In layer 2 mode, one node in your cluster assumes the responsibility of advertising all service IPs to the local network. From the network’s perspective, it simply looks like that machine has multiple IP addresses assigned to its network interface.
Under the hood, MetalLB responds to ARP requests for IPv4 services, and NDP requests for IPv6.
The major advantage of the layer 2 mode is its universality: it will work on any ethernet network, with no special hardware required, not even fancy routers.
In layer 2 mode, all traffic for all service IPs goes to one
node. From there, kube-proxy spreads the traffic to all the
service’s pods.
In that sense, layer 2 does not implement a load-balancer. Rather, it implements a failover mechanism so that a different node can take over should the current leader node fail for some reason.
Layer 2 mode has two main limitations you should be aware of: single-node bottlenecking, and potentially slow failover.
As explained above, in layer2 mode a single leader-elected node receives all traffic for all service IPs. This means that your cluster ingress bandwidth is limited to the bandwidth of a single node. This is a fundamental limitation of using ARP and NDP to steer traffic.
In the current implementation, failover between nodes depends on cooperation from the clients. When a failover occurs, MetalLB sends a number of gratuitous layer 2 packets (a bit of a misnomer - it should really be called “unsolicited layer 2 packets”) to notify clients that the MAC address associated with the service IPs has changed.
Most operating systems handle “gratuitous” packets correctly, and update their neighbor caches promptly. In that case, failover happens within a few seconds. However, some systems either don’t implement gratuitous handling at all, or have buggy implementations that delay the cache update.
All modern versions of major OSes (Windows, Mac, Linux) implement layer 2 failover correctly, so the only situation where issues may happen is with older or less common OSes.
To minimize the impact of planned failover on buggy clients, you should keep the old leader node up for a couple of minutes after flipping leadership, so that it can continue forwarding traffic for old clients until their caches refresh.
During an unplanned failover, the service IPs will be unreachable until the buggy clients refresh their cache entries.
If you encounter a situation where layer 2 mode failover is slow (more than about 10s), please file a bug! We can help you investigate and determine if the issue is with the client, or a bug in MetalLB.