Seamless Cisco SD-WAN Adoption: Integrating cEdge into Legacy Branch Environments

In the evolving landscape of enterprise networking, the push for agility, scalability, and performance has led many organizations to pursue SD-WAN adoption. However, modernization efforts often confront practical challenges, particularly in branches that still rely on legacy WAN technologies. Integrating SD-WAN in such environments necessitates thoughtful design choices that accommodate existing infrastructure while enabling a smoother transition.

One of the most common scenarios involves deploying a single cEdge router alongside a retained customer edge (CE) router. This approach ensures legacy WAN transport technologies, such as T1 or DSL, remain supported while introducing SD-WAN capabilities. It allows enterprises to progressively upgrade their networking architecture without compromising operational continuity.

Rationale for Retaining Legacy Routers

There are several compelling reasons for retaining CE routers during an SD-WAN rollout. First and foremost, legacy circuits like T1, xDSL, or Frame Relay, which are not supported by newer SD-WAN hardware (especially vEdge), may still provide necessary connectivity in remote or underserved regions. The existing CE routers are typically equipped to terminate such circuits.

Additionally, some CE routers host services like voice gateways or Wide Area Application Services (WAAS), which may not yet be fully available in SD-WAN software images. While modern platforms are quickly catching up, current limitations still demand hybrid designs in many environments.

This leads to a practical compromise: preserving the CE router for legacy service and transport terminations, while shifting LAN and SD-WAN control to a new cEdge platform. This allows enterprises to take advantage of SD-WAN’s centralized control, policy enforcement, and analytics, without dismantling vital infrastructure.

Architecture Overview: A Single cEdge Paired with a CE Router

In the single cEdge topology, the CE router continues to terminate legacy WAN connections, but relinquishes its LAN responsibilities. These are assumed by the cEdge, which becomes the default gateway for all client subnets. This requires a new interconnection between the cEdge and CE router, usually through a dedicated subnet—commonly referred to as Subnet C.

Subnet C may be implemented using a physical link or a logical subinterface via a southbound switch. Regardless of the physical medium, the design must ensure high availability and low latency between the two routers, as it will carry both user traffic and control data.

The CE router uses Subnet C to forward traffic to the cEdge, which then encapsulates the data in IPsec tunnels and sends it over the SD-WAN fabric. This logical handoff forms the crux of the hybrid approach, enabling secure and intelligent WAN routing without altering legacy circuits.

Subnet Allocation and Routing Fundamentals

In most deployments, the WAN providers assign /30 subnets for each transport—commonly labeled Subnets A and B. These are used to connect the CE router to the service provider’s PE devices. Meanwhile, Subnet C is provisioned as a private interconnect between the CE and cEdge.

From a routing perspective, the cEdge configures two default routes—one pointing to the CE’s IP address on Subnet C (for access to private WAN), and the other directed toward the ISP’s edge router (for internet access, if required). These routes are critical for forming IPsec tunnels over both transports.

Subnet C must also be advertised correctly within the SD-WAN environment. This enables other SD-WAN nodes to establish tunnels with the cEdge using its Subnet C interface. Typically, this requires including Subnet C in BGP announcements to the provider or manually adjusting static routes on upstream devices.

SD-WAN Tunnel Establishment and LAN Routing

Once routing is properly established, the cEdge forms secure IPsec tunnels across available transports. It leverages the SD-WAN controller infrastructure—vBond, vSmart, and vManage—to authenticate, configure, and manage these tunnels. The result is a dynamic, policy-aware overlay network that routes traffic based on application performance, link health, and security rules.

The cEdge also assumes routing responsibilities for all LAN-side subnets. In the typical configuration, it operates in VPN 1 and hosts the default gateway for local devices—referred to here as Subnet X. Additional VLANs can be supported through subinterfaces, each representing a separate broadcast domain.

These LAN routes are redistributed into OMP (Overlay Management Protocol) and advertised to other SD-WAN devices. This allows branch-to-branch or branch-to-data center communication to flow over the SD-WAN fabric, bypassing legacy routing paths.

Internet Traffic Flow and Security Considerations

In environments without a local firewall, internet-bound traffic must be handled with care. The cEdge can either backhaul such traffic to a central security stack or send it directly to the cloud using secure internet breakout. The latter is often facilitated by integrating with cloud-based security solutions like Zscaler or Umbrella.

This model offers greater flexibility and reduced latency, particularly for SaaS and cloud-hosted applications. However, it also places a greater emphasis on secure policies, URL filtering, DNS inspection, and malware defense at the SD-WAN edge.

Failure Scenarios and Site Reliability Challenges

While this single cEdge approach significantly modernizes the network, it introduces a new single point of failure. If the cEdge fails, the branch site loses both WAN and internet access. Such an outage can be catastrophic, depending on the business criticality of the site.

For low-priority branches or remote offices, this risk may be tolerable, particularly if hardware replacement SLAs are in place (ranging from 2-hour onsite support to next-business-day delivery). Still, most enterprises strive to eliminate such risks through redundancy, which necessitates a more robust design—such as a dual cEdge setup.

In its current form, this architecture lacks high availability. There’s no failover mechanism if the cEdge becomes unresponsive. Traffic routing halts, tunnels collapse, and LAN clients are effectively isolated. For this reason, implementing proactive monitoring, alerting, and health checks is paramount.

Migration Process and Implementation Guidelines

Transitioning from a purely legacy topology to this hybrid SD-WAN design involves several key steps:

Subnet Planning: Carefully allocate address space for Subnet C and ensure that it does not conflict with existing schemes.
Interface Configuration: Disable LAN interfaces on the CE router and configure the cEdge as the new default gateway for LAN clients.
Routing Synchronization: Establish BGP or static routes between the CE and the cEdge, ensuring end-to-end reachability.
Tunnel Formation: Register the cEdge with vManage, vBond, and vSmart, and form IPsec tunnels with remote SD-WAN nodes.
Policy Definition: Create policies for application routing, transport preferences, security zones, and traffic classification.
Testing and Validation: Conduct failover simulations, performance benchmarks, and policy enforcement checks.

Each of these steps should be carried out with methodical precision, preferably in a controlled lab or staging environment before production rollout.

Strategic Implications for Network Evolution

Deploying a single cEdge in conjunction with a CE router is not merely a stopgap—it’s a strategic bridge between legacy and future-state infrastructure. It enables organizations to explore SD-WAN’s capabilities incrementally, test policies, train operations teams, and refine designs before committing to full-scale deployment.

Moreover, it allows IT leaders to align network transformation with broader digital initiatives. Whether the goal is to support remote work, cloud migration, or edge computing, SD-WAN provides the necessary flexibility and control.

This hybrid model also underscores the importance of network modularity. By decoupling service roles (transport, routing, security), enterprises can adopt new technologies with less disruption and greater agility.

The Imperative for High Availability in Branch Networks

As organizations scale their SD-WAN deployments beyond initial proof-of-concept or pilot sites, the need for network resilience and continuity becomes paramount. Branch offices, regardless of size, often provide critical business functions that demand uninterrupted access to corporate resources, cloud services, and the internet. A single point of failure in a network device—especially at the WAN edge—can trigger costly outages and degrade user experience.

Overview of Dual cEdge Architecture with Legacy WAN Support

The dual cEdge topology retains the legacy CE router for continued termination of legacy WAN transports but introduces two cEdge routers into the branch site. These devices work in concert to provide active-active or active-standby redundancy for SD-WAN functions, ensuring that failure of any one device does not interrupt WAN or internet services.

A key component of this architecture is the use of VRRP (Virtual Router Redundancy Protocol) between the two cEdges. VRRP allows the routers to present a single virtual default gateway IP to LAN clients, with one router acting as the master and the other as a backup. If the primary cEdge experiences failure, VRRP triggers an automatic failover, promoting the backup cEdge to the master role.

Subnet Design and Interconnectivity

Unlike the single cEdge model which uses a /30 subnet for the interconnect between CE and cEdge, the dual cEdge design requires a larger subnet, typically a /29, to accommodate three devices: the legacy CE router and both cEdges.

Subnet A: Assigned by the WAN provider for private WAN transport, usually a /30 for CE to provider.
Subnet B: Used for public internet transport and, in this scenario, expanded to /29 or larger to assign individual IP addresses to each cEdge router for direct connectivity.
Subnet C: The private interconnect subnet between the CE and cEdges in VPN 0 (Transport VPN), also sized as a /29 to accommodate all three devices.
Subnet X: LAN-side subnets served by the cEdges in VPN 1.

The physical interconnects typically involve the southbound access switch, which manages the VLAN tagging and ensures traffic segmentation between the CE and the two cEdges. This switch facilitates seamless communication over Subnet C and the transport layer.

Routing and Traffic Flow Between Devices

In this design, the CE router continues to terminate legacy WAN transports and serves as a bridge to the SD-WAN overlay. Each cEdge establishes IPsec tunnels over both private and public transports, providing dual paths for data to traverse the SD-WAN fabric.

Two default routes exist on each cEdge, one for each transport type, pointing respectively to the ISP’s Provider Edge (PE) router IPs. The cEdges independently manage their tunnels, ensuring redundancy.

On the LAN side, the cEdges participate in VRRP, advertising a virtual IP address as the default gateway for client devices. The master cEdge handles active routing, while the backup remains ready to assume responsibility instantaneously if needed.

This approach not only eliminates the single point of failure from the previous design but also balances traffic loads, improving bandwidth utilization and providing seamless failover.

Advertisement and Route Management in SD-WAN Overlay

To maintain consistent routing and traffic flow across the overlay network, both cEdges redistribute connected LAN routes into the Overlay Management Protocol (OMP). OMP is the routing protocol native to the SD-WAN fabric, distributing routes, policies, and security information across the entire environment.

Route preference manipulation plays a critical role here: the routes advertised by the master VRRP cEdge are assigned a higher preference than those from the backup. This preference ensures that return traffic follows symmetrical paths, preserving session integrity and avoiding asymmetric routing challenges.

Additionally, the CE router’s legacy routes must be carefully managed to avoid leaking into the SD-WAN overlay improperly, which could cause routing loops or asymmetries. Controlled route redistribution between the underlay and overlay layers is critical to maintaining network stability.

Failover Scenarios and Operational Resilience

The dual cEdge design excels in providing robust failover capabilities. Consider the following scenarios:

Primary cEdge Failure: VRRP triggers a failover, promoting the secondary cEdge to master status. Existing tunnels re-route traffic with minimal disruption, and clients maintain uninterrupted connectivity.
Legacy CE Router Failure: Since the CE terminates legacy transports, its failure can impact legacy WAN connectivity. However, SD-WAN tunnels on the public transport path remain active if available, allowing some continuity.
Transport Link Failures: Each cEdge maintains independent tunnels on both private and public transports, so loss of one link does not disrupt connectivity entirely. Dynamic path selection reroutes traffic automatically.

These features significantly improve the branch’s uptime and user experience, minimizing operational risks and service-level agreement (SLA) violations.

Implementation Considerations and Best Practices

Deploying a dual cEdge topology alongside a legacy CE router involves careful planning and execution. Key considerations include:

Addressing and Subnetting: Confirm sufficient IP space for /29 subnets on transport and interconnect segments. Planning early avoids complex re-addressing later.
VRRP Configuration: Properly configure VRRP priorities, timers, and preemption to ensure rapid and deterministic failover.
Route Redistribution Policies: Implement strict route filtering and redistribution controls between underlay and overlay domains to prevent loops and asymmetry.
Switch Configuration: Ensure southbound access switches are configured to support required VLANs and subinterfaces for seamless connectivity.
Tunnel Management: Verify IPsec tunnels on both cEdges form correctly and can independently route traffic over all transport links.
Testing Failover: Conduct exhaustive failover and recovery tests simulating device, link, and power failures to validate operational readiness.
Monitoring and Alerting: Deploy monitoring tools capable of detecting VRRP state changes, tunnel status, and interface health in real-time.

These measures reduce operational complexity and help maintain the high availability promised by this architecture.

Operational Benefits and Business Impact

From a business perspective, deploying dual cEdges with a retained legacy CE router delivers several tangible benefits:

Improved Reliability: Redundant devices eliminate a single point of failure, minimizing downtime.
Performance Optimization: Traffic can be load-balanced across transports and devices, improving throughput and latency.
Service Continuity: Legacy WAN support continues without forcing premature migration or disruption.
Simplified Troubleshooting: Clear separation between legacy and SD-WAN functions facilitates targeted diagnostics.
Scalable Growth: The architecture supports gradual migration, allowing IT teams to scale SD-WAN features at their own pace.

Such resilience is particularly crucial for sites hosting customer-facing applications, voice services, or critical operational systems.

Challenges and Limitations to Consider

While dual cEdge deployment addresses many shortcomings of the single cEdge design, it is not without challenges:

Increased Complexity: Managing multiple devices, VRRP states, and inter-router routing policies demands more operational expertise.
Additional Hardware Costs: Procuring and maintaining a second cEdge increases CAPEX and OPEX.
Legacy Router Dependency: The CE router remains a potential bottleneck and single point of failure for legacy transports.
Route Redistribution Risks: Improperly configured route leaking can cause loops or asymmetric paths, impacting performance and stability.

Understanding these trade-offs is essential to designing an effective and maintainable branch network.

Preparing for Future Migration and Simplification

As SD-WAN platforms mature, many legacy services are gradually being ported into native SD-WAN images, reducing reliance on the CE router. Eventually, organizations can remove legacy devices entirely, simplifying branch topologies and lowering operational overhead.

The dual cEdge model positions enterprises well for this transition. It offers robust high availability today, while enabling incremental service migration tomorrow. As more functions become SD-WAN-native, the CE router’s role diminishes until it can be gracefully decommissioned.

During this transition, continuous alignment between network operations, security teams, and service providers is vital to coordinate changes, update routing policies, and maintain service levels.

Balancing Redundancy, Legacy Support, and SD-WAN Agility

Deploying dual cEdge routers alongside a retained legacy CE router offers a compelling balance between modern SD-WAN agility and legacy WAN transport continuity. It addresses critical high-availability concerns inherent to single cEdge designs, ensuring fault-tolerant connectivity for business-critical branch sites.

This design empowers enterprises to leverage advanced SD-WAN features such as dynamic path selection, application-aware routing, and centralized policy control, without sacrificing legacy service support. Thoughtful subnet design, VRRP configuration, and route management are essential to avoid pitfalls related to routing asymmetry or loops.

While the complexity and cost increase with dual devices, the operational benefits and reduced outage risks justify the investment in many enterprise environments. Ultimately, this hybrid design forms a vital step in the gradual evolution from legacy WAN infrastructures to fully integrated SD-WAN fabrics capable of supporting the demands of modern digital enterprises.

Retaining Legacy Services During SD-WAN Integration

In numerous branch scenarios, legacy services such as WAAS (Wide Area Application Services) or on-premise Voice systems must persist even as SD-WAN technology is introduced. These services, though aging, often represent significant investments and remain indispensable for critical business functions. Therefore, a network design must accommodate their continued operation without compromising the benefits of SD-WAN.

Deploying a single cEdge router alongside a CE router that hosts these services allows for a hybrid approach. This method respects the need for service continuity while providing the scalability and policy control of SD-WAN. The network must be meticulously segmented to isolate service processing from SD-WAN routing logic, preserving both operational clarity and security posture.

Designing the Interconnect: Subnets C and D

This deployment requires the creation of two distinct subnets: Subnet C in VPN 0, which provides the transport link between the cEdge and the CE router for WAN connectivity, and Subnet D in VPN 1, which facilitates LAN-side service routing.

Both subnets can be implemented using a single physical connection between the routers, with each subnet realized as a logical subinterface. Alternatively, distinct physical interfaces may be used if hardware permits. The Subnet C connection enables the cEdge to reach private WAN circuits terminated on the CE, while Subnet D allows the cEdge to receive LAN traffic that has been processed by the service node.

VRRP for Gateway Redundancy

In this setup, the CE router maintains its gateway role for LAN clients. VRRP is configured across Subnet X (the client subnet) between the CE and the cEdge, with the CE typically assuming the master role. This arrangement ensures all client traffic first encounters the service node, allowing for inspection, optimization, or voice-specific handling.

Northbound traffic is routed from the client to the CE, where any required services are applied. It is then forwarded across Subnet D to the cEdge’s VPN 1 interface. At this point, the cEdge encapsulates the traffic into the SD-WAN fabric for onward transport.

Ensuring Symmetry with Data Policies

To maintain symmetric service application, return traffic arriving from the SD-WAN overlay must also pass through the CE router. This is achieved using a data policy or route-map on the cEdge, which redirects traffic destined for Subnet X across Subnet D to the CE. The CE then processes this traffic as required before final delivery to LAN clients.

Without this redirection, the cEdge might respond directly to return traffic using its own interface in Subnet X, bypassing the CE and breaking symmetry. This could lead to inconsistent application of policies, particularly for stateful services such as firewalls or application optimization engines.

Failover Considerations and Transport Path Design

In the event of a CE router failure, the cEdge automatically becomes the VRRP master and assumes the gateway role. Traffic then bypasses the failed service node and proceeds directly to or from the SD-WAN overlay. While this temporarily suspends any legacy services, network connectivity remains intact, preserving business continuity.

If the private WAN transport is also terminated on the CE, the cEdge will lose access to this path during failure. Public internet tunnels, however, continue to function. To eliminate this vulnerability, private WAN circuits can be terminated directly on the cEdge, with the CE relegated purely to a service role.

Managing Routing and Advertisement Protocols

Careful consideration must be given to routing advertisements between the CE and cEdge routers. In VPN 1, the CE learns overlay routes via internal routing protocols such as OSPF or EIGRP peered with the cEdge. This dynamic exchange ensures that the CE can properly forward northbound traffic after service processing.

Meanwhile, the cEdge advertises connected and redistributed routes from VPN 1 into OMP, which are then shared across the SD-WAN fabric. It is imperative that these routes reflect the true path through the service node to ensure end-to-end policy compliance and service visibility.

This configuration reflects a balance between innovation and continuity—a mesh of modern SD-WAN capabilities woven delicately with the threads of legacy systems that, though aging, still serve vital operational purposes.

Dual cEdge Setup for Sites Requiring Legacy Services

Branch environments that still rely on vital legacy services but demand high availability require a more sophisticated network design. Combining dual cEdge routers with a CE services router provides a resilient architecture that not only supports service continuity but also minimizes the risk of network outages. This configuration enhances reliability, service performance, and traffic control while maintaining support for WAAS, voice, or any other hosted service.

By using VRRP, intelligent subnet segmentation, and seamless handoffs, this design ensures both SD-WAN performance and legacy compatibility. Each cEdge functions as a fully active node, and the CE router serves as the services node, making sure the network continues to operate smoothly even during device failures.

Extended Interconnect Using Subnets C and D

In this model, Subnet C (VPN 0) is used for transport-layer communication among CE, cEdge1, and cEdge2. A /29 subnet is recommended to accommodate all three devices. Subnet D (VPN 1) handles service-layer communication and should also be a /29 to allow routing flexibility.

These interconnects are often built using subinterfaces on physical links between the CE and both cEdges or trunked through the branch switch. Ensuring consistency in VLAN tagging, access policies, and routing paths across both subnets is essential to maintain operational harmony.

Traffic Flow Dynamics and VRRP Roles

Client devices reside in Subnet X and communicate through a virtual gateway IP shared between CE, cEdge1, and cEdge2 via VRRP. The CE router, as the VRRP master, processes inbound LAN traffic, applies services, and forwards it to the appropriate cEdge over Subnet D. The receiving cEdge then encapsulates and forwards the traffic into the SD-WAN fabric.

On the return journey, data enters through either cEdge, is policy-routed to the CE over Subnet D for symmetric service treatment, and is finally sent to the client on Subnet X. This ensures all traffic undergoes identical service processing in both directions, preserving session state and QoS.

Redundancy and Failover Mechanisms

Both cEdges maintain independent tunnels to SD-WAN transports. If the CE fails, the standby cEdge becomes the VRRP master, immediately assuming LAN gateway responsibilities. WAN services continue via SD-WAN tunnels, but service-layer functionality is bypassed unless failover services are mirrored on both cEdges.

If either cEdge fails, the remaining one maintains connectivity and VRRP adjusts accordingly. This configuration ensures that no single device failure can disrupt both internet and intranet connectivity, creating a robust, fault-tolerant network edge.

For further redundancy, dual transport links (public and private) can be terminated directly on both cEdges. This alleviates dependencies on the CE for WAN access, allowing services to remain intact even if one WAN link or service interface fails.

Coordinating Route Advertisements and Policies

Route management must be meticulously controlled to prevent asymmetry or routing loops. The CE and both cEdges should share routes in VPN 1 via a dynamic routing protocol like OSPF or EIGRP. These routes are then injected into OMP by each cEdge and distributed throughout the SD-WAN overlay.

Policy-based routing rules on the cEdges ensure that return traffic from the SD-WAN domain is directed through the CE before reaching the LAN. This redirection is vital to maintain full service symmetry.

Additionally, OMP route preferences and control policies should be configured to favor the VRRP master’s advertisements. This prevents unpredictable routing behaviors and enhances consistency during failovers or role changes.

Operational Considerations for Maintenance and Upgrades

With dual cEdges and an independent CE service node, planned maintenance can be executed with reduced risk. For example, software upgrades on one cEdge do not disrupt the site, as the secondary cEdge can continue tunnel operations and maintain routing duties.

Likewise, the CE router can be rebooted or replaced with minimal service interruption, provided redundancy paths are well-designed. Implementing warm standby or mirrored service capabilities on both cEdges further minimizes potential downtime.

Monitoring and logging across all devices must be synchronized. Real-time visibility into VRRP status, route advertisements, and interface statistics helps quickly identify and resolve anomalies. Tools integrated with SD-WAN controllers provide a single-pane-of-glass view that simplifies operations and enhances responsiveness.

Design Strengths and Forward Compatibility

This dual cEdge plus CE service model is an exemplary embodiment of transitional design. It gracefully accommodates the persistence of traditional network services while preparing the site for full SD-WAN adoption. As Cisco and other vendors continue to port services into SD-WAN-native platforms, the CE node can eventually be phased out, simplifying the topology without forcing immediate change.

The architecture allows organizations to adopt SD-WAN progressively, protecting their investment in existing services while benefiting from enhanced policy control, analytics, and scalability.

By weaving together redundancy, policy-driven routing, and legacy service accommodation, this network design fosters a resilient, adaptable branch fabric that stands resilient in the face of operational volatility and evolving technology landscapes.

Conclusion

In today’s evolving network landscape, integrating SD-WAN with existing legacy WAN infrastructures demands a thoughtful balance between innovation and continuity. The progressive approaches—from single cEdge integration to dual cEdge deployments with legacy routers—demonstrate practical pathways to achieve resilient, scalable, and manageable branch networks. These architectures ensure uninterrupted service by supporting legacy transports while unlocking the benefits of SD-WAN, such as enhanced traffic control, dynamic path selection, and centralized management.

Although complexity and costs may increase with redundancy and hybrid setups, the payoff in reliability and operational agility is significant. By adopting these phased integration models, organizations can protect their existing investments, minimize risk during migration, and prepare their networks for future advances. Ultimately, this strategic evolution fosters a robust, adaptive network environment that meets today’s demands and anticipates tomorrow’s challenges, enabling businesses to thrive in an increasingly connected world.