoint Slices: Distributes endpoint information across clusters

Implementation Options:

• Submariner: Implements the MCS API and handles both service discovery and network connectivity

• Istio/Service Mesh: Many service meshes now support multi-cluster service discovery

Advantages:

• Kubernetes-native approach

• Automated service discovery

• Support for standard Kubernetes features like health checks and load balancing

• Abstracts away underlying network complexity

Current Status:

The MCS API is still evolving, with implementations varying in maturity. It's part of the Kubernetes SIG-Multicluster work, aiming to standardize cross-cluster service communication.

Submariner: Dedicated Cross-Cluster Connectivity

Submariner is a CNCF project specifically designed to enable direct networking between multiple Kubernetes clusters.

Conceptual Overview:

Submariner creates secure tunnels between clusters, establishing a flat network that allows pods in one cluster to directly communicate with pods and services in remote clusters. It's a comprehensive solution that handles both the networking layer and service discovery aspects.

How It Works:

1. Deploy Submariner operators in each cluster

2. Designate one cluster as the broker (central coordination point)

3. Join each cluster to the broker, establishing encrypted IPsec tunnels

4. Submariner handles IP address management, ensuring non-overlapping CIDRs

5. The Lighthouse component provides DNS-based service discovery

Key Components:

• Gateway Nodes: Run the Submariner Gateway engine to establish secure tunnels

• Broker: Coordinates the exchange of connection information

• Route Agent: Manages routing rules on each node

• Lighthouse: Provides DNS discovery for cross-cluster services

• Service Discovery API: Implements the Kubernetes MultiCluster Services API

Advantages:

• Full stack solution covering both networking and service discovery

• Multiple connectivity options (including over the internet)

• Support for service discovery via the MultiCluster Services API

• TCP and UDP connectivity without requiring application changes

• NAT traversal capabilities

Disadvantages:

• Requires gateway nodes with public IP addresses for internet-based connectivity

• More networking overhead compared to application-layer solutions

Skupper: Application Layer Virtual Network

Skupper takes a different approach by creating an application-level virtual network without requiring changes to the underlying network infrastructure.

Conceptual Overview:

Unlike Submariner, which operates at the network layer (L3), Skupper works at the application layer (L7). It creates secure application-level connections between services across clusters without requiring VPN tunnels, special firewall rules, or cluster admin privileges.

How It Works:

1. Deploy Skupper routers in each namespace that needs cross-cluster communication

2. Create token-based links between Skupper router instances

3. Expose services you want to be accessible from other clusters

4. Skupper handles routing requests to the appropriate destination

Key Components:

• Skupper Router: Deployed as a pod in each namespace

• Router Console: Web interface for visualizing connections

• Service Interface: Allows services to be accessed across clusters

• Token-based Links: Secure connections between router instances

Advantages:

• Works without cluster admin privileges

• No need to modify underlying network infrastructure

• Operates through existing firewalls

• Minimal networking knowledge required

• Supports multi-cluster, multi-cloud, and hybrid cloud scenarios

Disadvantages:

• Application layer only (no direct pod-to-pod L3 connectivity)

• Higher latency compared to network-layer solutions

• Less transparent to applications than L3 solutions

Cross-Cluster Service Mesh Approach

Service meshes provide an application-level networking layer that can span multiple clusters:

Conceptual Overview:

A service mesh creates an overlay network of proxies (sidecars) that handle service-to-service communication. When extended across clusters, it provides unified service discovery, traffic management, and security.

How It Works:

1. Deploy service mesh control plane with multi-cluster configuration

2. Install service mesh data plane (proxies) in each cluster

3. Configure the mesh to share service registries across clusters

4. Services discover and communicate with each other through the mesh

Popular Service Mesh Options:

• Istio: Robust multi-cluster support with various deployment models

• Linkerd: Lighter-weight option with multi-cluster capabilities

• AWS App Mesh: AWS-native service mesh with cross-cluster support

• Consul Connect: HashiCorp's service mesh with multi-datacenter support

Advantages:

• Unified service discovery across clusters

• Advanced traffic management capabilities

• Consistent security policies (mTLS)

• Rich observability

• Works with or without direct network connectivity

Disadvantages:

• Increased complexity

• Performance overhead from proxies

• Learning curve for operations teams

Putting It All Together: Reference Architectures

Let's explore how these components fit together in different scenarios:

Reference Architecture 1: Transit Gateway + MCS API

In this architecture, Transit Gateway provides the network foundation, while the MultiCluster Services API handles service discovery:

1. Network Layer:

   • Transit Gateways connect VPCs across regions

   • Security groups allow cluster-to-cluster communication

   • Route tables direct traffic between VPCs

2. Service Discovery Layer:

   • MCS API controllers run in each cluster

   • Services are exported from source clusters

   • Services are imported in destination clusters

   • CoreDNS is configured to resolve cross-cluster service names

3. Communication Flow:

   • Pod in Cluster A wants to reach service-b in Cluster B

   • Pod looks up service-b using Kubernetes DNS

   • DNS returns endpoints for service-b in Cluster B

   • Pod connects to service-b using private IP routing through Transit Gateway

   • Traffic reaches pods backing service-b in Cluster B

Reference Architecture 2: VPC Peering + Service Mesh

This architecture uses VPC Peering for network connectivity and a service mesh for service discovery and advanced traffic management:

1. Network Layer:

   • VPC Peering connects cluster VPCs

   • Security groups allow mesh-to-mesh communication

   • Route tables direct traffic between VPCs

2. Service Mesh Layer:

   • Service mesh control plane spans clusters

   • Sidecars are injected alongside application pods

   • Mesh configuration defines multi-cluster service registry

3. Communication Flow:

   • Pod in Cluster A wants to reach service-b

   • Request goes to local sidecar proxy

   • Proxy consults mesh registry to locate service-b in Cluster B

   • Proxy routes request to service-b endpoints through VPC Peering

   • Remote proxy receives the request and forwards to local service-b pod

   • All communication is secured with mTLS

Implementation Considerations and Best Practices

Regardless of the approach you choose, consider these crucial aspects:

CIDR Planning

Careful IP address planning is essential for cross-cluster communication:

• Ensure non-overlapping CIDR blocks for pod and service networks across clusters

• Plan for future growth and additional clusters

• Document IP allocation to prevent future conflicts

• Consider using a tool like IPAM (IP Address Management) for larger deployments

Service Discovery and DNS Configuration

DNS is often the glue that makes cross-cluster communication work:

• Configure CoreDNS for stub domains to forward queries for remote cluster services

• Use consistent naming conventions for services across clusters

• Consider global DNS solutions like Route 53 Private Hosted Zones

• Implement DNS caching to reduce lookup latency

Security Considerations

Cross-cluster communication introduces new security challenges:

• Implement mTLS for all pod-to-pod communication

• Use network policies to restrict traffic flows within and between clusters

• Follow least-privilege principles for IAM roles

• Encrypt all cross-region traffic

• Implement consistent security policies across clusters

• Consider using service mesh authorization policies for fine-grained access control

Operational Readiness

Monitoring and troubleshooting cross-cluster setups require special attention:

• Implement distributed tracing across clusters

• Set up centralized logging for cross-cluster requests

• Monitor cross-cluster network metrics

• Create runbooks for troubleshooting connectivity issues

• Regularly test failover scenarios

Solution Comparison

Conclusion

Connecting Kubernetes pods across regions and VPCs requires addressing both the infrastructure networking layer and the Kubernetes service discovery layer. The MultiCluster Services API represents a significant step toward standardizing cross-cluster communication in Kubernetes, though it's still evolving.

For most organizations, the journey to multi-cluster connectivity begins with establishing solid network infrastructure (via Transit Gateway or VPC Peering) and then layering on service discovery mechanisms that fit their operational model—whether that's using internal load balancers with DNS, implementing the MCS API, or deploying a service mesh.

As your Kubernetes footprint grows globally, investing in a well-designed cross-cluster communication strategy will pay dividends in resilience, performance, and operational simplicity.

References

• Difference between VPC Peering and Transit Gateway

• Multi Cluster setup with Istio

• Multicluster Services API

• Submariner

• Skupper

• CoreDNS with Custom Stub Domains

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About the Author: Gourav Shah is a cloud infrastructure and devops architect specializing in distributed Kubernetes deployments for global enterprises.