Cilium Gateway API Part 3: Boundaries in a Multi-Control-Plane Cluster

Key takeaways

• The API-server entry point and application Gateway entry point need separate ports, addresses, health checks, and failure domains
• A production baseline sends external application traffic to dedicated infra or worker nodes rather than control-plane nodes
• The programmed Envoy data path can have a different failure lifecycle from the Kubernetes API and Cilium operator
• If control-plane nodes must receive Gateway traffic, verify scheduling, Envoy readiness, port conflicts, and security groups explicitly
• High availability requires redundant external entry, Envoy nodes, and backends across failure domains, not merely three control-plane nodes

Separate two load-balancing boundaries

• The API LB serves cluster management, while the Gateway LB serves application users
  • The API LB checks control-plane nodes on 6443/TCP and /readyz
  • The Gateway LB checks ingress nodes on 80/443 through an application health Route
  • Combining both roles into one VIP or target group couples access policy and incident diagnosis

• Use separate names and certificates for the two roles
  • api.cluster.example.com belongs in kubeconfig and Cilium k8sServiceHost
  • app.example.com belongs to the Gateway listener and user-facing certificate
  • Never reuse the API-server VIP as the public Gateway address

Prefer dedicated ingress nodes

• Dedicated infra nodes isolate application traffic capacity from the control plane
  • Envoy CPU, memory, and connections do not contend with API server and etcd resources
  • Internet or LB access can be restricted to infra-node security groups
  • Control-plane and Gateway maintenance can proceed independently

kubectl label node infra-a infra-b \
  role=infra component=gateway-api

kubectl get nodes \
  -l role=infra,component=gateway-api \
  -o custom-columns=NAME:.metadata.name,INTERNAL-IP:.status.addresses

• Cilium's Helm node selector limits listener placement specifically in hostNetwork mode

gatewayAPI:
  enabled: true
  hostNetwork:
    enabled: true
    nodes:
      matchLabels:
        role: infra
        component: gateway-api

• For Service mode, constrain traffic with external LB targets or BGP/L2 policy selectors
  • Distinguish where NodePort listens from where the external LB actually sends traffic
  • Align BGP advertisements and L2 announcements with the intended node and Service policy

Strengthen the controls when control-plane nodes are shared

• Small or control-plane-only clusters can use the same nodes for the Gateway data path

  • This is an operational cost and scale trade-off rather than a technical requirement
  • The design must accept that public traffic can compete with API server and etcd resources

• Inspect control-plane taints and standalone Envoy placement first

kubectl describe node cp-a | sed -n '/Taints:/,/Unschedulable:/p'
kubectl -n kube-system get pods -l k8s-app=cilium-envoy -o wide
kubectl -n kube-system get pods -l k8s-app=cilium -o wide

• Add a label that expresses public-ingress intent instead of reusing the generic control-plane role

kubectl label node cp-a cp-b cp-c gateway.cilium.io/expose=true

gatewayAPI:
  enabled: true
  hostNetwork:
    enabled: true
    nodes:
      matchLabels:
        gateway.cilium.io/expose: "true"

• Separate node ports and firewall policy by role
  • Allow 6443 only from administrative and cluster trust networks
  • Allow 80/443 from the external LB or the intended Internet policy
  • Never expose etcd 2379/2380, kubelet 10250, or Cilium management ports publicly

Test control-plane and data-plane failures separately

• One API-server failure should reduce management capacity without immediately stopping user traffic

  • Verify that existing Envoy configuration continues to serve when no Gateway change is needed
  • Observe operator leader transition and condition-update latency

• A complete API-server interruption can stop new Route, Secret, and Endpoint updates

  • Record the difference between existing traffic continuity and failed deployments
  • Set a maximum acceptable interval for certificate and Endpoint staleness

• One Gateway-node failure must be removed by the external LB or routing layer

  • A healthy control-plane quorum does not remove a dead ingress target
  • Place at least two Gateway nodes on separate zones or physical hosts

Record measurable availability criteria

• Measure recovery at each layer independently
  • API LB control-plane target removal time
  • Gateway LB or BGP ingress-node withdrawal time
  • DNS TTL and health-checked DNS propagation time
  • Envoy Endpoint update time

kubectl --request-timeout=5s get --raw=/readyz
kubectl get gateway -A
kubectl get httproute -A
curl --fail-with-body https://app.example.com/healthz

• Add resource and traffic controls when control-plane nodes are shared
  • Define Envoy resource requests, limits, and node reservations
  • Apply external connection and request-rate limits
  • Correlate API-server and etcd latency with Gateway traffic metrics

Recommended action

• Prefer three control-plane nodes plus at least two dedicated Gateway nodes across failure domains
• Document control-plane sharing as a small-cluster exception and require port-isolation and load-test evidence

References

• Cilium Gateway API underlying mechanics
• Cilium host network node selection
• Kubernetes highly available control plane
• Kubernetes control-plane communication