Advertising Kubernetes services over BGP

About advertising Kubernetes services over BGP

Calico supports advertising Kubernetes service cluster IPs over BGP, just as it advertises pod IPs. This means that, if your Calico deployment is configured to peer with BGP routers outside the cluster, those routers - plus any further upstream places that those routers propagate to - will be able to send traffic to a Kubernetes service cluster IP, and that traffic will be routed to one of the available endpoints for that service.

You may want to enable this feature if you would like Kubernetes services to be accessible outside of the cluster without the need for a dedicated load balancer.

Prerequisites

BGP peers

You must have established BGP connections to one or more routers outside of your Kubernetes cluster. See configuring BGP peers for more information.

For ECMP load balancing to services, the upstream routers must be configured to use BGP multipath.

Enabling service cluster IP advertisement

To enable this feature, first determine the cluster IP range in use for your cluster. This value is passed to the Kubernetes API server using the --service-cluster-ip-range option, and defaults to 10.0.0.0/24. See the API server reference guide for more information.

Then, set the CALICO_ADVERTISE_CLUSTER_IPS environment variable in the calico-node daemon set equal to the service cluster IP range for the cluster.

The following command will enable service cluster IP advertisement on a running cluster, assuming the default cluster IP range of 10.0.0.0/24.

kubectl patch ds -n kube-system calico-node --patch \
    '{"spec": {"template": {"spec": {"containers": [{"name": "calico-node", "env": [{"name": "CALICO_ADVERTISE_CLUSTER_IPS", "value": "10.0.0.0/24"}]}]}}}}’

Behavior

Calico implements external traffic policy for Kubernetes services using normal BGP route processing and ECMP routing.

  • traffic to the cluster IP for a service with externalTrafficPolicy: Local will be load-balanced across the nodes with endpoints for that service

  • traffic to the cluster IP for a service with externalTrafficPolicy: Cluster will be load-balanced across all the nodes in the cluster.

In order to implement this behavior, Calico does the following.

  • the service cluster IP CIDR (for example, 10.96.0.0/12) is advertised from every node in the cluster

  • for each active service with externalTrafficPolicy: Local, the cluster IP for that service is advertised as a /32 route from the nodes that have endpoints for that service.