Rip out any SMDS, Frame Relay, ATM or leased line-based WAN you have and let your service provider sell you an MPLS one. It’s fast, simple and transparent. Or at least that’s what they’ll tell you. But any technology that promises such a panacea is instantly under suspicion — nothing’s that good, surely?
Why should you want MPLS, and why are all the providers so eager to push it? And once you’ve signed up, can you breathe a sigh of relief and relax, or are there implications that you, the customer, should be aware of?
Service Provider Benefits
MPLS isn’t new. As a technology it’s been around for several years now. The original intent — to speed up routing processes by moving data based on simple labels rather than full IP address-based routing table decisions — was never fully realised. By the time the technology was viable, the development of ASICs had advanced such that ‘traditional’ routing was fast enough anyway. But this use of labels in a network core meant that providers could build one network and use it to offer completely independent and separate services to their customers, with no care for their IP addressing structures. Build once, sell many times — a concept any service provider will love.
It’s good for you too. MPLS services can be provided over any medium, and its adoption by carriers has led to a wide range of service offerings, from metro and national Ethernet (LES-type services) to full layer 3 IP VPNs. You can now get 10Mbps Ethernet connectivity between sites for less than you’d pay for a Megastream over the same distance — and because it’s Ethernet, you can basically extend your LAN over a geographically dispersed campus, giving you access to faster access and newer technologies.
Some issues to consider. If your provider offers you an EoMPLS service, make sure you go into the details of what you’re really being sold. The standard basic service does not give you full, transparent, Ethernet extension services, no matter what they say. By default, an EoMPLS pipe between two of your sites will not pass layer 2 traffic such as VTP, CDP or Spanning Tree BPDU frames. It’s not difficult for the provider to do that, but it will tend to be a ‘value-add’ (i.e. more expensive) option. Without it, you may find your extended LAN isn’t as transparent as you’d envisaged.
If you go for the more flexible layer 3 VPN service, you have to decide which routing protocol to run between your equipment and the providers. Some will only offer RIP, BGP or static routing, others are more amenable and will allow OSPF, for instance. There’s a lot more interaction required between yourself and your provider than you’re probably used to in a leased line environment, in relation to routing metrics, Quality of Service and multicast (more about that later).
If you’re running an OSPF network for instance, with all your sites in area 0, you’ll find that the redistribution process into the provider’s environment and back out will change your routing table metrics. That won’t be an issue as such — but watch out if you’re providing backup via a direct kilostream or ISDN circuit between sites, because you may well find that the backup route, which is a direct intra-area-0 hop, is seen as the preferred path, regardless of any administrative distances you try to impose. Not insurmountable, but will take a bit of head-scratching and tuning until you get it looking the way you want.
Quality of Service, which is arguably of less importance in a 1Gbps backboned LAN, becomes more significant with slower speeds — even 10Mbps. Make sure you know how (or if) your provider will honour the QoS parameters you set. As a minimum, they mustn’t change them. Will they use your settings to map to the MPLS EXP bits used for QoS in their network? Or will they use the manual process of setting access lists to map traffic to their QoS classes (and how many will they offer)? Which will work, but if you add a new server or application, how much notice will they need to update their configurations? Just how dynamic is that?
And then there’s multicast. Becoming more prevalent in financial applications, software downloads and audio and video streaming. Until very recently, the only way to support multicast over an MPLS network was for the provider to build GRE tunnels between every source-receiver pair. Not too bad in a hub and spoke set up, although still a pain, but a complete management nightmare if you need any-to-any connectivity, which meant a lot of the providers just didn’t offer it. However, thanks to a new technology, called MVPN (Multicast VPN), providers now have the ability to dynamically provide multicast support over MPLS networks. Its simple to set up and it actually works.
It also requires upgrades to the provider’s whole network, only a couple of vendors’ equipment supports it (and even then not as well as they might — this really is leading edge we’re talking about here), and so far only two providers in the UK are capable of offering it, although it’s in the labs of all of them. So don’t hold your breath.
Yes, MPLS is great — it’s easy for the provider to run, and does offer you great flexibility at actually pretty good prices. But ask to see the small print, and make sure you know exactly what the service will and will not offer so that you don’t get caught out.
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