Learn the differences between Software-defined wide area networks (SD-WAN) and Multiprotocol Label Switching (MPLS) protocol in supporting your multi-site connectivity. In this article, we provide tabular side-by-side comparison, and explain the pros, cons and benefits of each solution.
This article is meant to help the network administrators faced with a decision to choose between MPLS and SD-WAN to connect remote office or data center locations. We begin with an overview of each before we compare the pros and cons of each service and help you select the technology that is right for your needs and budget. Let us start with some background information.
In the second half of the 2010s, “software-defined” architectures led to several major changes in the networking industry. In simple terms, “software-defined” refers to the virtualization and programmatic configuration of network infrastructure, which can greatly improve network performance, agility, and monitoring capabilities.
Software-defined separates the control plane management of network devices from the data plane (also called underlay) that forwards network traffic. The control plane consists of controllers (the name varies based on the vendor) that have a complete view of the network and can program the underlay network devices.
Software-defined networking creates a software overlay network that abstracts away the underlying hardware used to transport traffic between endpoints. There are multiple applications for the software-defined technology in the network space:
The last technology in the list, SD-WAN, has begun to displace the once-dominant Multi-protocol Label Switching (MPLS) as the go-to WAN connectivity solution for many organizations. However, MPLS is far from a thing of the past, and in some cases, SD-WAN and MPLS can be used in tandem.
To help you better understand the modern WAN landscape and decide which solution is right for you, this article explores the pros and cons of MPLS and SD-WAN. But before we get too far, let’s take a high-level look at how the two stack up:
MPLS is a routing technique that is based on simple labels and operates “between” Layer 2 and Layer 3 of the OSI (open systems interconnection) Model.
To get a better idea of how MPLS works, let’s take a step back and look at how “traditional” routing works.
When a packet -- which inherently has a source and destination address -- traverses a network, routers check their routing tables to see what the next-hop IP is from the destination IP in the packet header. Once it is known, the packet is forwarded to the next-hop router, and the process repeats until the packet reaches its destination.
MPLS streamlines this process and allows the devices to forward the packets based on the label present on the packet. The label has an associated path through the network which allows better control of the traffic flow. Based on the information found in the MPLS header, different classes of service characteristics can be applied to the traffic identified by the label.
Because of its performance advantages and the fact MPLS vendors provide private dedicated circuits to route traffic over, MPLS became a popular technology for WAN connectivity beginning in the 1990s. For enterprises looking to provision connectivity between multiple WAN locations at scale, MPLS was the go-to solution.
To better conceptualize why MPLS links are considered “private”, take a look at the diagram below. It shows an MPLS network where two customers, A and B, have separate circuits. The red links are isolated to customer A and the blue links are isolated to customer B.
Software defined wide area network (SD-WAN) is a software-defined approach to WAN connectivity.
With SD-WAN, you are given the flexibility to use multiple types of underlying connections: MPLS, broadband, and LTE. SD-WAN then creates a control plane “over the top” of these underlay technologies that abstract away the hardware. This allows for streamlined provisioning, simple failover, and significant operational flexibility. Additionally, application visibility and advanced network performance monitoring give SD-WAN an edge for WAN connectivity compared to other solutions.
In the past, the role of the WAN was to allow the users from branches or campuses to access the applications hosted in the data center. For this to happen in a reliable and secure way, MPLS circuits were commissioned from service providers.
Legacy WAN architectures also consist of Internet circuits which sometimes work in active/backup mode with MPLS circuits. With this kind of architecture, Internet connections are present only in central points of the network, most likely the data centers through which users can access the Internet.
However, the rise of cloud-computing and remote work lead to challenges that MPLS could not elegantly nor economically solve. Moreover, the high-costs of MPLS bandwidth at a time when bandwidth consumption was growing rapidly led to demand for reliable alternatives. This is where SD-WAN comes in.
The MPLS problems SD-WAN solutions aim to solve include:
SD-WAN is a virtual WAN architecture formed by establishing encrypted tunnels between sites. These tunnels form the overlay.
While specific implementations vary, each SD-WAN solution includes:
The diagram below shows a standard SD-WAN topology where communication between branches and the data center can happen via MPLS or broadband connections and the branches can access cloud applications directly.
Every SD-WAN solution has a centralized management from which configuration policies are pushed to the edge devices to accomplish the intended operation of the network. The policies can be applied globally or per device.
For instance, the operator might decide that for a certain type of traffic, a specific link must be used if it meets specific requirements in terms of latency, packet loss or other criteria that is available with that SD-WAN solution. This is called dynamic path selection and allows the traffic steering based on the link conditions.
At this point, some of the upsides of SD-WAN should be clear. To see why it is becoming such a popular choice in the era of cloud connectivity, remote work, and bandwidth-hungry apps, let’s take a closer look at the benefits we have not yet delved into.
MPLS circuits are expensive. While MPLS circuits have their place in the WAN, most of them can be decommissioned and replaced with other types of connections: broadband or LTE. The price of MPLS circuits is considerably higher compared to other transport mediums. Further, geographical location can drive the price of MPLS circuits up even more.
With SD-WAN, all the underlying transport methods can be used concurrently by load balancing traffic across all the available links. This not only increases the available bandwidth, but also provides high availability and active-active failover capabilities.
Many applications used on a daily basis are hosted in the cloud. SD-WAN allows direct internet access at the branch avoiding the traffic to be backhauled to a central location before it can exit to the Internet. Not only is bandwidth saved for other critical applications that can only be accessed in the datacenter, but also you also improve performance by having lower latency.
To control the traffic, you need to know what your traffic is. With SD-WAN, packet inspection enables deep network visibility. Further, the software-defined nature means it is simple to make granular changes to routing policies and optimize traffic based on specific requirements.
Common SD-WAN use cases include:
Now that we’ve explored MPLS and SD-WAN in-depth, let’s take a look at the pros and cons of each WAN connectivity solution.
Now let’s take a look at the pros and cons of SD-WAN.
So, should you choose SD-WAN or MPLS?
Frankly, in many cases SD-WAN far outstrips MPLS if you’re starting from scratch. However, while that is true in most cases, it is far from an absolute.
There are situations when MPLS is required to ensure business continuity. Some use cases simply need an SLA and can justify the additional MPLS costs to get it. As reliable as the Internet is today, it is still a best effort service. In those cases, it may make sense to stick with MPLS or -- even better from an operational standpoint -- use it as an underlay for SD-WAN.
Remember, there’s never a one-size-fits-all answer. Weigh the pros and cons of each of these popular WAN connectivity solutions, and make the right decision for your business.
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