When it comes to wide area networks (WANs), MPLS clearly rules the roost. Seventy-four percent of the organizations that participated in Nemertes' 2011-2012 Computing and Communications benchmark have successfully deployed MPLS/IP-VPN services. I'm referring, of course, to Layer-3 IP VPNs, in which carrier routers peer with customer edge (CE) routers, and communications between the carrier and the enterprise happen at the IP layer.
Why are companies using MPLS? Here are some MPLS advantages:
- Outsourced routing. With MPLS, the carrier handles WAN routing. This means users don't have to, which makes MPLS operationally simpler than managing a large routed network. This means, in turn, that companies can keep fewer WAN/router engineers on staff.
- Any-to-any connectivity. Applications, like voice and video, feature any-to-any traffic patterns. MPLS makes it easy to interconnect sites with each other to handle these types of traffic patterns.
- Built-in support for Quality of Service (QoS). Virtually all commercial MPLS services feature multiple levels of QoS, meaning that users can specify latency, jitter and packet loss minimum thresholds for each type of traffic (voice, video, email, bulk file transfers, etc.). This means that, for example, the MPLS network treats latency-sensitive traffic, like voice and video, preferentially over less-sensitive traffic.
- Service-level agreements (SLAs) with delivery guarantees. Like most commercial-caliber services, MPLS comes with guarantees from the carrier when it comes to installation times and availability. That's in contrast to consumer-grade Internet, which is typically offered on a best-effort basis.
Where in the WAN is MPLS used?
How are companies using MPLS? Unsurprisingly, given these MPLS advantages, the most common use for MPLS is to connect branch offices to data centers and interconnect branch offices to each other. Sixty-seven percent of benchmark participants use MPLS for this purpose. Just over half -- 52% -- are using it to interconnect data centers, and 20% say they're using it to connect remote sites.
Figure 1: The three-tiered WAN architecture
This conforms to what Nemertes calls the three-tiered WAN architecture. (See Figure 1.) The notion behind the three-tiered WAN is that there's no one-size-fits-all WAN technology. Instead, some technologies and services work better in certain network tiers than others -- and most organizations deploy a mix of two or more technologies to cover all three tiers.
MPLS advantages compared to WAN connectivity alternatives
As noted, MPLS is great at -- and widely used for -- interconnecting data centers and branches, and branches to each other. That level of connectivity comprises Tier 2 in the WAN architecture. What about the other two tiers?
For Tier 1 -- interconnnecting data centers -- the most common alternative to MPLS is carrier Ethernet. Carrier Ethernet is an umbrella term that covers point-to-point Ethernet, metro Ethernet, and any-to-any topologies like virtual private LAN service (VPLS). The common denominator to all these services is that they operate at Layer 2 -- where the enterprise handles all routing.
The big advantage to carrier Ethernet services is that the bandwidth is low cost (relative to the data rate) and the services are easy to set up. The downside is that carrier Ethernet services don't scale to the same order of magnitude of sites as MPLS. Because MPLS is a routed network, it can support tens of thousands of sites (and more). Carrier Ethernet, in contrast, is effectively a giant bridged network, which can't scale easily beyond a few hundred sites. So, for a small set of high-bandwidth sites like data centers, carrier Ethernet may be a better fit than MPLS. And, in fact, the most common use for Ethernet is in this Tier 1 scenario: interconnecting data centers.
At the opposite end of the WAN there's Tier 3: connecting to remote and small offices. Most companies already use Internet services to connect at least some of their remote and small offices to the WAN. Sometimes these are business-caliber Internet services (with SLAs); other times they're consumer-grade "best effort" offerings. To ensure privacy, companies may deploy encryption, including secure socket layer (SSL), IPsec, or proprietary application-layer security.
The big benefits the Internet confers over MPLS in this Tier-3 scenario are cost and ubiquity. Internet services are typically considerably cheaper than MPLS (though commercial-grade services, with SLAs, can approach MPLS in cost). And it's rare to find a location that doesn't feature at least some flavor of Internet access.
The downside? Unreliability. Consumer-grade Internet services, in particular, can be spotty, both in service quality (where the service is working, but poorly) or outright availability (where there's an outage).
So for locations where best-effort services is good enough, QoS isn't required, and ubiquity trumps reliability, Internet services make a good alternative.
The bottom line? MPLS advantages include the connectivity of data centers to branch offices, or branches to each other. For point-to-point links requiring a lot of bandwidth, or remote offices where cheap connectivity is the most important factor, Carrier Ethernet and Internet services, respectively, are solid alternatives.