This article concludes this series by discussing point to point and ring topologies for Fibre Channel.
Introduction
In the previous article in this series, I discussed the Fibre Channel switched fabric topology. As I mentioned in that article, switched fabric is by far the most common Fibre Channel topology used in Storage Area networks. Even so, there are two additional Fibre Channel topologies that I wanted to show you.The Point to Point Topology
Point to point is by far the simplest Fibre Channel topology. It is so simple in fact, that its simplicity renders it unsuitable for use in SAN environments.
Point to point topology can best be thought of as a direct connection between two Fiber Channel nodes. In this topologies the N_Port from one node is connected to the N_Port of another. The cable that is used to make the connection performs a cross over so that the traffic being transmitted from the first node gets sent to the receive port on the second node. Likewise, the second node’s transmit port sends traffic to the first node’s receive port. The process is very similar to connecting two Ethernet devices together without a switch by using a crossover cable.
As you can see, a point to point topology is extremely simple in that there are no switches used. The down side to using point to point connectivity is that this type of topology severely limits your options because the design can’t be scaled to service more complex storage requirements without switching to a different topology.
Arbitrated Loop Topology
The other type of topology that is sometimes used with Fibre Channel is known as an arbitrated loop. This type of topology is also sometimes referred to simply as Loop or as FC-AL.
The Arbitrated Loop topology has been historically used as a low cost alternative to the switched fabric topology that I discussed in the previous article. Switched fabric topologies can be expensive to implement because of their reliance on Fibre Channel switches. In contrast, the arbitrated loop topology does not use switches.
It is worth noting that today Fibre Channel switches are less expensive than they once were, which makes the use of switched fabric more practical than it was a few years ago. The reason why I mention this is because in a SAN environment you really should be using a switched fabric. A switched fabric provides the highest level of flexibility and the highest resiliency when a component failure occurs. Even so, an arbitrated loop can be a valid option in smaller organizations with limited budgets, so I wanted to at least talk about it.
Just as the fabric topology can be implemented (cabled) in several different ways, so too can the ring topology. Although the phrase ring topology implies that devices will be cabled together in a ring, this concept does not always hold true.
The first way in which a ring topology can be cabled is in a ring. In doing so, the Fibre Channel devices are arranged in a circle (at least from a cabling standpoint anyway) and each device in the circle has a physical connection to the device to its left and to the device to its right.
This type of design has one major disadvantage (aside from the limitations that are shared by all forms of the ring topology). That disadvantage is that the cabling can become a single point of failure for the ring. If a cable is damaged or unplugged then the entire ring ceases to function. This occurs because there is no direct point to point connection between devices. If one device wants to communicate with another device then the transmission must be passed from device to device until it reaches its intended destination.
Another way in which the ring topology can be implemented is through the use of a centralized Fibre Channel hub. From a cabling prospective, this topology is not a ring at all, but rather a star topology. Even so, the topology is still defined as a ring topology because it makes use of NL_Ports (node loop ports) rather than the N_Ports that are used with a switched star topology.
So why would an organization use a Fibre Channel hub as a part of a ring topology? It’s because using a hub prevents the ring’s cabling from becoming a single point of failure. If a cable is broken or unplugged it will cause the associated device to become inaccessible, but the hub ensures that the affected device is bypassed and that the rest of the ring can continue to function. This would not be the case without the Fibre Channel hub. If the same failure were to occur on a Fibre Channel ring that was not based around a hub then the entire ring would cease to function.
The other advantage to using a Fibre Channel hub is that the hub can increase the ring’s scalability. I will talk more about scalability in a moment, but for right now I’m sure that some of you are curious as to the cost of a Fibre Channel hub. The price varies among vendors and is based on the number of ports on the hub. However, prices for Fibre Channel hubs start at less than a hundred dollars, and higher end hubs typically cost less than a thousand dollars. By way of comparison, some low end Fibre Channel switches cost less than five hundred dollars, but most cost several thousand.
Public Loops
Occasionally Fibre Channel loops use a design known as a public loop. A public loop is a hub based Fibre Channel loop that is also tied into a switched fabric. In this type of topology, the devices within the ring connect to the hub using NL_Ports. However, the hub itself is also equipped with a single FL_Port that connects the loop to a single port on a Fibre Channel switch. Needless to say, this is a low performance design since the switch port’s bandwidth must be shared by the entire ring.Scalability
Arbitrated loops are limited to a total of 127 ports. When a hub is not used, each device in the loop requires two ports, because it must link to the device to its left and the device to its right. When a hub is used then each device only requires a single port, so the loop could theoretically accommodate up to 127 devices (although hardware limitations often limit the actual number of devices that can be used).
On the lower limit, an arbitrated loop can have as few as two devices. Although a loop consisting of two devices is cabled similarly to a point to point topology, it is a true ring topology because unlike a point to point topology, NL_Ports are being used.
One last thing that you need to know about the ring topology is that it is a serial architecture and the ring’s bandwidth is shared among all of the devices on the ring. In other words, only once device can transmit at a time. This is a stark contrast to a switched fabric in which multiple communications can occur simultaneously.
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