Multiprotocol label switching (MPLS) is a common method for constructing the connections between local area networks (LANs) that make up wide area networks (WANs). Using specialized routers, MPLS sends packets along predetermined network paths, improving upon the typical way the Internet works. These predetermined network paths can be used as the connective tissue that comprises a WAN and allow multiple virtual WANs to coexist over a shared network backbone. However, they take quite a bit of time to set up, can be expensive, and require a contracted service from a carrier or telecommunications company.
A software-defined WAN (SD-WAN) is a large network that connects LANs using software, not hardware. SD-WANs do not require any specialized equipment for routing. They run over the regular Internet, making them cheaper to implement than other networking methods.
The SD-WAN model does not exclude the usage of MPLS — MPLS can be one of the networking methods used in an SD-WAN — but overall SD-WANs are often more flexible and cost-effective by comparison.
SD-WAN vs. MPLS: A real-world analogy
To understand the differences between software-defined connections and MPLS connections, consider the difference between a railroad service and a passenger bus line.
Railroads have specialized routes set up via train tracks, and only trains that belong to the railroad can use the tracks. Because trains can stay on these tracks and often do not have to stop until they reach their destination, train transport is fairly fast and reliable.
However, railroads require a significant upfront investment to build the specialized routes (the train tracks) that the vehicles require. A massive surge in passengers or cargo may exceed the railroad's capacity, because only a certain number of trains can use the tracks at once. And if the railroad wishes to add more routes, it must build more track — which requires obtaining permits, negotiating with property owners, and expensive track construction. While a railroad line may be direct, it costs a lot to build and is not very flexible.
Conversely, a bus line travels across a massive network of roads that also serve many other types of vehicles. Buses operated by the line do not have to take the same route to their destination every time; they can route around areas of heavy traffic or add more stops as needed. Carrying more passengers is easy: the bus line can simply use more buses, since there is no defined limit to how many buses can be on the road at a given time.
Because traffic can vary so much, bus travel time can vary. And because the roads were not specifically constructed to serve the needs of the bus line, routes are less direct than those of the railroad service. However, if the line wants to offer more routes, it does not need to build new roads; it only needs to purchase more buses, and it can add routes over preexisting roads. The bus line is less direct than the railroad service, but it costs less to operate and is more flexible.
Like railroad tracks, MPLS connections are dedicated only to the users of those connections. They are more direct and more reliable than the public Internet. However, they require the purchasing of expensive hardware (similar to the laying of railroad tracks), and their routes cannot change very easily. Meanwhile, SD-WANs are built on existing paths (the public Internet) and can easily increase their routes and the number of users served, like the bus line.
What are some SD-WAN benefits compared to MPLS?SD-WANs do not rely on specialized hardware. MPLS requires configuring specialized routers to forward packets correctly. SD-WANs can run using any networking hardware.
SD-WANs have no inherent bandwidth limits. Because MPLS connections are more or less set in stone (unless they are reconfigured), there is a hard limit over how much capacity can be provisioned over an MPLS connection at once. SD-WAN connections can add capacity as required by combining multiple connections and leveraging the fastest connectivity available.
SD-WANs are service provider-agnostic. MPLS requires organizations to use the same carrier at all WAN-connected sites because MPLS connections have to be configured in physical routers in the adjacent network. SD-WAN connections run over the regular Internet; any ISP can support an SD-WAN connection.
SD-WAN routing is more flexible. SD-WAN can take advantage of multiple connectivity options including broadband Internet connections, private lines, and 5G. It can direct traffic and failover between all available connectivity options. MPLS services typically require dedicated private line connections from the service provider.
SD-WANs integrate more easily with the cloud. Connecting to the cloud via MPLS is a specialized service offered by some MPLS service providers for some cloud providers. With MPLS, connecting with the cloud requires constructing a direct route to that cloud provider's infrastructure.
What are some SD-WAN drawbacks compared to MPLS?MPLS offers more granular control over where packets go. Typically, data packets on the Internet take different routes depending on how routers on the path are forwarding packets at that time — but MPLS routes are only updated manually. MPLS packets never deviate from the defined route since they always are forwarded between the same routers and networks. Depending on the routing method used, network traffic on an SD-WAN likely will not always take the same route, and some packets may be lost in transit, as is the case with most Internet traffic.
MPLS is sometimes more reliable. MPLS traffic is usually given a higher priority over service provider backbone networks relative to Internet traffic. MPLS services often include quality-of-service (QoS) guarantees. SD-WAN leverages best effort Internet services and may experience occasional packet loss. However, most SD-WAN services compensate for this by intelligently steering traffic away from lossy connections.
MPLS is better for real-time applications. MPLS often includes multiple classes of service which are preserved throughout the WAN. This makes it easier to ensure real-time applications have a better experience across the WAN.
How does SD-WAN compare to the network-as-a-service (NaaS) model?
Network-as-a-service (NaaS) is a cloud service model in which organizations rent networking services from a cloud provider instead of setting up their own networks. Users connect to their applications directly through a virtual network, and they do so via any Internet connection. SD-WANs still require hardware setup; NaaS only requires Internet connectivity.
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