You need to connect several of your sites together. Various Service Providers have given you quotes for their favoured option, but which suit your requirements best? You don't want to limit yourself, but you don't want to pay over the odds, either. We look at what the telcos might not tell you about your choices.

If any of these sites are international, you might not have so many options. Frame Relay is still about the only connection type you can, more or less, rely on being able to get in some of the more exotic places you might have sites (or satellite, but in that case you’ll just be bringing it to a UK site in a point to point set up).

But if you don't work for an oil company that insists on you providing excellent data services to a hut in the middle of a swamp in Nigeria, then you do seem to be pretty spoilt for choice.

Leased line services have been around a long time. They give you a nice, dedicated amount of bandwidth on a point to point basis but they’re expensive, since you tend to get charged by the mile (sorry, kilometre). And if you need resilience, you buy two.

Connecting to a telco's Frame Relay, ATM or SMDS network means you pay for a connection from your site to their network based on access connection speed, not distance. These are very flexible when it comes to connecting multiple sites and you can set up VCs with different security and QoS features as it suits.

The newer WAN options now available include the likes of MPLS, which is basically the IP-based successor to Frame, ATM etc, and the so-called LAN Extension Service (LES) circuits.

With LES (also known as Metro or National Ethernet), you extend your Ethernet LAN to a remote site, connecting two switches together over the LES circuit so it just looks like a bigger campus network. These services are reasonably new, and are becoming increasingly popular for connecting sites within a city, say. The technology is Ethernet - no need for routing if you don’t want it - and the prices are very attractive. The telco gives you an RJ-45 socket on the wall and away you go. Anything from sub-megabit to Gigabit rates are possible (depending on what your provider wants to offer)

MPLS VPNs are the most hyped up connectivity option just now. Since they can offer both Layer 2 and Layer 3 VPN services, can carry voice and video as well as data, and offer speeds from tens of Kbps to multi-Gigabit, they do seem to be the way of the future. Service Providers like MPLS since they can run multiple customers over one network securely and it's easy to add PoPs to bring in new customer sites.

The limitations
So why should you care - all you want is the equivalent of a piece of string between sites, right? You’re not bothered how the telco provides it, as long as it's at the right price.

Well, not really. First off, what is it you really want? If you're just connecting two sites together, do you want a routed link between them, as you get with a leased line, or would it be better to have a LES-type circuit, so that you can just hang the second site off your main one using a switch and just extend your campus?

If you have a whole bunch of sites, chances are you have one main one - where your data centre is - that the rest connect to in a hub and spoke. But what if you have two or three main sites? Or need every site to be able to communicate with each other? Yes, you can do it via the central hub site, but maybe it'd be better to have a meshed arrangement, if you can. And again, do you want a routed WAN, or could you benefit from a simulated Layer 2 topology, so that it looks like all sites connect to one Ethernet 'cloud'?

This might narrow your options down a bit. Ethernet transparency isn't something you can get from an ATM or Frame Relay network, so if this is what you want you're looking at a LES service or Layer 2 VPN. You can use leased line, Frame, or even another LES circuit to connect into the provider's PoP.

Some providers will only offer you a LES service within a set distance - say a 25 mile radius. Others will offer it from one end of the country to the other. You'll need to decide exactly what you want to do with this type of connection (it's a very good choice for the likes of Data Centre backup, by the way, or for extending Storage Area Networks). Just because it's an Ethernet service doesn't necessarily mean that it will behave as your campus LAN does. It's extra effort for the provider to carry your Spanning Tree BPDU packets (and CDP and VTP if you have Cisco switches) for instance, so that might not be included in the basic service. You'll need to make sure they do offer this service, and at what extra cost.

For anything more than point to point, you'll be tempted by MPLS. A couple of things to be aware of here: most UK MPLS networks operated today by service providers cannot natively support multicast traffic. There are a couple that can, and do, but the technology is still pretty new.

For the rest, multicast traffic can be passed over their MPLS networks by building GRE tunnels, but this is a real administrative nightmare for the providers, so many won't do it unless it's for just a couple of sites. Make sure you know how they offer this service. If you don't need multicast this year, chances are it'll be vital you support it next, and you don't want to have to be looking for a new provider.

Layer 2 VPNs over MPLS are good at point to point and even point to multipoint (hub and spoke) topologies, but the network vendors have been struggling with true multipoint to multipoint capabilities. True, any to any connectivity is now possible using a technology termed Virtual Private LAN Service (VPLS), but again it's very new and not integrated into many offerings. Quiz your provider on how it plans to provide this service, and at what cost. It's worth noting that Layer 3 VPNs don't have this sort of restriction, since the routing just takes care of it.

Which leads to another point to note. If you have a layer 3 VPN service from a provider, you will have to route to their PE (or CPE if it's a managed service). Different providers allow the use of different routing protocols here - if you care, make sure you discuss it with them first.

Making Your choice
So, decide what sort of service you need, based on which applications will run over it, find out if any telco offers that service in the locations you need, and what they'll charge for the little things you might think are included in the basic service. That's before you start discussing SLAs, QoS provision, resiliency and change control mechanisms.

Networks are more complex than they used to be. The piece of string has to do a lot more these days - and you’re going to have to be aware of what it can and can’t do.

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