Here’s the **20% of SD-WAN that covers 80% of what you need to know**, with a focus on **practical knowledge for senior network roles** and seamless integration with your IPSec expertise:

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### **SD-WAN Crash Course: The 20% That Matters**  
**Goal:** Understand **core SD-WAN concepts**, how they differ from traditional WAN, and how they integrate with IPSec.

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## **1. SD-WAN vs Traditional WAN**  
| **Feature**          | **Traditional WAN (MPLS/VPN)** | **SD-WAN** |
|----------------------|-------------------------------|------------|
| **Cost**             | Expensive (MPLS circuits)      | Cheaper (uses Internet + broadband) |
| **Agility**          | Manual config changes         | Centralized, automated policies |
| **Performance**      | Predictable but rigid         | Dynamic path selection (jitter/loss-aware) |
| **Security**         | Relies on IPSec/MPLS          | Built-in encryption (IPSec, TLS) |
| **Topology**         | Hub-and-spoke                 | Any-to-any, mesh |

**Key Takeaway:**  
- SD-WAN **decouples control plane from hardware**, allowing dynamic traffic routing over **any transport (MPLS, LTE, broadband)**.

---

## **2. SD-WAN Core Components**  
### **(1) Edge Devices (CPE)**  
- **e.g., Cisco vEdge, FortiGate, VeloCloud**  
- Sit at branch offices, apply policies, and encrypt traffic.  

### **(2) Orchestrator (Controller)**  
- **e.g., Cisco vManage, VMware Orchestrator**  
- **Centralized policy management** (no CLI needed!).  

### **(3) Overlay Tunnels**  
- **Encrypted tunnels** (IPSec, GRE, DTLS) between edges.  
- Uses **TLOC (Transport Locator)** = Public IP + Color (e.g., `INET`, `MPLS`).  

### **(4) Underlay Transport**  
- **Any WAN link**: MPLS, Internet, LTE, 5G.  

---

## **3. How SD-WAN Works (The 80% You Need)**  
### **(1) Path Selection**  
- **Dynamic multi-path steering**: Chooses best path based on:  
  - **Application SLA** (e.g., VoIP → low latency).  
  - **Real-time metrics** (jitter, packet loss, latency).  

**Example Policy:**  
```plaintext
IF (Application == VoIP) AND (Latency > 50ms) → SWITCH to backup link  
```

### **(2) Zero-Touch Provisioning (ZTP)**  
- Plug in a device → auto-configures via orchestrator.  

### **(3) Application-Aware Routing**  
- **DPI (Deep Packet Inspection)** identifies apps (e.g., Teams, SAP).  
- **QoS prioritization** (VoIP > YouTube).  

### **(4) Security Integration**  
- **IPSec for all overlays** (mandatory for Internet links).  
- **Cloud-based firewalls** (e.g., FortiGate, Zscaler).  

---

## **4. SD-WAN + IPSec Integration**  
- **SD-WAN uses IPSec for secure tunnels** but adds:  
  - **Automated key rotation** (no manual PSK updates).  
  - **Tunnel bonding** (combines multiple links for throughput).  

**Key Difference:**  
- Traditional IPSec VPN = **static tunnels**.  
- SD-WAN IPSec = **dynamic, SLA-driven tunnels**.  

---

## **5. SD-WAN Troubleshooting (Top 5 Issues)**  
| **Issue**                     | **Debug Command**                     | **Fix** |
|-------------------------------|--------------------------------------|---------|
| **Tunnels not coming up**     | `show sdwan tunnel` (Cisco)          | Check underlay reachability |
| **Poor VoIP quality**         | `show sdwan app-route stats`         | Adjust SLA thresholds |
| **Orchestrator sync failure** | `show sdwan control connections`     | Verify certs/connectivity |
| **Traffic taking wrong path** | `show sdwan policy-service-path`     | Fix application-aware rules |
| **High latency on backup**    | `show sdwan interface`               | Enable FEC (Forward Error Correction) |

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## **6. SD-WAN vs. DMVPN (Common Interview Qs)**  
**Q: When would you use SD-WAN over DMVPN?**  
- **SD-WAN**: When you need **application-aware routing + centralized management**.  
- **DMVPN**: When you need **scalable IPSec tunnels but don’t need SaaS optimization**.  

**Q: Can SD-WAN replace IPSec?**  
- **No!** SD-WAN **uses** IPSec for encryption but adds intelligence on top.  

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## **7. Lab Practice (Quick Wins)**  
1. **Simulate link failure** in GNS3/EVE-NG → Watch SD-WAN switch paths.  
2. **Prioritize VoIP traffic** over YouTube.  
3. **Break the orchestrator** → Observe fallback to local policies.  

**CLI Examples (Cisco Viptela):**  
```bash
show sdwan control connections  # Check orchestrator status  
show sdwan app-route stats      # Verify path selection  
clear sdwan tunnel              # Force tunnel re-establishment  
```

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## **8. Interview Cheat Sheet**  
✅ **SD-WAN = Automation + Application-Aware Routing + Multiple Underlays**.  
✅ **IPSec is still used, but dynamically managed**.  
✅ **Key metrics: Jitter (<30ms), Latency (<150ms), Packet Loss (<1%)**.  
✅ **Orchestrator is the brain; edges are the muscle**.  

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### **Where to Go Next?**  
1. **Deep dive into your vendor’s SD-WAN** (Cisco, Fortinet, VMware).  
2. **Learn cloud-integrated SD-WAN** (AWS Transit Gateway, Azure Virtual WAN).  
3. **Study real-world designs** (e.g., "How SD-WAN replaces MPLS").  

Need a **deep dive on a specific SD-WAN vendor** or **mock scenarios**? Let me know! 🚀