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As a cluster administrator, you can add the MetalLB Operator to your cluster so that when a service of type LoadBalancer is added to the cluster, MetalLB can add a fault-tolerant external IP address for the service. The external IP address is added to the host network for your cluster.

When to use MetalLB

Using MetalLB is valuable when you have a bare-metal cluster, or an infrastructure that is like bare metal, and you want fault-tolerant access to an application through an external IP address.

You must configure your networking infrastructure to ensure that network traffic for the external IP address is routed from clients to the host network for the cluster.

After deploying MetalLB with the MetalLB Operator, when you add a service of type LoadBalancer, MetalLB provides a platform-native load balancer.

MetalLB Operator custom resources

The MetalLB Operator monitors its own namespace for two custom resources:

MetalLB

When you add a MetalLB custom resource to the cluster, the MetalLB Operator deploys MetalLB on the cluster. The Operator only supports a single instance of the custom resource. If the instance is deleted, the Operator removes MetalLB from the cluster.

AddressPool

MetalLB requires one or more pools of IP addresses that it can assign to a service when you add a service of type LoadBalancer. When you add an AddressPool custom resource to the cluster, the MetalLB Operator configures MetalLB so that it can assign IP addresses from the pool. An address pool includes a list of IP addresses. The list can be a single IP address, a range specified in CIDR notation, a range specified as a starting and ending address separated by a hyphen, or a combination of the three. An address pool requires a name. The documentation uses names like doc-example, doc-example-reserved, and doc-example-ipv6. An address pool specifies whether MetalLB can automatically assign IP addresses from the pool or whether the IP addresses are reserved for services that explicitly specify the pool by name.

After you add the MetalLB custom resource to the cluster and the Operator deploys MetalLB, the MetalLB software components, controller and speaker, begin running.

MetalLB software components

When you install the MetalLB Operator, the metallb-operator-controller-manager deployment starts a pod. The pod is the implementation of the Operator. The pod monitors for changes to the MetalLB custom resource and AddressPool custom resources.

When the Operator starts an instance of MetalLB, it starts a controller deployment and a speaker daemon set.

controller

The Operator starts the deployment and a single pod. When you add a service of type LoadBalancer, Kubernetes uses the controller to allocate an IP address from an address pool.

speaker

The Operator starts a daemon set with one speaker pod for each node in your cluster.

For layer 2 mode, after the controller allocates an IP address for the service, each speaker pod determines if it is on the same node as an endpoint for the service. An algorithm that involves hashing the node name and the service name is used to select a single speaker pod to announce the load balancer IP address. The speaker uses Address Resolution Protocol (ARP) to announce IPv4 addresses and Neighbor Discovery Protocol (NDP) to announce IPv6 addresses.

Requests for the load balancer IP address are routed to the node with the speaker that announces the IP address. After the node receives the packets, the service proxy routes the packets to an endpoint for the service. The endpoint can be on the same node in the optimal case, or it can be on another node. The service proxy chooses an endpoint each time a connection is established.

MetalLB concepts for layer 2 mode

In layer 2 mode, the speaker pod on one node announces the external IP address for a service to the host network. From a network perspective, the node appears to have multiple IP addresses assigned to a network interface.

The speaker pod responds to ARP requests for IPv4 services and NDP requests for IPv6.

In layer 2 mode, all traffic for a service IP address is routed through one node. After traffic enters the node, the service proxy for the CNI network provider distributes the traffic to all the pods for the service.

Because all traffic for a service enters through a single node in layer 2 mode, in a strict sense, MetalLB does not implement a load balancer for layer 2. Rather, MetalLB implements a failover mechanism for layer 2 so that when a speaker pod becomes unavailable, a speaker pod on a different node can announce the service IP address.

When a node becomes unavailable, failover is automatic. The speaker pods on the other nodes detect that a node is unavailable and a new speaker pod and node take ownership of the service IP address from the failed node.