Configuration

MetalLB remains idle until configured. This is accomplished by creating and deploying a config map into the same namespace (metallb-system) as the deployment.

There is an example config map in manifests/example-config.yaml, annotated with explanatory comments.

If you’ve named the config map config.yaml, you can deploy the manifest with kubectl apply -f config.yaml.

The specific configuration depends on the protocol(s) you want to use to announce service IPs. Jump to:

• Layer 2 configuration
• BGP configuration
• Advanced configuration

Layer 2 configuration

Layer 2 mode is the simplest to configure: in many cases, you don’t need any protocol-specific configuration, only IP addresses.

Layer 2 mode does not require the IPs to be bound to the network interfaces of your worker nodes. It works by responding to ARP requests on your local network directly, to give the machine’s MAC address to clients.

For example, the following configuration gives MetalLB control over IPs from 192.168.1.240 to 192.168.1.250, and configures Layer 2 mode:

apiVersion: v1
kind: ConfigMap
metadata:
  namespace: metallb-system
  name: config
data:
  config: |
    address-pools:
    - name: default
      protocol: layer2
      addresses:
      - 192.168.1.240-192.168.1.250

BGP configuration

For a basic configuration featuring one BGP router and one IP address range, you need 4 pieces of information:

• The router IP address that MetalLB should connect to,
• The router’s AS number,
• The AS number MetalLB should use,
• An IP address range expressed as a CIDR prefix.

As an example, if you want to give MetalLB the range 192.168.10.0/24 and AS number 64500, and connect it to a router at 10.0.0.1 with AS number 64501, your configuration will look like:

apiVersion: v1
kind: ConfigMap
metadata:
  namespace: metallb-system
  name: config
data:
  config: |
    peers:
    - peer-address: 10.0.0.1
      peer-asn: 64501
      my-asn: 64500
    address-pools:
    - name: default
      protocol: bgp
      addresses:
      - 192.168.10.0/24

Advertisement configuration

By default, BGP mode advertises each allocated IP to the configured peers with no additional BGP attributes. The peer router(s) will receive one /32 route for each service IP, with the BGP localpref set to zero and no BGP communities.

You can configure more elaborate advertisements by adding a bgp-advertisements section that lists one or more custom advertisements.

In addition to specifying localpref and communities, you can use this to advertise aggregate routes. The aggregation-length advertisement option lets you “roll up” the /32s into a larger prefix. Combined with multiple advertisement configurations, this lets you create elaborate advertisements that interoperate with the rest of your BGP network.

For example, let’s say you have a leased /24 of public IP space, and you’ve allocated it to MetalLB. By default, MetalLB will advertise each IP as a /32, but your transit provider rejects routes more specific than /24. So, you need to somehow advertise a /24 to your transit provider, but still have the ability to do per-IP routing internally.

Here’s a configuration that implements this:

apiVersion: v1
kind: ConfigMap
metadata:
  namespace: metallb-system
  name: config
data:
  config: |
    peers:
    - peer-address: 10.0.0.1
      peer-asn: 64501
      my-asn: 64500
    address-pools:
    - name: default
      protocol: bgp
      addresses:
      - 198.51.100.0/24
      bgp-advertisements:
      - aggregation-length: 32
        localpref: 100
        communities:
        - no-advertise
      - aggregation-length: 24
    bgp-communities:
      no-advertise: 65535:65282

With this configuration, if we create a service with IP 198.51.100.10, the BGP peer(s) will receive two routes:

• 198.51.100.10/32, with localpref=100 and the no-advertise community, which tells the peer router(s) that they can use this route, but they shouldn’t tell anyone else about it.
• 198.51.100.0/24, with no custom attributes.

With this configuration, the peer(s) will propagate the 198.51.100.0/24 route to your transit provider, but once traffic shows up locally, the 198.51.100.10/32 route will be used to forward into your cluster.

As you define more services, the router will receive one “local” /32 for each of them, as well as the covering /24. Each service you define “generates” the /24 route, but MetalLB deduplicates them all down to one BGP advertisement before talking to its peers.

The above configuration also showcases the bgp-communities configuration section, which lets you define readable names for BGP communities that you can reuse in your advertisement configurations. This is completely optional, you could just specify 65535:65281 directly in the configuration of the /24 if you prefer.

Limiting peers to certain nodes

By default, every node in the cluster connects to all the peers listed in the configuration. In more advanced cluster topologies, you may want each node to connect to different routers. For example, if you have a “rack and spine” network topology, you likely want each machine to peer with its top-of-rack router, but not the routers in other racks.

You can limit peers to certain nodes by using the node-selectors attribute of peers in the configuration. The semantics of these selectors are the same as those used elsewhere in Kubernetes, so refer to the labels documentation on the Kubernetes website.

For example, this is a (somewhat contrived) definition for a peer that will only be used by machines:

• With hostname hostA or hostB, or
• That have the rack=frontend label, but not the label network-speed=slow:

peers:
- peer-address: 10.0.0.1
  peer-asn: 64501
  my-asn: 64500
  node-selectors:
  - match-labels:
      rack: frontend
    match-expressions:
    - key: network-speed
      operator: NotIn
      values: [slow]
  - match-expressions:
    - key: kubernetes.io/hostname
      operator: In
      values: [hostA, hostB]

Configuring the BGP source address

When a host has multiple network interfaces or multiple IP addresses configured on one interface, the host’s TCP/IP stack usually selects the IP address that is used as the source IP address for outbound connections automatically. This is true also for BGP connections.

Sometimes, the automatically-selected address may not be the desired one for some reason. In such cases, MetalLB supports explicitly specifying the source address to be used when establishing a BGP session:

peers:
- peer-address: 10.0.0.1
  peer-asn: 64501
  my-asn: 64500
  source-address: 10.0.0.2
  node-selectors:
  - match-labels:
      kubernetes.io/hostname: node-1

The configuration above tells the MetalLB speaker to check if the address 10.0.0.2 exists locally on one of the host’s network interfaces, and if so - to use it as the source address when establishing BGP sessions. If the address isn’t found, the default behavior takes place (that is, the kernel selects the source address automatically).

Advanced address pool configuration

Controlling automatic address allocation

In some environments, you’ll have some large address pools of “cheap” IPs (e.g. RFC1918), and some smaller pools of “expensive” IPs (e.g. leased public IPv4 addresses).

By default, MetalLB will allocate IPs from any configured address pool with free addresses. This might end up using “expensive” addresses for services that don’t require it.

To prevent this behaviour you can disable automatic allocation for a pool by setting the auto-assign flag to false:

# Rest of config omitted for brevity
address-pools:
- name: cheap
  protocol: bgp
  addresses:
  - 192.168.144.0/20
- name: expensive
  protocol: bgp
  addresses:
  - 42.176.25.64/30
  auto-assign: false

Addresses can still be specifically allocated from the “expensive” pool with the methods described in the usage section.

Handling buggy networks

Some old consumer network equipment mistakenly blocks IP addresses ending in .0 and .255, because of misguided smurf protection.

If you encounter this issue with your users or networks, you can set avoid-buggy-ips: true on an address pool to mark .0 and .255 addresses as unusable.