Advanced L2 configuration

Limiting the set of nodes where the service can be announced from

In L2 mode, only one node is elected to announce the IP from.

Normally, all the nodes where a Speaker is running are eligible for any given IP.

There can be scenarios where only a subset of the nodes are exposed to a given network, so it can be useful to limit only those nodes as potential entry points for the service IP.

This is achieved by using the node selector in the L2Advertisement CR.

graph TD
    metallBA-->|announces|subnetA(Subnet A)
    metallBB-->|announces|subnetA(Subnet A)
    metallBC-->|announces|subnetB(Subnet B)

    subgraph NodeA
        metallBA("MetalLB<br>Speaker")
    end
    subgraph NodeB
        metallBB("MetalLB<br>Speaker")
    end
    subgraph NodeC
        metallBC("MetalLB<br>Speaker")
    end

In this example, NodeA and NodeB are exposed to the subnet A, whereas node C is exposed to subnet B.

In order to limit the set of nodes for a given advertisement, the node selector must be set:

apiVersion: metallb.io/v1beta1
kind: L2Advertisement
metadata:
  name: example
  namespace: metallb-system
spec:
  ipAddressPools:
  - first-pool
  nodeSelectors:
  - matchLabels:
      kubernetes.io/hostname: NodeA
  - matchLabels:
      kubernetes.io/hostname: NodeB

apiVersion: metallb.io/v1beta1
kind: L2Advertisement
metadata:
  name: example
  namespace: metallb-system
spec:
  ipAddressPools:
  - second-pool
  nodeSelectors:
  - matchLabels:
      kubernetes.io/hostname: NodeC

In this way, all the IPs coming from first-pool will be reachable only via NodeA and NodeB, and only one of those node will be chosen to expose the IP.

On the other hand, IPs coming from second-pool will be exposed always via NodeC.

Specify network interfaces that LB IP can be announced from

In L2 mode, by default a metallb speaker announces the LoadBalancer IP from all the network interfaces of a node. We can use interfaces in L2Advertisement to select a subset of them.

apiVersion: metallb.io/v1beta1
kind: L2Advertisement
metadata:
  name: example
  namespace: metallb-system
spec:
  ipAddressPools:
  - third-pool
  interfaces:
  - eth0
  - eth1

This L2Advertisement will make MetalLB announce the Services associated to IPs from third-pool only from the interfaces eth0 and eth1 of all nodes.

The interfaces selector can also be used together with nodeSelectors. In this example, the IPs from fourth-pool will be announced only from eth3 of NodeA:

apiVersion: metallb.io/v1beta1
kind: L2Advertisement
metadata:
  name: example
  namespace: metallb-system
spec:
  ipAddressPools:
  - fourth-pool
  nodeSelectors:
  - matchLabels:
      kubernetes.io/hostname: NodeA
  interfaces:
  - eth3

apiVersion: metallb.io/v1beta1
kind: L2Advertisement
metadata:
  name: example-advertisement9
  namespace: metallb-system
spec:
  interfaces:
  - eno1
---
apiVersion: metallb.io/v1beta1
kind: L2Advertisement
metadata:
  name: example-advertisement10
  namespace: metallb-system
spec:
  ipAddressPools:
  - pool1
  nodeSelectors:
  - matchLabels:
      kubernetes.io/hostname: hostB
  interfaces:
  - ens18