Software-defined wide area networking, or SD-WAN, uses software abstraction to create a virtual overlay network that sits on top of the underlying WAN infrastructure. This gives organizations greater control over distributed network architectures and enables a high degree of automation. SD-WAN can be deployed in a hub-and-spoke model or a variety of mesh topologies depending on the organization’s requirements and capabilities.
In this guide are the most common SD-WAN deployment models with diagrams, pros and cons, and best practices to help organizations choose the right topology.
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Hub-and-spoke SD-WAN deployment models
In the hub-and-spoke model, large data centers serve as centralized hubs that all SD-WAN traffic must flow through. The spokes are the campuses, branch offices, and edge sites that connect to each hub in order to access the enterprise WAN.
The hub-and-spoke SD-WAN deployment model can be executed in a couple of different ways:
In this SD-WAN deployment model, the spokes for a particular hub are unable to communicate with each other, they can only interact with the central hub. The networking logic for this deployment model is relatively simple compared to the others. It’s also useful when spoke sites need to be isolated from each other for security, privacy, or regulatory reasons.
Spoke-to-spoke via hub
In this deployment, spoke sites can communicate with each other, but all of their traffic must pass through the central hub first. Again, this makes SD-WAN networking logic a little simpler than allowing direct communication between sites. It also allows spoke traffic to pass through a centralized firewall first so that traffic can be inspected and security policies can be applied without needing to deploy firewall appliances at each spoke site.
Hub-and-spoke pros and cons
The hub-and-spoke approach is relatively easy to deploy and manage because all SD-WAN traffic must flow through a limited number of hub data centers. There’s no need to configure, secure, and load-balance network traffic between the numerous spoke sites that comprise an enterprise network, so organizations that are adopting SD-WAN for the first time tend to favor this approach.
However, since all spoke traffic needs to pass through a centralized hub, this creates bottlenecks that can impact network performance for all connected sites. Centralized hubs also represent single points of failure for the spokes that rely on them for enterprise network access – if a hub goes down due to a cybersecurity breach, ISP outage, or equipment failure, then spokes will lose their ability to communicate with each other or access enterprise network resources. Plus, since all traffic and data are aggregated in centralized hubs, they present attractive targets to cybercriminals, who can easily get all the valuable data they want in a single location instead of needing to move around the network.
Hub-and-spoke best practices
If you’re considering the hub-and-spoke approach, it’s important to have a business continuity plan that accounts for hub failures. For example, you should have some sort of network failover option (such as cellular LTE) in case the ISP connection goes down. You should also be able to redirect traffic from one hub to another if the first hub gets overloaded or experiences a failure. In addition, hubs should be deployed with out-of-band (OOB) management so network admins have 24/7 management access even if the SD-WAN connection goes down.
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Mesh SD-WAN deployment models
In the mesh deployment model, campuses, branch offices, and edge sites have some ability to communicate with each other directly, without their traffic needing to pass through a hub first. The degree to which sites can interact with each other depends on the type of mesh topology that’s deployed.
Full mesh deployment
In a full mesh deployment, there is any-to-any communication between edge and branch sites using overlay tunnels. There are often still hub data centers to handle requests for centralized enterprise resources, but most other requests are handled by services deployed at the network edge (either at the sites themselves or in the cloud).
Full mesh pros and cons
The full mesh topology enables faster communication between sites. It also allows organizations to deploy applications and services at the edge of the network where data is being generated and used, which is known as edge computing. There are also no single points of failure in the full mesh SD-WAN architecture, which reduces the risk of an outage and shortens the duration of outages that do occur. In addition, data and applications are distributed across many different locations, which can limit the damage caused by security breaches – instead of finding all your valuable data in a single server, cybercriminals have to jump around the network, increasing the chances they’ll be caught.
However, the full mesh topology is the most challenging to implement, manage, and optimize because there are so many moving parts involved. Each site needs a full stack of networking and security solutions because they aren’t relying on a centralized hub. Increasing the complexity of the SD-WAN architecture also increases the risk that mistakes will be made during configurations, deployments, and updates.
Partial mesh deployment
In this topology, some spokes can directly communicate with each other, and some services are deployed at the edge. However, there isn’t a full decentralization of applications and security, so some traffic still needs to pass through a hub.
Partial mesh pros and cons
Many organizations arrive at a partial mesh deployment on their journey from a hub-and-spoke deployment to a full mesh. A partial mesh topology reduces some of the load on centralized hubs and firewalls by moving certain services to the edge and allowing some direct site-to-site communications, which can improve performance. The organization can still rely on some centralized security functionality, reducing the number of security appliances or cloud-based services needed to protect the edge.
However, a partial mesh doesn’t completely address the limitations of a more centralized hub-and-spoke deployment. Hubs still represent potential bottlenecks and points of failure in the SD-WAN architecture. Plus, there’s still a lot of data centralization, meaning that storage and data processing servers in hubs will make attractive targets to hackers.
Hybrid mesh deployment
In a hybrid mesh, many hubs are deployed in a mesh topology, which means there is frequent communication and a high level of redundancy. However, branch and edge sites are still deployed as spokes, which means all their traffic goes through the hub mesh.
Hybrid mesh pros and cons
A hybrid mesh deployment ensures that no single hub acts as a single point of failure, enabling a high degree of redundancy and load balancing. Spoke sites can’t directly interact with each other, which reduces the complexity of the SD-WAN topology and decreases the number of security appliances to purchase and manage. Bottlenecking is also less of an issue because traffic from spokes doesn’t have to pass through a single designated hub, and can instead be distributed across multiple hubs.
However, a hybrid mesh topology is still fairly centralized, so you don’t get all the security and performance benefits of a decentralized full mesh deployment. Hubs are still valuable targets for cybercriminals, edge sites still need to send data to hubs for processing, and spokes cannot access enterprise network resources without going through the hub mesh. If that hub mesh is unavailable for any reason, spoke sites will be completely cut off.
A regional mesh allows organizations to implement different deployment models for different regions. For instance, one region may have a spoke-to-spoke deployment, while another has a partial mesh. When traffic from one region needs to go to another, it must pass through a regional hub first.
Regional mesh pros and cons
The regional mesh model gives large or highly distributed organizations the flexibility to use deployments that meet the specific requirements of each region. For example, there may be a cluster of branch offices that process classified data for the defense industry, so the organization uses the spoke-to-hub-only deployment to isolate those sites from the rest of the enterprise network. In another region, the org may run a lot of edge computing for machine learning applications and need there to be fast and seamless communication between edge data centers. In that case, they might deploy a full mesh topology in that specific region.
However, this flexibility can also create a lot of management complexity. Admins need to learn, manage, and troubleshoot different configurations, network logic, and security rules for different regions. Plus, each region is subject to the downsides and limitations of whichever deployment model is used there. In addition, regional hubs often still act as potential bottlenecks and points of failure between each region and the enterprise network.
Mesh best practices
All of the best practices for the hub-and-spoke model also apply to mesh SD-WAN deployments. There are a couple of additional considerations for organizations considering a more decentralized deployment, including:
Cloud-delivered security – Mesh deployments typically require security solutions deployed at the edge, so that traffic doesn’t need to pass through a centralized firewall. However, purchasing appliances for each location is expensive and adds to the complexity of the architecture. Solutions like SSE (Security Service Edge) allow you to use your SD-WAN technology to send edge traffic through security services in the cloud. SSE takes many of the capabilities of traditional security appliances and combines them into a cloud-delivered stack, so organizations can get enterprise-level protection without needing to deploy additional hardware or route traffic through a central bottleneck.
Network automation – Manually configuring and controlling a complex mesh network increases the risk of human error. Luckily, SD-WAN’s software abstraction makes it easier to use automated scripts and playbooks to handle a lot of the tedious, repetitive tasks that are prone to mistakes such as device configurations and ACL (access control list) updates. Automation also means that work can get done faster and without putting additional pressure on overworked network teams.
Centralized orchestration – Mesh topologies have so many moving parts, and needing to manage each one of them separately makes it impossible to holistically control, optimize, and secure the deployment. Centralized orchestration gives network teams a common interface they can use to create, organize, deploy, monitor, and troubleshoot automated and manual SD-WAN workflows. That means the orchestrator should be vendor-neutral so it can dig its hooks into every device, application, and service in every SD-WAN site in your deployment.
Vendor neutrality – On that note, vendor neutrality is also a best practice because of how it simplifies and strengthens automation infrastructure. Many vendors lock you into using specific tools, applications, and devices, which makes automation pipelines more complicated and brittle — if one component breaks, the whole infrastructure goes down. Using vendor-neutral tools and orchestration platforms means you don’t have to inflate your toolkit or overcomplicate your automation pipeline, improving resiliency.
Choosing the right SD-WAN deployment model
Every SD-WAN topology comes with its own advantages and limitations, so organizations must assess their requirements and capabilities to determine which one to deploy. For example, if your team doesn’t have any SD-WAN experience, then jumping right into a full mesh deployment may not be practical. If you need your edge sites to have real-time data processing abilities (for example, in healthcare applications), then you may want to consider a less centralized mesh topology.
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