Advanced L2 configuration - Metallb

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.

{{<mermaid align="center">}} graph TD metallBA-->|announces|subnetA(Subnet A) metallBB-->|announces|subnetA(Subnet A) metallBC-->|announces|subnetB(Subnet B)

subgraph NodeA
    metallBA("MetalLB

Speaker") end subgraph NodeB metallBB("MetalLB Speaker") end subgraph NodeC metallBC("MetalLB 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:

yaml

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

yaml

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.

yaml

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:

yaml

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

{{% notice note %}} The IP from a given IPAddressPool is advertised using the union of all the L2Advertisements referencing it. This means that the interfaces used to advertise an IP will be the union of the interfaces selected by all the L2Advertisements.

For example:

yaml

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

The above YAML indicates that MetalLB should advertise the VIPs of all IPAddressPools including pool1 from the interface eno1 of all nodes, and also advertise the VIPs in pool1 from ens18 of hostB.

In other words, if MetalLB chooses hostB to announce the VIP of pool1, the Speaker should announce the VIP from the interfaces ens18 and eno1; if it chooses other nodes, the Speaker should announce the VIP only from the interface eno1. {{% /notice %}}

{{% notice warning %}} The interface selector won't affect how MetalLB is choosing the leader for a given L2 IP. This means that if it elects a leader where the selected interface is not available, the service won't be announced. The cluster administrator is responsible to use the combination of interfaces selector and node selector to avoid the problem. {{% /notice %}}

Limiting the advertisement to specific services

By default, an L2Advertisement applies to all services that receive an IP from the associated pools. You can use serviceSelectors to limit the advertisement to only services that match specific label selectors.

This is useful when you want different L2 configurations for different services. For example, you might want to announce certain services only from specific interfaces or nodes based on their labels.

yaml

apiVersion: metallb.io/v1beta1
kind: L2Advertisement
metadata:
  name: frontend-services
  namespace: metallb-system
spec:
  ipAddressPools:
  - first-pool
  interfaces:
  - eth0
  serviceSelectors:
  - matchLabels:
      tier: frontend

yaml

apiVersion: metallb.io/v1beta1
kind: L2Advertisement
metadata:
  name: backend-services
  namespace: metallb-system
spec:
  ipAddressPools:
  - first-pool
  interfaces:
  - eth1
  serviceSelectors:
  - matchLabels:
      tier: backend

With this configuration, services labeled tier: frontend that received an IP from first-pool will be announced from interface eth0, while services labeled tier: backend that received an IP from first-pool will be announced from interface eth1.

Multiple selectors in the serviceSelectors list are combined with OR logic - a service matching any of the selectors will be advertised via that advertisement.