Advanced BGP 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 with multiple BGPAdvertisements 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: metallb.io/v1beta1
kind: IPAddressPool
metadata:
  name: first-pool
  namespace: metallb-system
spec:
  addresses:
  - 198.51.100.10/24
apiVersion: metallb.io/v1beta1
kind: BGPAdvertisement
metadata:
  name: local
spec:
  ipAddressPools:
  - first-pool
  aggregationLength: 32
  localpref: 100
  communities:
  - 65535:65282
apiVersion: metallb.io/v1beta1
kind: BGPAdvertisement
metadata:
  name: external
spec:
  ipAddressPools:
  - first-pool
  aggregationLength: 24

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.

Additionally, we can define community aliases in order to have descriptive names for the communities, to be used in place of the two 16 bits format.

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 machine to peer with its top-of-rack router, but not the routers in other racks. 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.

graph BT subgraph "" metallbA("MetalLB
Speaker") end subgraph "" metallbB("MetalLB
Speaker") end subgraph "" metallbC("MetalLB
Speaker") end subgraph "" metallbD("MetalLB
Speaker") end metallbA-->torA(ToR Router) metallbB-->torA(ToR Router) metallbC-->torB(ToR Router) metallbD-->torB(ToR Router) torA-->spine(Spine Router) torB-->spine(Spine Router)

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:
apiVersion: metallb.io/v1beta2
kind: BGPPeer
metadata:
  name: example
  namespace: metallb-system
spec:
  myASN: 64512
  peerASN: 64512
  peerAddress: 172.30.0.3
  peerPort: 180
  nodeSelectors:
  - matchLabels:
      rack: frontend
    matchExpressions:
    - key: network-speed
      operator: NotIn
      values: [slow]
  - matchExpressions:
    - key: kubernetes.io/hostname
      operator: In
      values: [hostA, hostB]

Announcing the Service from a subset of nodes

It is possible to limit the set of nodes that are advertised as next hops to reach the service IPs. This is achieved by using the node selector in the BGPAdvertisement CR.

graph TD metallBA-->|connects|torA(Router) metallBA-->|announces|torA(Router) metallBB-->|connects|torA(Router) metallBB-->|announces|torA(Router) metallBC-->|connects|torA(Router) subgraph NodeA metallBA("MetalLB
Speaker") end subgraph NodeB metallBB("MetalLB
Speaker") end subgraph NodeC metallBC("MetalLB
Speaker") end

In this example, the service is announced only from NodeA and NodeB. Note that this feature is orthogonal to the BGP peer node selector, as it’s performed at service (IPAddressPool) level.

MetalLB will still follow the BGPPeer configuration to choose if a certain node must be peered with a certain router.

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

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

In this way, all the IPs coming from first-pool will be reacheable only via NodeA and NodeB.

Announcing the Service to a subset of peers

By default, every service IP is advertised to all the connected peers. It is possible to limit the set of peers a service IP is advertised to. This is achieved by using the peers in the BGPAdvertisement CR.

graph TD PoolA("PoolA
198.51.100.10/24") style PoolA fill:orange metalLB("MetalLB Speaker")-->|announces|PeerA metalLB("MetalLB Speaker")-->|announces|PeerB metalLB("MetalLB Speaker")-->|announces|PeerB metalLB("MetalLB Speaker")-->|announces|PeerC linkStyle 0 stroke-width:2px,fill:none,stroke:orange; linkStyle 1 stroke-width:2px,fill:none,stroke:orange; linkStyle 2 stroke-width:2px,fill:none,stroke:lightgreen; linkStyle 3 stroke-width:2px,fill:none,stroke:lightgreen; PoolB("PoolB
198.51.200.10/24") style PoolB fill:lightgreen

In this example, a service IP from PoolA is announced only to PeerA and PeerB, while a service IP from PoolB is announced only to PeerB and PeerC.

In order to limit the set of peers for a given advertisement, the peers must be set:

apiVersion: metallb.io/v1beta1
kind: BGPAdvertisement
metadata:
  name: example
spec:
  ipAddressPools:
  - PoolA
  peers:
  - PeerA
  - PeerB

In this way, all the IPs coming from PoolA will be advertised only to PeerA and PeerB.

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:

apiVersion: metallb.io/v1beta2
kind: BGPPeer
metadata:
  name: example
  namespace: metallb-system
spec:
  myASN: 64512
  peerASN: 64512
  peerAddress: 172.30.0.3
  peerPort: 179
  sourceAddress: 172.30.0.2
  nodeSelectors:
  - matchLabels:
      kubernetes.io/hostname: node-1

The configuration above tells the MetalLB speaker to check if the address 172.30.0.2 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).

In most cases the source-address field should only be used with per-node peers, i.e. peers with node selectors which select only one node.

By default, a BGP peer configured under the peers configuration section runs on all speaker nodes. It is likely meaningless to use the source-address field in a peer configuration that applies to more than one node because two nodes in a given network usually shouldn’t have the same IP address.

Community Aliases

It’s possible to define aliases for BGP Communities used when advertising. This is done by using the Community CRD that allows to associate a name to a given BGP community:

apiVersion: metallb.io/v1beta1
kind: Community
metadata:
  name: communities
  namespace: metallb-system
spec:
  communities:
  - name: vpn-only
    value: 1234:1
  - name: NO_ADVERTISE
    value: 65535:65282

After defining an alias, it can be used in the BGPAdvertisement in place of its two 16 bits number format:

apiVersion: metallb.io/v1beta1
kind: BGPAdvertisement
metadata:
  name: local
spec:
  ipAddressPools:
  - first-pool
  aggregationLength: 32
  localpref: 100
  communities:
  - vpn-only

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