Let's step back for a minute. So far in this series of blog posts on DCI, I've been focusing on extending the Layer 2 domain between data centers with the goal of supporting hot migrations — ie, moving a virtual machine between sites while it's online and servicing users.

Is that the only objective with DCI?

Here's a topic that comes up more and more now that FabricPath is getting more exposure and people are getting more familiar with the technology: Can FabricPath be used to interconnecting data centers?

FabricPath has some characteristics that make it appealing for DCI. Namely, it extends Layer 2 domains while maintaining Layer 3 — ie, routing — semantics. End host MAC addresses are learned via a control plane, FP frames contain a Time To Live (TTL) field which purge looping packets from the network, and there are no such thing as blocked links — all links are forwarding and Equal Cost Multi-Pathing (ECMP) is used within the fabric. In addition, since FabricPath does not mandate a particular physical network topology, it can be used in spine/leaf architectures within the data center or point-to-point connections between data centers.

Sounds great. Now what are the caveats?

This is the third article in my series on Data Center Interconnection (DCI). In the first (Why is there a "Wrong Way" to Interconnect Data Centers?) I wrote about the risks associated with DCI when the method chosen is to stretch Layer 2 domains between the data centers. In the second article (DCI: Why is Stretched Layer 2 Needed?) I wrote about why the need exists for stretching Layer 2 domains between sites and also touched on why it's such a common element in many DCI strategies.

In the previous article in this DCI series (Why is there a "Wrong Way" to Interconnect Datacenters?) I explained the business case for having multiple data centers and then closed by warning that extending Layer 2 domains was a path to disaster and undermined the resiliency of having two data centers.

Why then is stretching Layer 2 a) needed and b) a go-to maneuver for DCI.

Let's look at it from two points of view: technology and business.

There's certainly a lot of focus on data center interconnection (DCI) right now. And understandably so since there are many trends in the industry that are making IT organizations look at data center redundancy. Among these trends are:

1. The business is saying to IT that they require their IT services to be available at all times. In effect the business is saying that they want to be shielded from technology issues, maintenance windows, and unplanned downtime because the IT services they consume (not all of them mind you, but certainly some of them) are so critical to running the business that they cannot be without them (or, they cannot be without them for whatever period of time it would take IT to recover the service).
2. The technical ability to move workloads between sites thanks to the near ubiquity of features like vMotion and Live Migration. The ability to pick up an application and swing it over to another location makes item #1 above far less daunting to IT shops and lowers the barrier to adoption.

In this post I'm going to talk about how IT can address item #1 above — the business need — in a manner that does not introduce hidden risk into the environment. This is a common conversation that a lot of IT organizations are having right now but unfortunately the easiest and most obvious outcome from those conversations is not always the one with the least risk.

In the second post of this DCI mini series, I'll talk more about item #2 since that's the one that drives a lot of the technical requirements that have to be met when delivering the overall solution to address #1.