Flannel — How It Works

VXLAN overlay, subnet lease allocation, and backend mechanics.

VXLAN Overlay

sequenceDiagram
    participant PodA as Pod A (Node 1, 10.244.0.2)
    participant Flannel1 as flanneld (Node 1)
    participant VTEP1 as VXLAN VTEP (flannel.1)
    participant Network as Physical Network (UDP 8472)
    participant VTEP2 as VXLAN VTEP (flannel.1)
    participant PodB as Pod B (Node 2, 10.244.1.3)

    PodA->>VTEP1: Packet to 10.244.1.3
    VTEP1->>VTEP1: Encapsulate in VXLAN header
    VTEP1->>Network: UDP 8472 → Node 2
    Network->>VTEP2: Receive VXLAN packet
    VTEP2->>VTEP2: Decapsulate
    VTEP2->>PodB: Deliver original packet

Subnet Lease Model

Each node gets a /24 subnet from the cluster CIDR (e.g., 10.244.0.0/16). Flannel stores leases in etcd or the Kubernetes API (via kube-subnet-mgr).

Node	Subnet Lease	VTEP IP
Node 1	10.244.0.0/24	10.244.0.0
Node 2	10.244.1.0/24	10.244.1.0
Node 3	10.244.2.0/24	10.244.2.0

Sources

• Flannel README
• Backends