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Wednesday, January 23, 2013

Storage 101 - Part 4

Storage 101 - Part 2
Storage 101 - Part 3

This article continues the discussion of Storage Area Networking by discussing Fibre Channel switches.


In my previous article in this series, I talked about some of the fabric topologies that are commonly used in Storage Area Networks. In case you missed that particular article, a fabric is essentially either a single switch or a collection of switches that are joined together to form the Storage Area Network. The way in which the switches are connected forms the basis of the topologies that I discussed in the previous article.

Fibre Channel Switches

Technically a SAN can be based on either iSCSI or Fibre Channel, but Fibre Channel SANs are far more common than iSCSI SANs. Fibre Channel SANs make use of Fibre Channel switches.

Before I get too far into a discussion on Fibre Channel switches, I need to explain that although a fabric is defined as one or more switches used to form a storage area network, the fabric and the SAN are not always synonymous with one another. The fabric is the basis of the SAN, but a SAN can consist of multiple fabrics. Typically multi fabric SANs are only used in large, enterprise class organizations.

There are a couple of reasons why an organization might opt to use a multi fabric SAN. One reason has to do with storage traffic isolation. There might be situations in which an organization needs to isolate certain storage devices from everything else either for business reasons or because of a regulatory requirement.

Another reason why some organizations might choose to use a multi-fabric SAN is because doing so allows an organization to overcome limitations inherent in Fibre Channel. Like Ethernet, Fibre Channel limits the total number of switches that can be used on a network. Fibre Channel allows for up to 239 switches to be used within a single fabric.

Given this limitation it is easy to assume that you can get away with building a single fabric SAN so long as you use fewer than 239 switches. For the most part this idea holds true, but there some additional limitations that must be taken into consideration with regard to fabric design.

Just as the Fibre Channel specification limits the number of switches that can be used within a fabric, there are also limitations to the total number of switch ports that can be supported. Therefore, if your Fibre Channel fabric is built from large switches with lots of ports then the actual number of switches that you can use will likely be far fewer than the theoretical limit of 239.

Fibre Channel Switching Basics

One of the first things that you need to know about Fibre Channel switches is that not all switches are created equally. Fibre Channel is a networking standard and every Fibre Channel switch is designed to adhere to that standard. However, many of the larger switch manufacturers incorporate proprietary features into their switches. These proprietary features are not directly supported by the Fibre Channel specification.

That being the case, the functionality that can be achieved within a SAN varies widely depending upon the switches that are used within the SAN. It is perfectly acceptable to piece a SAN together using Fibre Channel switches from multiple vendors. Doing so is fairly common in fact, simply because of how quickly some vendors offer and then discontinue various models of switches. For example, an organization might purchase a Fibre Channel switch and later decide to expand their SAN by adding an additional switch. By the time that the second switch is needed the vendor who supplied the previously existing switch might have stopped making that model of switch. Hence the organization could end up using a different model of switch from the same vendor or they might choose to use a different vendor’s switch.

When a fabric contains switches from multiple vendors (such as HP and IBM) the fabric is said to be heterogeneous. Such situations are also sometimes referred to as an open fabric. When a SAN consists of one or more open fabrics, the switch’s proprietary features must usually be disabled. This allows one vendor’s Fibre Channel switch to work with another vendor’s switch since each switch adheres to a common set of Fibre Channel standards.

The alternative of course is to construct a homogeneous fabric. A homogenous fabric is one in which all of the switches are provided by the same vendor. The advantage of constructing a homogenous fabric is that the switches can operate in native mode, which allows the organization to take full advantage of all of the switch’s proprietary features.

The main disadvantage to using a homogenous fabric is something that I like to call vendor lock. Vendor lock is a situation in which an organization uses products that are provided by a single vendor. When this goes on for long enough, the organization becomes completely dependent upon the vendor. Vendor dependency can lead to inflated pricing, poor customer service, and ongoing sales pressure.

Regardless of which vendor’s switches you choose to use, Fibre Channel switches generally fall into two different categories – Core and Edge.

Core switches are also sometimes called Director switches. They are used primarily in situations in which redundancy is essential. Typically a core switch is built into a rack mounted chassis that uses a modular design. The reason why the switch has a modular design is for redundancy. A core switch is generally designed to prevent individual components within the switch from becoming single points
of failure.

The other type of switch is called an edge switch. Edge switches tend to have fewer configuration options and less redundancy than core switches. However, some edge switches do have at least some degree of redundancy built in.

It is important to understand that the concepts of core switches and edge switches are not a part of the Fibre Channel specification. Instead, vendors market various models of switches as either core switches or edge switches based on how they intend for a particular switch model to be used. The terms core and edge give customers an easy way to get a basic idea of what they can expect from the switch.

SAN Ports

I plan to talk in detail about switch ports in Part 5 of this series, but for right now I wanted to introduce you to the concept of Inter Switch Linking. Fibre Channel switches can be linked to one another through the use of an Inter Switch Link (ISL). ISLs allow storage traffic to flow from one switch to another.

As you will recall, I spent some time earlier talking about how some vendors build proprietary features into their switches that will only work if you use switches from that vendor. Some of these features come into play with regard to ISLs.

ISLs are a Fibre Channel standard, but some vendors use ISLs in a non-standard way. For example, most switch vendors support a form of ISL aggregation in which multiple ISL ports can be used together to emulate a single very high bandwidth ISL. Cisco refers to this as EISL, whereas Brocade refers to it as trunking. The point is that if you want to use ISL aggregation you will have to be vendor consistent with your Fibre Channel switches.


In this article I have tried to familiarize you with some of the basics of Fibre Channel switches. In Part 5, I plan to talk about Fibre Channel switch ports.


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