About MetalLB and the MetalLB Operator

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.

Conceptual diagram for MetalLB and layer 2 mode

The preceding graphic shows the following concepts related to MetalLB:

  • An application is available through a service that has a cluster IP on the 172.130.0.0/16 subnet. That IP address is accessible from inside the cluster. The service also has an external IP address that MetalLB assigned to the service, 192.168.100.200.

  • Nodes 1 and 3 have a pod for the application.

  • The speaker daemon set runs a pod on each node. The MetalLB Operator starts these pods.

  • Each speaker pod is a host-networked pod. The IP address for the pod is identical to the IP address for the node on the host network.

  • The speaker pod on node 1 uses ARP to announce the external IP address for the service, 192.168.100.200. The speaker pod that announces the external IP address must be on the same node as an endpoint for the service and the endpoint must be in the Ready condition.

  • Client traffic is routed to the host network and connects to the 192.168.100.200 IP address. After traffic enters the node, the service proxy sends the traffic to the application pod on the same node or another node according to the external traffic policy that you set for the service.

  • If node 1 becomes unavailable, the external IP address fails over to another node. On another node that has an instance of the application pod and service endpoint, the speaker pod begins to announce the external IP address, 192.168.100.200 and the new node receives the client traffic. In the diagram, the only candidate is node 3.

Layer 2 and external traffic policy

With layer 2 mode, one node in your cluster receives all the traffic for the service IP address. How your cluster handles the traffic after it enters the node is affected by the external traffic policy.

cluster

This is the default value for spec.externalTrafficPolicy.

With the cluster traffic policy, after the node receives the traffic, the service proxy distributes the traffic to all the pods in your service. This policy provides uniform traffic distribution across the pods, but it obscures the client IP address and it can appear to the application in your pods that the traffic originates from the node rather than the client.

local

With the local traffic policy, after the node receives the traffic, the service proxy only sends traffic to the pods on the same node. For example, if the speaker pod on node A announces the external service IP, then all traffic is sent to node A. After the traffic enters node A, the service proxy only sends traffic to pods for the service that are also on node A. Pods for the service that are on additional nodes do not receive any traffic from node A. Pods for the service on additional nodes act as replicas in case failover is needed.

This policy does not affect the client IP address. Application pods can determine the client IP address from the incoming connections.

Limitations and restrictions

Support for layer 2 only

When you install and configure MetalLB on OKD 4.9 with the MetalLB Operator, support is restricted to layer 2 mode only. In comparison, the open source MetalLB project offers load balancing for layer 2 mode and a mode for layer 3 that uses border gateway protocol (BGP).

Support for single stack networking

Although you can specify IPv4 addresses and IPv6 addresses in the same address pool, MetalLB only assigns one IP address for the load balancer.

When MetalLB is deployed on a cluster that is configured for dual-stack networking, MetalLB assigns one IPv4 or IPv6 address for the load balancer, depending on the IP address family of the cluster IP for the service. For example, if the cluster IP of the service is IPv4, then MetalLB assigns an IPv4 address for the load balancer. MetalLB does not assign an IPv4 and an IPv6 address simultaneously.

IPv6 is only supported for clusters that use the OVN-Kubernetes network provider.

Infrastructure considerations for MetalLB

MetalLB is primarily useful for on-premise, bare metal installations because these installations do not include a native load-balancer capability. In addition to bare metal installations, installations of OKD on some infrastructures might not include a native load-balancer capability. For example, the following infrastructures can benefit from adding the MetalLB Operator:

  • Bare metal

  • VMware vSphere

MetalLB Operator and MetalLB are supported with the OpenShift SDN and OVN-Kubernetes network providers.

Limitations for layer 2 mode

Single-node bottleneck

MetalLB routes all traffic for a service through a single node, the node can become a bottleneck and limit performance.

Layer 2 mode limits the ingress bandwidth for your service to the bandwidth of a single node. This is a fundamental limitation of using ARP and NDP to direct traffic.

Slow failover performance

Failover between nodes depends on cooperation from the clients. When a failover occurs, MetalLB sends gratuitous ARP packets to notify clients that the MAC address associated with the service IP has changed.

Most client operating systems handle gratuitous ARP packets correctly and update their neighbor caches promptly. When clients update their caches quickly, failover completes within a few seconds. Clients typically fail over to a new node within 10 seconds. However, some client operating systems either do not handle gratuitous ARP packets at all or have outdated implementations that delay the cache update.

Recent versions of common operating systems such as Windows, macOS, and Linux implement layer 2 failover correctly. Issues with slow failover are not expected except for older and less common client operating systems.

To minimize the impact from a planned failover on outdated clients, keep the old node running for a few minutes after flipping leadership. The old node can continue to forward traffic for outdated clients until their caches refresh.

During an unplanned failover, the service IPs are unreachable until the outdated clients refresh their cache entries.

Incompatibility with IP failover

MetalLB is incompatible with the IP failover feature. Before you install the MetalLB Operator, remove IP failover.

Additional resources