MetalLB hooks into your Kubernetes cluster, and provides a network load-balancer implementation. In short, it allows you to create Kubernetes services of type “LoadBalancer” in clusters that don’t run on a cloud provider, and thus cannot simply hook into paid products to provide load-balancers.
It has two features that work together to provide this service: address allocation, and external announcement.
In a cloud-enabled Kubernetes cluster, you request a load-balancer, and your cloud platform assigns an IP address to you. In a bare metal cluster, MetalLB is responsible for that allocation.
MetalLB cannot create IP addresses out of thin air, so you do have to give it pools of IP addresses that it can use. MetalLB will take care of assigning and unassigning individual addresses as services come and go, but it will only ever hand out IPs that are part of its configured pools.
How you get IP address pools for MetalLB depends on your environment. If you’re running a bare metal cluster in a colocation facility, your hosting provider probably offers IP addresses for lease. In that case, you would lease, say, a /26 of IP space (64 addresses, and provide that range to MetalLB for cluster services.
Alternatively, your cluster might be purely private, providing services to a nearby LAN but not exposed to the internet. In that case, you could pick a range of IPs from one of the private address spaces (so-called RFC1918 addresses), and assign those to MetalLB. Such addresses are free, and work fine as long as you’re only providing cluster services to your LAN.
Or, you could do both! MetalLB lets you define as many address pools as you want, and doesn’t care what “kind” of addresses you give it.
Once MetalLB has assigned an external IP address to a service, it needs to make the network beyond the cluster aware that the IP “lives” in the cluster. MetalLB uses standard routing protocols to achieve this: ARP, NDP, or BGP.
In layer 2 mode, one machine in the cluster takes ownership of the service IPs, and uses standard address discovery protocols (ARP for IPv4, NDP for IPv6) to make those IPs reachable on the local network. From the LAN’s point of view, the announcing machine simply has multiple IP addresses.
The layer 2 mode sub-page has more details on the behavior and limitations of layer 2 mode.
In BGP mode, all machines in the cluster establish BGP peering sessions with nearby routers that you control, and tell those routers how to forward traffic to the service IPs. Using BGP allows for true load balancing across multiple nodes, and fine-grained traffic control thanks to BGP’s policy mechanisms.
The BGP mode sub-page has more details on BGP mode’s operation and limitations.