I spent a long time creating my first Spark bot, Zpark. The first commit was in August and the first release was posted in January. So, six months elapsed time. It's also over-engineered. I mean, all it does is post messages back and forth between a back-end system and some Spark spaces and I ended up with something so complex that I had to draw a damn block diagram in the user guide to give people a fighting chance at comprehending how it works.

Its internals could've been much simpler. But that was part of the point of creating the bot: examining the proper architecture for a scalable application, learning about new technologies for building my own API, learning about message brokers, pulling my hair out over git's eccentricities and ultimately, having enough material to write this blog post.

In this post I'm going to break down the different functional components of Zpark, discuss what each does, and why-or not-that component is necessary. If I can achieve one goal, it will be to retire to a tropical island ASAP. If I can achieve a second goal, it will be to give aspiring bot creaters (like yourself, presumably) a strong mental model of a Spark bot to aid their development.

NSF and GR are two features in Layer 3 network elements (NEs) that allows two adjacent elements to work together when one of them undergoes a control plane switchover or control plane restart.

The benefit is that when a control plane switchover/restart occurs, the impact to network traffic is kept to a minimum and in most cases, to zero.

Presented by: Russ White, LinkedIn

In this post I'm going to look at the characteristics of OSPF and EIGRP when used in a Dynamic Multipoint VPN (DMVPN). I will do my best not to play favorites and instead stick to the facts (yes, I do have a preference :-). To that end I will back everything up with data from my lab. The focus areas of the comparison will be:

• Scalability of the hub router's control plane
• Overall control plane stability
• Traffic engineering

This post won't go into any background on how DMVPN works. If you're not yet familiar with DMVPN, I recommend watching these introductory videos by Brian McGahan. This post also does not do a deep dive on OSPF or EIGRP. I'm making the assumption that you're already familiar with the different LSA types in OSPF and general functions of EIGRP.

After reading this post you should be able to describe the pros and cons of OSPF and EIGRP in the three areas listed above and incorporate this knowlege into a DMVPN design.

Mohamed Anwar asked the following question on my post "4 Types of Port Channels and When They're Used".

"I need a clarification, where if a member link fails, what will happen to the traffic already sent over that link ? Is there any mechanism to notify the upper layer about the loss and ask it to resend ? How this link failure will be handled for data traffic and control traffic ?"

— Mohamed Anwar

I think his questions are really important because he hits on two really key aspects of a failure event: what happens in the data plane and what happens in the control plane.

A network designer needs to bear both of these aspects in mind as part of their design. Overlooking either aspect will almost always open the network up to additional risk.

I think it's well understood that port channels add resiliency in the data plane (I cover some of that in the previous article). What may not be well understood is that port channels also contribute to a stable control plane! I'll talk about that below. I'll also address Mohamed's question about what happens to traffic on the failed link.

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.