What is an MPLS Network? A Short Explanation

An MPLS network is any network that uses multi-protocol label switching (MPLS) to help get traffic to the right places via appropriate paths.

MPLS is used extensively by Network Service Providers (such as Internet Service Providers) in their own networks, as well as by mid-to-large organisations in their Wide Area Networks (WANs).

MPLS Networking Basics

As with most computer networking, traffic flows are split into data packets. When these packets enter an MPLS network, one or more labels is applied to them.  

As these packets are sent through the MPLS Network, each routers look at the outer-most packet to decide what to do next. These routers may add (aka 'push') additional labels, swap labels, or remove ('pop') labels.

Finally, when a packet is about to leave the network, the remaining MPLS labels (added by THAT network) are removed. From then on, the packet will be routed based on other factors, for example based on the IP address a packet is being sent to.

Why use MPLS networks?

The original idea behind MPLS was that having every router look up routing information for every packet was stupid. It made far more sense to shift that comparatively time-consuming task to the outer-most routers which would apply labels to the packets. Inner-most routers (known as label switch routers) would then quickly transmit the data based solely on the labels. Later in the journey, the outer-most routers would get rid of any remaining labels. As certain custom hardware used in routers (Application-Specific Integrated Circuits) got faster, the speed benefit of this approach diminished. However, there were several other reasons to use MPLS.

MPLS is protocol-agnostic and allows packets to be labelled with a Quality of Service/Class of Service tag. As a result, MPLS makes it possible for data packets to be sent over lots of different types of network links, with MPLS providing a useful layer of abstraction. Different types of traffic (voice calls, web traffic etc) could be sent over a single converged network, with different MPLS labels or MPLS packet Class of Service tags applied to different types of traffic. Service providers and their customers could just have one network for all their traffic, rather than having to maintain separate networks for voice, data etc. This simplifies support and cuts costs.

Converged networks that can support traffic prioritisation and traffic engineering... such as most MPLS networks... are popular with network operators because the aggregation of traffic onto a single network allows spare bandwidth to be pooled, creating the opportunity for new bandwidth hungry services to be added at zero or minimal marginal cost, as the required network capacity may already be in place.  

A third reason, and one that doesn't tend to get mentioned much, is that MPLS makes life easier for service providers. A feature known as MPLS Fast ReRoute allows them to reroute traffic more quickly in the event that a given link goes down - reducing the visibility of network link downtime to end-customers. MPLS also made it easier for paths to be set up across the network, for example using RSVP-TE (Resource Reservation Protocol - Traffic Engineering) to provisionally reserve bandwidth across the provider's core network for use in a given customer's WAN.

How Are MPLS Networks Used?

Service providers use them to carry their customers' voice and data flowing over a single network. In many cases, these connections are provided by multiple underlying telecoms firms (carriers), including over layer 2 AND layer 3 connections. This is particular true for international WANs (where different carriers operate in different countries - limiting the local connectivity options available). It's also true for carrier-independent ISPs who rely on a variety of carriers - cherrypicking the best value options at each site or choosing to use multiple carriers to enhance resilience.

Large organisations occasionally use MPLS in-house. However, most firms that use MPLS tend to ask an MPLS Network Provider to set it up instead. Occasionally, MPLS will be used to create point-to-point links (virtual leased lines), where the MPLS encapsulation hides the underlying complexity of how the data gets from A to B. More often, it's used to create a wide-area-network. The MPLS hides the underlying complexity of the WAN links. If it's a layer 2 WAN, the WAN will look as though the organisation has just bought some very long ethernet cables and plugged them into an ethernet switch.

The organisation is able to apply different Class of Service options to different types of traffic. Typically, 'voice' traffic such as phone calls will be prioritised over more general traffic, as such traffic is delay-intolerant. Although companies could already configure their on-premise routers to prioritise Voice, an MPLS WAN enables them to apply those settings across the entire WAN, including at any hub.  

International organisations tend to use MPLS WANs as these allow them to apply settings uniformly across their network, even though the underlying connectivity providers (carriers) may vary from country to country. A global MPLS WAN can hide the underlying complexity, making network management and monitoring easier. 

MPLS vs SD-WAN - A Phoney War, Full of Half Truths

Vendors and technology marketers like to focus on the latest greatest thing, and paint everything that came before as outdated. Currently, MPLS is being trashed, while Software-Defined Wide Area Networks (SD-WANs) are being feted as its glorious successor.

As both an MPLS network provider AND an SDN-WAN provider, let us set the record straight.

MPLS is in greater use than SD-WANs. That's likely to remain the case for years, and possibly forever, because SD-WAN hardware is comparatively costly. In many cases, the additional cost might be better spent on upgrading connectivity to individual sites, instead of paying for fancy packet-inspecting routers to determine whether to send a given packet of traffic over a leased line or a broadband connection.

SD-WANs and MPLS aren't really direct substitutes. SD-WANs are more focused on connection bonding and deep packet inspection.

Often, MPLS is painted as expensive in comparison to SDWAN, based on the premise that dedicated leased lines (characterised as MPLS links) can be fully or partially replaced with cheap contended broadband connections.

Unfortunately, the idea works less well than many SDWAN providers would have you believe. For a start, leased lines in the UK typically offer up to 1Gbps in both directions. Broadband typically struggles to deliver even 20Mbps upstream. Secondly, broadband connections tend to have higher latency, so it's not always a good substitute. Thirdly, the pricing structure of leased lines means that comparatively little extra spent on a leased line can lead to a big increase in its available bandwidth.

SD-WANs do make sense for large organisations - particularly ones which have lots of sites and almost no-one technical at most sites. SDWANs are primarily selected about centralising control and enabling network changes to be made to lots of sites rapidly.

Often, the deep packet inspection isn't really about routing at all. It's about enabling the firm to restrict which web sites and applications can use their network. MPLS doesn't offer those functions. 

How Much does an MPLS Network Cost?

That very much depends on the locations being connected, the amount of bandwidth you're after, whether the links are dedicated or contended.

To get a quote for an MPLS network, give us a call on 020 7847 4510.

It's not MPLS itself that tends to cost the money - at least in the core network. The high-spec networking equipment western Network Service Providers use in their core networks often supports MPLS, whether or not the provider intends to use that functionality.

If you decide to have an end-to-end MPLS solution, it will likely require more expensive routers at each of your sites than if the MPLS encapsulation had ended at the provider's end of your leased line.

MPLS WANs and Avoidance of Unnecessary VPNs

In the early 2000's organisations with many sites would get internet connections for each site, then create IPsec VPN tunnels over the top. This remains a valid and popular option.

MPLS WANs can provide a more scalable alternative.

Let's say an organisation has 40 sites. With the standard VPN approach, all 40 sites need a device that can support the VPN - typically a VPN-capable firewall. In contrast, with an MPLS WAN, with MPLS ending at the provider's end of the site's circuits, no sites would need their own VPN device. Many organisations might opt to get one VPN device anyway, to let staff working from home connect to their employer's corporate network. With an MPLS WAN, the organisation could avoid having to buy 40 VPN devices.

Note that if a 41st site is added, under the traditional VPN approach, all 40 VPN devices need to be told to accept connections from the 41st VPN device. With MPLS WANs, the network extends the 40-site WAN to the 41st site, without any on-premise hardware having to be updated.

MPLS Networks and Traffic Engineering

The shortest route between two points isn't always the best route for traffic to take.

Paths can become congested - causing traffic to be queued or dropped.

When particular routes are congested, it often makes sense to reroute traffic over alternative routes, so that only the most delay-intolerant traffic travels over the fastest route, while less urgent traffic takes a more circuitous route. For commercial reasons, network operators may also choose to reserve bandwidth for particular customers, even when its not required. Customers might also choose to prioritise particular types of traffic over others.

In broad terms, MPLS networks apply different labels or Class of Service tags to traffic to help the network determine which path a particular packet should take. MPLS routers talk to each other, using Label Distribution Protocol (LDP), building and maintaining their own database of label-switched paths.

RSVP-TE (Resource Reservation Protocol - Traffic Engineering) may also be used.

Bear in mind that MPLS networks aren't limited to applying just one MPLS label to traffic. Often, the end-customer (or the MPLS provider on their behalf) may apply MPLS labels to traffic on a given corporate WAN. Then, within the MPLS provider's own network, additional labels are applied, so RSVP-TE can be used.