How MetalLB works in Kubernetes

7/2/2021

Can anybody explain me how MetalLB gets IP addresses in a Kubernetes environment? I have installed Kubernetes cluster in GCP compute engines. I have provided a range of Internal IP addresses in MetalLB ConfigMap.

NAME       	STATUS   INTERNAL-IP   EXTERNAL-IP     
instance-1  Ready    10.140.0.20   56.169.53.26    
instance-2  Ready    10.140.0.21   57.11.92.241   
instance-3  Ready    10.140.0.22   54.7.255.253

In my case the IP range which I gave in the CM was 10.140.0.30-10.140.0.40

It works as expected but I want to know how MetalLB get's IP addresses.

-- Dusty
kubernetes
load-balancing
metallb

1 Answer

7/5/2021

to summarize my comments:

MetalLB in layer 2 mode is deploying on each node a Speaker Pod which responds to ARP(IPv4) and NDP(IPv6) requests.

If you now connect to the IP, which your Kubernetes Service with type: LoadBalancer got from the range you have defined in the MetalLB configuraton, your client will send out an arp-request who-has <IP-Service>, tell <IP-Client> to the Network. Since the Speaker Pods are listening to arp-requests, they'll answer with reply <IP-Service> is-at <node-MAC-address-of-the-leader>.

It does not mean, that your Pod is running on that Node which the Mac-Address is resolved, only the MetalLB "leader" is running on this one. Your request will pass then over to the Kube-Proxy which is aware where your Pod lives.

Also keep in mind:

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.

https://metallb.universe.tf/concepts/layer2/#load-balancing-behavior

-- CLNRMN
Source: StackOverflow