What You Need To Know About A MPLS Network

There's no one "MPLS Service". A lot of what you get will depend how the service provider has Engineered and built their core network. Bear in mind that many carriers don't own the whole network, but will piece together a service from other carriers networks, or will interconnect with other carriers to extend their reach.

Cell-mode MPLS was mentioned: basically this is ATM which has been retro-fitted with MPLS. Be careful with this for VoIP applications because it can use bandwidth very inefficiently.

MPLS can support QoS, but many services aren't engineered with this, or only with very basic prioritisation. Also the services are very often structured to reduce the potential complexity and to ensure the network can cope. Bear in mind a typical MPLS router can only carry a percentage of "high-priority, realtime" traffic. If everyone sends all their data as high priority then the benefit is lost, and the network may suffer. Usually QoS is provided as a small number of service classes, typically 3 or 4.

The biggest bottleneck in any such service is normally the tail circuit to each of your premises. If you move from a T1 mesh to a MPLS service then you will likely find that some sites need more bandwidth than others. Tracking the requirement for this bandwidth is usually your problem, although the service provides may give you some reporting tools to assist with this. I would avoid service providers who cannot offer this as it will make it very difficult for you to manage your bandwidths.

If you factor in multiple service classes then your management of these tail circuits gets more complex as you no only have to work out how much bandwidth is required for each tail circuit, but how much of it should be reserved for each service class.

Regarding resilience, within the service providers core, the service is normally highly resilient to failures. However, when failures do occur, very often (depending on how the service is engineered) the rerouting can take a second or two. During this network reconvergance you will lose packets. Depending on the protocol your traffic uses this can be unimportant or devastating. For instance, some VPN and VoIP services don't survive this well.

Normally resilience is not automatically provided all the way to the customer. Typically you will have one tail circuit and one router at each site. If either fails (or if the Service provider's PE router has problems) you will lose service to the site, totally.

If this is an issue, you need to factor in dual connections. There's multiple ways of doing this, and different service providers will offer different options. Make sure you get your Network Engineer involved as the devil is in the detail here, and some options which sound like they provide a fantastic level of resilience may not be as good as they sound, depending on how your internal network is configured.

And, of course, the key to all of this is SLAs: what do they offer? What happens if they break them? How do they report them to you?

Generally speaking, MPLS services are a great way to run a multi-site data network including VoIP services. I have seen many carriers and their customers doing this successfully for years.

Strictly speaking MPLS does not provide QoS. QoS is done by prioritising traffic, and most IP routers, even those on the backbone of the Internet, can do this. The difference is whether they are configured to do this or not. In an MPLS network MPLS is provided by standard router features. MPLS technology (specifically Traffic Engineering) gives the carrier better control over how this traffic is prioritised and routed (and restored in case of network fault). All this does is give them the confidence to support SLAs.

As I mentioned, "QoS" is provided as a set of "service classes". Typically these are things like "real-time", "high-priority" and "everything else". Mapping actual traffic into these classes can be done in a few different ways, but this is largely up to you to control. For instance, you could quite easily put web-browsing traffic into "real time" although this would normally be a dumb thing to do.

I would suggest the case for MPLS in terms of performance, cost and continuity against 'traditional' or 'legacy' data networks is now pretty robust, i.e. MPLS provides significant advantages in all 3 areas.

The key considerations when migrating include provider selection, access media (e.g. using Ethernet rather than SDH/SONET), the decision on procuring a managed or unmanaged service (often called wires-only) and the providers ability to map their CoS/QoS to the applications you need to support. This is especially important if you are operating any proprietary applications.

There is also an increasing trend to use WAN Optimisation/application management solutions either as a value added service from the provider or from an alternative integrator or indeed doing it in house. This is important say for voice or applications such as CITRIX.

MPLS providers also now offer a whole suite of value added services such as integrated internet, managed network based firewalls and remote user support. If these are important to you make sure the providers demonstrate how this is achieved.

In selecting your provider ensure they have good geographic coverage in your areas and experience within you market segment. I always recommend taking up 3 references. Equally I think it is wise to understand how important a client you will be to the provider; it's all well and good using the market leader (say according to Gartner) but you'll often get a better service from a provider who values and really wants your business.

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