This is one of the most common Senior OpenShift Architect interview questions. Interviewers are looking for your understanding of what happens from the moment a server powers on until an application pod is running.

A strong answer should cover Ignition → RHCOS → kubelet → CRI-O → Control Plane → Scheduler → CNI → Pod.

Complete OpenShift Boot Sequence

Power ON
   │
   ▼
BIOS / UEFI
   │
   ▼
Boot Loader (GRUB)
   │
   ▼
RHCOS Kernel
   │
   ▼
systemd starts services
   │
   ▼
Ignition (first boot only)
   │
   ▼
Machine Config Daemon
   │
   ▼
CRI-O + kubelet start
   │
   ▼
kubelet registers with API Server
   │
   ▼
Node becomes Ready
   │
   ▼
Operators reconcile cluster
   │
   ▼
Scheduler assigns Pods
   │
   ▼
CRI-O pulls images
   │
   ▼
OVN configures networking
   │
   ▼
Pod starts

Step 1 — Server Power On

The physical server or VM starts.

The sequence begins with:

• BIOS/UEFI
• CPU initialization
• Memory initialization
• Disk detection
• NIC initialization

After POST (Power-On Self Test), the firmware loads the bootloader.

Step 2 — Bootloader (GRUB)

GRUB loads:

• Linux kernel
• initramfs

For OpenShift this is:

Red Hat CoreOS (RHCOS)

The kernel initializes:

• CPU scheduler
• Memory manager
• Filesystems
• Drivers
• Network interfaces

Step 3 — systemd Starts

After the kernel mounts the root filesystem:

systemd becomes PID 1.

It starts services such as:

• NetworkManager
• CRI-O
• kubelet
• chronyd
• sshd
• Machine Config Daemon

Step 4 — Ignition (First Boot Only)

One of OpenShift’s biggest differences from a standard Kubernetes deployment is Ignition.

On the first boot:

The node downloads its Ignition configuration.

The configuration contains:

• SSH keys
• kubelet configuration
• certificates
• hostname
• networking
• systemd units
• MachineConfig references

Once applied, Ignition is not run again. Ongoing node configuration is handled by the Machine Config Operator (MCO) and Machine Config Daemon (MCD).

Step 5 — Machine Config Operator

Now the Machine Config Daemon starts.

It checks:

Desired configuration
↓
Current configuration
↓
Need changes?
↓
Apply configuration
↓
Reboot (if required)

It configures:

• kernel arguments
• sysctl
• kubelet config
• registries.conf
• CA certificates
• chrony
• SSH configuration

Step 6 — CRI-O Starts

OpenShift uses CRI-O as its container runtime.

CRI-O:

• starts container runtime
• communicates with kubelet
• manages OCI containers
• uses runc (or crun) underneath

At this point there are still no application pods.

Step 7 — kubelet Starts

kubelet is the node agent.

Its responsibilities include:

• registering the node
• monitoring pods
• starting containers
• reporting status
• managing volumes

The kubelet loads:

• client certificates
• kubeconfig
• bootstrap configuration

Step 8 — TLS Bootstrap

The kubelet authenticates to the API Server.

kubelet
      │
      │ CSR
      ▼
API Server
      │
      ▼
Machine Approver
      │
      ▼
Certificate Approved

After approval:

The node receives a client certificate.

Step 9 — Node Registration

The kubelet creates a Node object in the Kubernetes API.

Example:

oc get nodes

Initially:

NotReady

because networking is not yet configured.

Step 10 — Operators Start Reconciling

This is where OpenShift differs significantly from upstream Kubernetes.

Cluster Operators begin reconciling the desired state.

Examples:

• Authentication Operator
• Ingress Operator
• DNS Operator
• Image Registry Operator
• Monitoring Operator
• Network Operator
• Machine Config Operator

The Cluster Version Operator (CVO) ensures the correct versions of all platform components are deployed.

Step 11 — OVN-Kubernetes Configures Networking

The Cluster Network Operator deploys OVN-Kubernetes.

Each node starts:

ovnkube-node
OVS
OVN Controller

OVN creates:

• virtual switches
• logical routers
• GENEVE tunnels
• routing tables
• pod interfaces

Now the node becomes:

Ready

because networking is available.

Step 12 — Scheduler Assigns Pods

The scheduler watches for pods without a node assignment.

Example:

spec:
  nodeName: null

The scheduler evaluates:

• CPU
• Memory
• Taints
• Tolerations
• Affinity
• Anti-affinity
• Node selectors
• Topology spread constraints

Then writes:

nodeName: worker-2

into the Pod specification.

Step 13 — kubelet Receives the Pod

The kubelet on the assigned node watches the API server and detects the new Pod.

Example:

Pod Assigned
↓
kubelet notices
↓
Start Pod

Step 14 — Image Pull

kubelet asks CRI-O to start the container.

CRI-O:

Check local cache
↓
Image exists?
↓
Yes → Run
↓
No
↓
Pull image
↓
Verify digest
↓
Store image

The image is pulled from:

• Quay
• OpenShift Image Registry
• Docker Hub
• Harbor
• Artifactory
• another OCI-compatible registry

Step 15 — Pod Sandbox Creation

CRI-O creates:

• network namespace
• PID namespace
• IPC namespace
• mount namespace
• cgroups
• SELinux labels
• seccomp profile

This forms the pod sandbox.

Step 16 — OVN Creates the Pod Network

OVN:

• allocates an IP address
• creates the veth pair
• connects the pod to Open vSwitch
• updates routing
• applies NetworkPolicies
• configures GENEVE encapsulation for cross-node traffic

Now the pod can communicate with:

• Services
• Other pods
• External networks (subject to policy)

Step 17 — Volume Mounts

kubelet mounts:

• Secrets
• ConfigMaps
• PVCs
• CSI volumes
• projected volumes
• service account token

Step 18 — Container Starts

CRI-O executes:

runc create
↓
runc start

The container process starts.

Step 19 — Health Checks

kubelet begins running probes:

• Startup probe (if configured)
• Liveness probe
• Readiness probe

Only after the readiness probe succeeds does the pod receive traffic through a Service.

Step 20 — Service Endpoints Updated

The EndpointSlice controller updates the Service.

Pod Ready
↓
EndpointSlice updated
↓
kube-proxy / OVN
↓
Service updated
↓
Traffic flows

At this stage:

• The application is reachable inside the cluster via a Service.
• If exposed with an OpenShift Route or Kubernetes Ingress, external clients can also reach it.

End-to-End Flow (Interview Summary)

Power On
   │
BIOS/UEFI
   │
GRUB
   │
Linux Kernel
   │
systemd
   │
Ignition (first boot)
   │
Machine Config Daemon
   │
CRI-O
   │
kubelet
   │
TLS Bootstrap
   │
API Server
   │
Node Registered
   │
OVN Networking
   │
Node Ready
   │
Scheduler
   │
kubelet
   │
CRI-O Pull Image
   │
Pod Sandbox
   │
Network Setup
   │
Volume Mounts
   │
Container Starts
   │
Readiness Probe
   │
Service Endpoints
   │
Application Running

Interview Tip

For a senior architect role, emphasize these OpenShift-specific differentiators from upstream Kubernetes:

• RHCOS provides an immutable, container-focused operating system.
• Ignition performs one-time provisioning on first boot.
• The Machine Config Operator (MCO) manages ongoing node configuration and orchestrates safe reboots when required.
• The Cluster Version Operator (CVO) coordinates platform upgrades and lifecycle management.
• Cluster Operators continuously reconcile core platform components such as networking, ingress, authentication, DNS, and monitoring.
• OVN-Kubernetes provides the default software-defined networking, using GENEVE tunnels, logical switches, and logical routers for pod connectivity.
• Security Context Constraints (SCCs) enforce pod security policies before workloads are admitted.

This level of detail demonstrates not only Kubernetes knowledge but also a solid understanding of the OpenShift platform lifecycle and architecture, which is exactly what interviewers expect from a senior OpenShift engineer or architect.