In case you missed the memo, MPLS now is the technology of choice for WAN underpinnings. More than half of the companies I work with on a regular basis say they're using MPLS or planning to in the near future - and that number increases dramatically among companies that are large (with more than US$1 billion in annual revenue) and/or have global operations. That's why I've been spending the last few columns detailing best practices for migrating to an MPLS-based WAN, for those who are still in transition.

MPLS isn't the whole story, however. A less-obvious but fascinating corollary is the demise of the three-tiered architecture that dominated WANs from roughly 1995 until about last year. In case you've forgotten, here's how it worked: Tier 1 was the high-speed interconnects (ATM or dark fibre) linking data centres, contact centres and large headquarters facilities. Tier 2 was the core WAN architecture, typically frame relay, which connected larger sites with the majority of sites. Tier 3 was the mishmash of connectivity options - dial-up Internet VPN links, very-small-aperture terminals and so on - used to link remote and mobile sites into the core WAN.

What happened over the past few years is that data centre consolidation, branch-office proliferation, the growth in broadband and the spread of MPLS have combined to fuse that tiered architecture into a much flatter design - one that relies on MPLS-based services to most sites in the network, from branch or remote offices to data centres.

Let's start with data centre consolidation. As I've noted in previous columns, most companies have consolidated their data centres over the past 12 months - and most will continue to do so during the next 12 months. That means ultimately we'll arrive at an architecture based on two to four data centres linked by a range of high-speed network technologies. While dense wavelength division multiplexing over dark fibre is a common choice (and some companies are still using ATM) as a data centre interconnection, companies increasingly are moving to high-speed (OC-1 or OC-3) MPLS-based services.

Not every company is jumping on the bandwagon of MPLS between data centres, however. Some high-end firms, such as financial services organisations, report that without a substantial number of branch offices to connect to, MPLS doesn't offer a price-performance advantage over, say, dark fibre.

Speaking of branch offices, I'm seeing roughly a 10 percent annual increase in the number of branch offices. That's a lot. The vast majority of these newer branch offices have broadband (T-1 rates and faster) connections. That's a sea change from yesteryear, when branch offices typically were served by 56K to fractional-T1 connections. Once again, MPLS-based services (which, unlike frame relay, scale from T1 to OC-X speeds), are a perfect fit.

Finally, companies increasingly are moving to highly redundant Internet connections as a way to link their mobile users, business partners and customers. More on that development in a bit.

The bottom line? The old-school, three-tiered architecture is fading away, replaced by a flat MPLS mesh linking everything from data centres to branch offices.