Antrea Multi-cluster Architecture

An Antrea Multi-cluster ClusterSet includes a leader cluster and multiple member clusters. Antrea Multi-cluster Controller needs to be deployed in the leader and all member clusters. A cluster can serve as the leader, and meanwhile also be a member cluster of the ClusterSet.

The diagram below depicts a basic Antrea Multi-cluster topology with one leader cluster and two member clusters.

ClusterSet is a placeholder name for a group of clusters with a high degree of mutual trust and shared ownership that share Services amongst themselves. Within a ClusterSet, Namespace sameness applies, which means all Namespaces with a given name are considered to be the same Namespace. The ClusterSet Custom Resource Definition(CRD) defines a ClusterSet including the leader and member clusters information.

The ClusterClaim CRD is used to claim a ClusterSet with a unique ClusterSet ID, and to claim the cluster itself as a member of a ClusterSet with a unique cluster ID.

The MemberClusterAnnounce CRD declares a member cluster configuration to the leader cluster.

The Common Area is an abstraction in the Antrea Multi-cluster implementation provides a storage interface for resource export/import that can be read/written by all member and leader clusters in the ClusterSet. The Common Area is implemented with a Namespace in the leader cluster for a given ClusterSet.

Antrea Multi-cluster Controller implements ClusterSet management and resource export/import in the ClusterSet. In either a leader or a member cluster, Antrea Multi-cluster Controller is deployed with a Deployment of a single replica, but it takes different responsibilities in leader and member clusters.

In the leader cluster, Multi-cluster controller watches and validates the ClusterSet and Clusterclaim CRs, and initializes the ClusterSet. It also validates the MemberClusterAnnounce CR created by a member cluster and adds the cluster to the ClusterSet CR’s member cluster list.

In a member cluster, Multi-cluster controller watches exported resources (e.g. ServiceExports, Services, Multi-cluster Gateways), encapsulates an exported resource into a ResourceExport and creates the ResourceExport CR in the Common Area of the leader cluster.

In the leader cluster, Multi-cluster Controller watches ResourceExports created by member clusters (in the case of Service and ClusterInfo export), or by the ClusterSet admin (in the case of Multi-cluster NetworkPolicy), converts ResourceExports to ResourceImports, and creates the ResourceImport CRs in the Common Area for member clusters to import them. Multi-cluster Controller also merges ResourceExports from different member clusters to a single ResourceImport, when these exported resources share the same kind, name, and original Namespace (matching Namespace sameness).

Multi-cluster Controller in a member cluster also watches ResourceImports in the Common Area of the leader cluster, decapsulates the resources from them, and creates the resources (e.g. Services, Endpoints, Antrea ClusterNetworkPolicies, ClusterInfoImports) in the member cluster.

For more information about multi-cluster Service export/import, please also check the Service Export and Import section.

Antrea Multi-cluster Controller implements Service export/import among member clusters. The above diagram depicts Antrea Multi-cluster resource export/import pipeline, using Service export/import as an example.

Given two Services with the same name and Namespace in two member clusters - and foo.ns.cluster-b.local, a multi-cluster Service can be created by the following resource export/import workflow.

• User creates a ServiceExport foo in Namespace ns in each of the two clusters.
• Multi-cluster Controller in the leader cluster sees the ResourcesExports in the Common Area, including the two for Service foo: cluster-a-ns-foo-service, cluster-b-ns-foo-service; and the two for the Endpoints: cluster-a-ns-foo-endpoints, cluster-b-ns-foo-endpoints. It then creates a ResourceImport ns-foo-service for the multi-cluster Service; and a ResourceImport ns-foo-endpoints for the Endpoints, which includes the exported endpoints of both cluster-a-ns-foo-endpoints and cluster-b-ns-foo-endpoints.
• Multi-cluster Controller in each member cluster watches the ResourceImports from the Common Area, decapsulates them and gets Service ns/antrea-mc-foo and Endpoints ns/antrea-mc-foo, and creates the Service and Endpoints, as well as a ServiceImport foo in the local Namespace ns.

Antrea started to support Multi-cluster Gateway since v1.7.0. User can choose one K8s Node as the Multi-cluster Gateway in a member cluster. The Gateway Node is responsible for routing all cross-clusters traffic from the local cluster to other member clusters through tunnels. The diagram below depicts Antrea Multi-cluster connectivity with Multi-cluster Gateways.

Antrea Agent is responsible for setting up tunnels between Gateways of meamber clusters. At the moment, Multi-cluster Gateway only works with Antrea encap mode. The tunnels between Gateways use Antrea Agent’s configured tunnel type. All member clusters in a ClusterSet need to deploy Antrea with the same tunnel type.

The Multi-cluster Gateway implementation introduces two new CRDs Gateway and . Gateway includes the local Multi-cluster Gateway information including: internalIP for tunnels to local Nodes, and gatewayIP for tunnels to remote cluster Gateways. ClusterInfoImport includes Gateway and network information of member clusters, including Gateway IPs and Service CIDRs. The existing esource export/import pipeline is leveraged to exchange the cluster network information among member clusters, generating ClusterInfoImports in each member cluster.

Let’s use the ClusterSet in the above diagram as an example. As shown in the diagram:

1. Cluster A has a client Pod named pod-a running on a regular Node, and a multi-cluster Service named antrea-mc-nginx with ClusterIP 10.112.10.11 in the default Namespace.
2. Cluster B exported a Service named nginx with ClusterIP 10.96.2.22 in the default Namespace. The Service has one Endpoint 172.170.11.22 which is pod-b’s IP.
3. Cluster C exported a Service named nginx with ClusterIP 10.11.12.33 also in the default Namespace. The Service has one Endpoint 172.10.11.33 which is pod-c’s IP.

The multi-cluster Service antrea-mc-nginx in cluster A will have two Endpoints:

• Service’s ClusterIP 10.11.12.33 from cluster C.

When the client Pod pod-a on cluster A tries to access the multi-cluster Service antrea-mc-nginx, the request packet will first go through the Service load balancing pipeline on the source Node node-a2, with one endpoint of the multi-cluster Service being chosen as the destination. Let’s say endpoint 10.11.12.33 from cluster C is chosen, then the request packet will be DNAT’d with IP 10.11.12.33 and tunnelled to the local Gateway Node node-a1. node-a1 knows from the destination IP (10.11.12.33) the packet is multi-cluster Service traffic destined for cluster C, and it will tunnel the packet to cluster C’s Gateway Node node-c1, after performing SNAT and setting the packet’s source IP to its own Gateway IP. On node-c1, the packet will go through the Service load balancing pipeline again with an endpoint of Service nginx being chosen as the destination. As the Service has only one endpoint - 172.10.11.33 of pod-c, the request packet will be DNAT’d to 172.10.11.33 and tunnelled to node-c2 where pod-c is running. Finally, on node-c2 the packet will go through the normal Antrea forwarding pipeline and be forwarded to pod-c.

At this moment, Antrea does not support Pod-level policy enforcement for cross-cluster traffic. Access towards multi-cluster Services can be regulated with Antrea ClusterNetworkPolicy toService rules. In each member cluster, users can create an Antrea ClusterNetworkPolicy selecting Pods in that cluster, with the imported Mutli-cluster Service name and Namespace in an egress toService rule, and the Action to take for traffic matching this rule. For more information regarding Antrea ClusterNetworkPolicy (ACNP), refer to .