UCS Hardware: What are Fabric Interconnects and IOMs?

When most people hear the word fabric, they tend to think of beautiful hand-made quilts and fancy t-shirts. Not IT folk, though. Not at all. To IT folk, a fabric is a tightly interwoven set of connections and network cables.

Suppose you're an IT tech in a Cisco household. In that case, you might be aware of the terms fabric interconnects and fabric extenders, too. If you're not familiar with those terms, you might be interested in learning what they are. So, let's answer that question today: what are fabric interconnects and IOMs.

What is a Fabric?

A fabric is simply a network. While the term fabric is used to describe other topologies, you'll most likely encounter it when referencing a software-defined network.

Most software-defined networks have multiple, redundant connections between servers, switches and routers. For example, depending on the specific structure of a network, each switch will connect to another switch, and each server will connect to at least two other switches.

This creates a lot of different connections. So when the network diagram is made for this type of network, the chart will look like a piece of fabric because of all of those interwoven connections.

A network fabric is not restricted to only the world of Cisco. So it's important to know that you will run into other types of network fabrics, too. For the purposes of this article, though, we will be referring to a network fabric created with Cisco UCS products.

What is a Fabric Interconnect?

Fabric interconnects mostly exist in the Cisco world. They are devices that connect servers and other network-attached equipment to the rest of the network. Fabric interconnects are typically found in the UCS line of Cisco products and services.

That sounds an awful lot like a network switch, doesn't it? If you picked that out from that first paragraph, congrats! You're halfway to understanding what a fabric interconnect is. In its simplest form, a fabric interconnect is like a network switch, but there is more to it. Let's explain.

We mentioned above that a fabric interconnect is part of the Cisco UCS line of products and services. The Cisco UCS system is a compartmentalized set of equipment designed to fit together in a specific type of chassis. This unified system should reduce costs while making management easier and network communication more reliable.

The Cisco UCS line of equipment is mostly built around a specialized chassis. These chassis come in different sizes and look very similar to a typical network rack. Each chassis is designed to house either blade-style servers or full-sized servers. These servers are also specialized pieces of equipment and are part of the Cisco UCS product line.

The fabric interconnect always lives at the top of these UCS chassis, and is like a specialized switch. The most significant difference is that each connection is always active. These fabric interconnects typically have more bandwidth than traditional switches, too.

What makes these fabric interconnects so special is the management software built into them. The UCS management platform is Cisco's special sauce. It makes the software-defined network easy to manage.

For instance, experienced network engineers know how difficult it is to connect two distant sites together over a WAN utilizing the public internet. However, the UCS management platform makes that process a few-click procedure.

UCS has reliability and failover built-in, too. A network fabric is called a fabric because conceptually, there are so many connections between each node, or network endpoint, that the diagram of that network looks like a piece of woven fabric. Each fabric interconnect in a UCS connected system requires that two connections are made between each other fabric interconnect. If one link goes down, the other will still be functional. That kind of network model prevents outages.

What is an IOM?

What is an IOM? An IOM is a piece of networking equipment belonging to the Cisco UCS family of networking equipment that connects devices to fabric interconnects. An IOM is also known as a fabric extender. However, for the remainder of this article, we will refer to IOMs as fabric extenders instead of IOMs.

We mentioned above that a fabric extender is a unique piece of equipment used to connect devices to fabric interconnects. Does that sound like any other piece of networking equipment?

If you thought of a network card, you would be right. A fabric extender is conceptually like a regular network card. There are differences, though.

A fabric extender is more like a virtualized network card that lives on a piece of hardware. For instance, the Cisco UCS B-series products are specialized blade server chassis with different half or full mount positions for blade servers. Blade servers do not have built-in network cards. Instead, the blade server chassis has built-in power and network distribution hubs for all blade servers in the network chassis.

These network distribution 'blocks' are fabric extenders in the UCS world. In the case of the fabric extender included with B-series chassis, the extender consists of virtual network cards, too. Each virtual network card can provide up to 255 network addresses. That means each blade server can host up to 255 different VMs with unique network addresses — without utilizing technologies like virtual switches a la' Hyper-V.

Each piece of UCS equipment will have two fabric extenders. UCS hardware always connects to two fabric extenders for redundancy. Those fabric extenders then connect to multiple fabric interconnects. And thus, our fabric is woven, and we create its fringe edges.

Final Thoughts

This article is only an introduction to fabric extenders and fabric interconnects. Network fabrics, network interconnects, and network extenders are complicated topics. It's impossible to definitively explain UCS equipment in 1000+ words. So, here's what we suggest.

First, take a look at Cisco's documentation. Specifically, look at this article. Of course, Cisco's documentation is technical, so be prepared to decode technical jargon and translate it into something more familiar to you. Nonetheless, that article specifically is an excellent resource for learning what UCS is as a whole from a 10,000-foot view.

After reading that article, consider looking at CBT Nugget's Cisco UCS Essentials course. CBT Nuggets has many other online networking courses that explain how to install and manage Cisco UCS equipment. Still, that course takes a deeper dive into the fundamentals of UCS. If you need to learn how the UCS architecture works, that course is an excellent resource as well.