Wide Area Networks (WAN)

From expensive dedicated circuits to intelligent, software-defined overlays. The WAN connects the enterprise to the world.

1. MPLS (Multi-Protocol Label Switching)

MPLS sits between Layer 2 and Layer 3 (often called "Layer 2.5"). It replaces slow IP lookups with fast label swapping.

L2 Header Ethernet

MPLS Label 20 Bits

Exp CoS

S BoS

TTL 8 Bits

L3 Packet IP Header + Data

Label Operations

PUSH: Ingress Router (PE) adds a label to an incoming IP packet.
SWAP: Core Router (P) swaps the incoming label for an outgoing label. No IP lookup performed.
POP: Penultimate Hop Router (PHP) removes the top label before sending to the Egress PE.

Engineer's Notebook: Penultimate Hop Popping (PHP)

The router before the final destination removes the label (Implicit Null). This saves the final router (Egress PE) from doing two lookups (Label lookup -> "It's gone" -> IP lookup). The Egress PE receives a pure IP packet.

2. MPLS L3VPNs

Service Providers use MPLS to keep customer traffic separate. Customer A and Customer B can use the same IP space (e.g., 10.0.0.0/8) without conflict.

Key Components

VRF (Virtual Routing and Forwarding): A separate routing table for each customer.
RD (Route Distinguisher): A 64-bit prefix added to the customer's IP (e.g., 100:1 + 10.1.1.0/24) to make it globally unique within the provider core.
RT (Route Target): Extended BGP community that controls import/export policy. "Export routes from Site A with tag Red. Import routes with tag Red into Site B."
MP-BGP (VPNv4): The protocol that carries these labeled routes across the provider core.

3. The Shift to SD-WAN

MPLS is reliable but expensive and rigid. Software-Defined WAN (SD-WAN) decouples the control plane and allows using any transport (LTE, Broadband, MPLS) securely.

Architecture Layers

Layer	Function	Component
Orchestration	Onboarding, ZTP (Zero Touch Provisioning), Licensing.	vBond (Cisco), Orchestrator (Velocloud)
Management	GUI Dashboard, Policy Configuration, Analytics.	vManage
Control Plane	Distributes routes and keys (OMP - Overlay Management Protocol).	vSmart (The "Brain")
Data Plane	Forwards actual traffic. Establishes IPsec tunnels.	vEdge / cEdge routers

Engineer's Notebook: Application-Aware Routing

SD-WAN routers constantly measure Latency, Jitter, and Packet Loss on all links using BFD probes. You define the policy:

"Voice traffic must go over MPLS unless Jitter > 30ms, then switch to Internet VPN."

This dynamic steering (DPI) ensures optimal performance without manual intervention.