The implementation of a **Dual Hub with Dual DMVPN (Dynamic Multipoint Virtual Private Network)** layout offers an efficient solution for enterprises seeking enhanced control over routing and redundancy. Although slightly more complex to set up compared to single-hub architectures, it delivers robust failover capabilities, optimized routing, and improved bandwidth utilization.
This article will compare the setup and considerations for deploying a dual-hub, dual-DMVPN layout on older routers and the improvements observed in the latest Cisco IOS, post 15.9(3)M10.
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### **Understanding Dual Hub with Dual DMVPN Architecture**
A **dual-hub, dual-DMVPN topology** involves two DMVPN clouds where:
- Each hub router is connected to a dedicated DMVPN subnet (or "cloud").
- Spoke routers connect to both DMVPN subnets via two GRE tunnel interfaces, creating routing redundancy.
This architecture enables better load balancing and more refined control over the routing protocol's decision-making process. By adjusting interface-level configurations such as bandwidth, cost, and delay, the spoke routers can prioritize one hub over the other in normal operation, while still providing seamless failover if one hub becomes unavailable.
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### **Challenges in Older Cisco Routers**
On routers running older Cisco IOS versions, there were several challenges when configuring and managing a dual-hub DMVPN layout:
1. **Limited Scalability and Performance**
Older devices struggled with scaling DMVPN configurations involving multiple tunnels due to hardware constraints. GRE tunnel termination and encryption demanded significant processing power, leading to potential performance bottlenecks.
2. **Static Workarounds for Routing Protocol Metrics**
Manual tweaking of routing protocol metrics was often cumbersome. While EIGRP, OSPF, and BGP are commonly used in DMVPN setups, achieving fine-grained control over routing decisions often required complex configurations, which could become error-prone.
3. **Configuration Complexity**
Implementing QoS (Quality of Service), bandwidth controls, and routing adjustments across the two hubs and multiple spokes required detailed planning and was difficult to maintain.
4. **Security Enhancements**
Early IOS versions lacked the advanced security features needed for securely handling dynamic spoke-to-spoke communication.
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### **Advantages of Cisco IOS 15.9(3)M10 and Later**
Cisco IOS 15.9(3)M10 introduces multiple enhancements that simplify the deployment and management of dual-hub, dual-DMVPN topologies. Here's how:
#### **1. Improved DMVPN Performance**
The newer IOS versions are optimized for modern hardware, providing:
- Better GRE and IPsec performance for handling multiple DMVPN clouds.
- Enhanced throughput for encrypted traffic.
- Support for new crypto algorithms that strengthen VPN security.
#### **2. Enhanced Routing Protocols**
Routing protocols now feature:
- **EIGRP DMVPN Stub Routing:** Reduces unnecessary routing updates to spokes, improving performance and efficiency.
- **Faster Convergence:** In the event of hub or tunnel failures, routing protocol convergence is quicker, minimizing downtime.
#### **3. Simplified QoS Configuration**
QoS policies can now be more easily applied to DMVPN interfaces, enabling:
- Dynamic prioritization of traffic across the tunnels.
- Better handling of bandwidth limitations and traffic shaping on spoke-to-hub links.
#### **4. Dynamic Metric Adjustments**
The newer IOS versions allow for better control over routing protocol metrics such as delay, cost, and bandwidth. This simplifies configuring the routing preferences for spoke routers:
- **Primary Hub Preference:** By setting a lower OSPF cost or EIGRP delay for the preferred hub, spokes automatically prioritize it.
- **Seamless Failover:** The secondary hub takes over without additional manual intervention.
#### **5. Security Enhancements**
- **IPsec VTI Integration:** Simplifies the configuration of secure tunnels.
- **IKEv2 Support:** Enhances tunnel establishment with faster and more secure key exchanges.
#### **6. Centralized Management with SD-WAN**
While not DMVPN-specific, newer Cisco IOS versions support integration with Cisco SD-WAN, enabling centralized configuration and monitoring of DMVPN clouds for larger deployments.
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### **Configuration Steps: New vs. Old IOS**
#### **Old IOS Configuration Highlights**
1. Manually configure two GRE tunnels on each spoke router.
2. Establish NHRP (Next Hop Resolution Protocol) mappings for both DMVPN clouds.
3. Fine-tune routing metrics for hub preference.
4. Configure IPsec profiles for secure communication.
#### **New IOS Configuration Highlights**
1. Use enhanced NHRP commands for dynamic mapping and spoke-to-spoke tunneling.
2. Simplify IPsec integration with VTIs (Virtual Tunnel Interfaces).
3. Leverage EIGRP DMVPN stub routing or optimized OSPF configurations for faster convergence.
4. Enable new QoS policies for dynamic traffic prioritization.
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### **Best Practices for Dual Hub, Dual DMVPN Setup**
1. **Plan Routing Metrics:** Clearly define primary and backup hub preferences using bandwidth or delay settings on the interfaces.
2. **Monitor and Optimize Performance:** Use tools like NetFlow or SNMP to monitor traffic and troubleshoot any bottlenecks.
3. **Implement Redundancy:** Ensure hubs are located in different geographic locations to avoid single points of failure.
4. **Test Failover:** Simulate hub failures to verify that spokes seamlessly reroute traffic to the backup hub.
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### **Conclusion**
The transition to Cisco IOS 15.9(3)M10 and later brings significant improvements for dual-hub, dual-DMVPN architectures. Enhanced routing protocol performance, simplified configurations, and robust security make it easier to deploy and maintain such setups. While older routers and IOS versions were functional, they often required more manual intervention and suffered from performance limitations. The newer advancements ensure that enterprises can achieve the full potential of DMVPN for secure, scalable, and efficient connectivity.
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