Update tech_docs/networking/sdwan.md

tech_docs/networking/sdwan.md

@@ -95,6 +95,103 @@ Would you like a **sample config snippet** for any of these sections?
 
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+To **bring up an SD-WAN fabric**, you need to configure key components correctly. Below is a **concise, step-by-step breakdown** of the essentials, along with **critical design considerations**.
+
+---
+
+### **1. Underlay Network (VPN 0 - Transport VPN)**
+- **Purpose**: Handles **control-plane traffic** (OMP, DTLS/TLS tunnels between devices).  
+- **Key Configurations**:  
+  - **Interfaces**: Assign WAN interfaces (e.g., MPLS, Internet, LTE) to VPN 0.  
+  - **Routing**:  
+    - Static routes (for simple setups).  
+    - BGP/OSPF (for dynamic underlay routing in larger deployments).  
+  - **TLOC Extensions**: Define public/private IPs for tunnel endpoints.  
+- **Design Considerations**:  
+  - **Dual Underlay**: Use at least **two transport types** (e.g., MPLS + Internet) for redundancy.  
+  - **TLOC Preference**: Prioritize cheaper/faster links (e.g., MPLS over LTE).  
+
+---
+
+### **2. Overlay Network (OMP Routing)**
+- **Purpose**: Distributes routes and policies across the fabric.  
+- **Key Configurations**:  
+  - **OMP (Overlay Management Protocol)**: Advertises routes between vSmart controllers and edges.  
+  - **Route Policies**: Control which prefixes are shared (e.g., only corporate LAN routes).  
+- **Design Considerations**:  
+  - **Route Aggregation**: Minimize prefixes advertised to vSmart (e.g., summarize branch LANs).  
+  - **TLOC Redundancy**: Assign multiple TLOCs per route for failover.  
+
+---
+
+### **3. Service VPNs (VPN 1-511)**
+- **Purpose**: Segments user/data traffic (e.g., VoIP, guest Wi-Fi).  
+- **Key Configurations**:  
+  - **VRF Creation**: Define VPNs (e.g., `vpn 10` for corporate LAN).  
+  - **Interface Assignment**: Assign LAN interfaces to the correct VPN.  
+  - **Route Leaking**: If needed, allow controlled traffic flow between VPNs (via centralized policies).  
+- **Design Considerations**:  
+  - **QoS Tagging**: Apply DSCP markings per VPN (e.g., EF for VoIP in `vpn 20`).  
+  - **Security Policies**: Restrict inter-VPN communication (e.g., guest Wi-Fi in `vpn 30` can’t reach `vpn 10`).  
+
+---
+
+### **4. Internet Breakout (VPN 512)**
+- **Purpose**: Local internet access (DIA) from branches.  
+- **Key Configurations**:  
+  - **NAT & Firewall**: Enable NAT overload (PAT) for private→public IP translation.  
+  - **PBR (Policy-Based Routing)**: Steer specific traffic (e.g., Zoom) to VPN 512.  
+- **Design Considerations**:  
+  - **Security**: Apply **umbrella/ZTNA** for secure internet access.  
+  - **Backup Path**: If VPN 512 fails, fall back to centralized internet via VPN 0.  
+
+---
+
+### **5. Management & Control Plane (VPN 0)**
+- **Purpose**: Connects vEdges to controllers (vManage, vSmart, vBond).  
+- **Key Configurations**:  
+  - **Controller IPs**: Ensure vEdges can reach vManage/vSmart/vBond over VPN 0.  
+  - **Certificate Auth**: Use device certificates for secure onboarding.  
+- **Design Considerations**:  
+  - **Out-of-Band (OOB) Management**: Use a separate OOB network for high availability.  
+  - **Geo-Redundancy**: Deploy controllers in multiple regions.  
+
+---
+
+### **6. Security Policies**
+- **Purpose**: Enforce traffic rules (e.g., blocking, inspection).  
+- **Key Configurations**:  
+  - **Zone-Based Firewall**: Assign interfaces to zones (e.g., "inside," "outside").  
+  - **Application-Aware Policies**: Block high-risk apps (e.g., Tor, Netflix).  
+- **Design Considerations**:  
+  - **Default-Deny**: Start with "deny all," then allow only needed traffic.  
+  - **IPS/IDS**: Enable for internet-bound traffic (VPN 512).  
+
+---
+
+### **7. High Availability (HA)**
+- **Design Considerations**:  
+  - **Dual vSmarts**: Avoid single points of failure.  
+  - **Active/Standby Edges**: Use VRRP for LAN-side HA.  
+  - **Cloud Gateway Redundancy**: For cloud-onramp (e.g., AWS/Azure).  
+
+---
+
+### **Summary Checklist**
+| **Step** | **Action** | **Critical Design Tip** |
+|----------|-----------|-------------------------|
+| **1. Underlay** | Configure VPN 0 interfaces & routing | Dual transports (MPLS + Internet) |
+| **2. Overlay** | Set up OMP & route policies | Summarize routes to reduce overhead |
+| **3. Service VPNs** | Define VPNs 1-511 & assign interfaces | Use QoS for VoIP/VC traffic |
+| **4. Internet** | Configure VPN 512 for DIA | Add ZTNA/umbrella for security |
+| **5. Management** | Ensure controllers are reachable via VPN 0 | OOB management for resiliency |
+| **6. Security** | Apply firewall/IPS policies | Default-deny approach |
+| **7. HA** | Deploy redundant controllers/edges | Active/standby for critical sites |
+
+Would you like a **sample config snippet** for any of these sections?
+
+---
+
 Yes, you're absolutely correct. In Cisco SD-WAN (formerly Viptela), **VPN 0** is indeed referred to as the **"front-door VRF" (FD-VRF)**. This is a critical concept in the architecture. Let me break down why it's special and how to properly configure it.
 
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