A Crash Course in Storage Area Networking (Part 5)

by [Published on 9 Oct. 2012 / Last Updated on 9 Oct. 2012]

This article continues the discussion of Fibre Channel SANs by discussing Fibre Channel topologies and ports.

If you would like to read the other parts in this article series please go to:

In the previous article in this series, I spent some time talking about Fibre Channel switches and how they work together to form a switched fabric. Now I want to turn my attention to the individual switch ports.

One of the big ways that Fibre Channel differs from Ethernet is that not all switch ports are created equally. In an Ethernet environment, all of the Ethernet ports are more or less identical. Sure, some Ethernet ports support a higher throughput than others and you might occasionally encounter an uplink port that is designed to route traffic between switches, but in this day and age most Ethernet ports are auto sensing. This means that Ethernet ports are more or less universal, plug and play network ports.

The same cannot be said for Fibre Channel. There are a wide variety of Fibre Channel switch port types. The most common type of Fibre Channel switch port that you will encounter is known as an N_port, which is also known as a Node Port. An N_port is simply a basic switch port that can exist on a host or on a storage device.

Node ports exist on hosts and on storage devices, but in a SAN environment traffic from a host almost always passes through a switch before it gets to a storage device. Fibre Channel switches do not use N_ports. If you need to connect an N_port device to a switch then the connection is made through an F_port (known as a Fabric Port).

Another common port type that you need to know about is an E_port. An E_port is an Expansion Port. Expansion ports are used to connect Fibre Channel switches together, much in the same way that Ethernet switches can be connected to one another. In Fibre Channel an E_port link between two switches is known as an ISL or Inter switch link.

One last port type that I want to quickly mention before I move on is a D_port. A D_port is a diagnostic port. As the name implies, this port is used solely for switch troubleshooting.

It is worth noting that these are the most basic types of ports that are used in Fibre Channel SANs. There are about half a dozen more standard port types that you might occasionally encounter and some of the vendors also define their own proprietary port types. For example, some Brocade Fibre Channel switches offer a U_port, or Universal Port.

Right about now I’m sure that many of you must be wondering why there are so many additional port types and why I haven’t addressed each one individually. In a Fibre Channel SAN the types of ports that are used depend largely on the Fibre Channel topology that is being used.

Fibre Channel topologies loosely mimic the topologies that are used by other types of networks. The most common topology is the switched fabric topology. The switched fabric topology uses the same basic layout as most Ethernet networks. Ethernet networks use a lot of different names for this topology. It is often referred to a hub and spoke topology or a star topology.

Regardless of the name, the basic idea behind this topology is that each device is connected to a central switch. In the case of Ethernet, each network node is connected to an Ethernet switch. In the case of Fibre Channel, network nodes and storage devices are all connected to a switch. The switch port types that I discussed earlier are all found in switched fabric topologies.

It is important to understand that describing a switched fabric topology as a star or as a hub and spoke topology is a bit of an over simplification. When you describe a network topology as a star or as a hub and spoke, the assumption is that there is a central switch and that each device on the network ties into that switch.

Although this type of design is a perfectly viable option for Fibre Channel networks, most Fibre Channel networks use multiple switches that are joined together through E_ports. Often times a single switch lacks a sufficient number of ports to accommodate all of the network devices, so multiple switches must be joined together in order to provide enough ports for the devices.

Another reason why the star or hub and spoke model is a bit of an over simplification is because in a SAN environment it is important to provide full redundancy. That way, a switch failure cannot bring down the entire SAN. In some ways a redundant switched fabric could still be thought of as a star or a hub and spoke topology, it’s just that the redundancy requirement creates multiple “parallel stars”.

To give you a more concrete idea of what I am talking about, check out the diagrams below. Figure A shows a basic switched fabric that completely adheres to the star or hub and spoke topology. In this figure you can see that hosts and storage devices are all linked to a central Fibre Channel switch.


Figure A: This is the most basic example of a switched fabric.

In contrast, the Fibre Channel network shown in Figure B uses the same basic topology, but with redundancy. The biggest difference between the two diagrams is that in Figure B, each host and each storage device is equipped with two Host Bus Adapters. There is also a second switch present. Each host and storage device maintains two separate Fibre Channel connections – one connection to each switch. This prevents there from being any single points of failure on the SAN. If a switch were to fail then storage traffic is simply routed through the remaining switch. Likewise, this design is also immune to the failure of a host bus adapter or a Fibre Channel cable, because the network is fully redundant.


Figure B: This is a switched fabric with redundancy.

As you look at the diagram above, you will notice that there is no connection between the two Fibre Channel switches. If this were a real Fibre Channel network then the switches would in all likelihood be equipped with expansion ports (E_ports). Even so, using them is unnecessary in this situation. Remember, our goal is to provide redundancy, not just to increase the number of available F_ports.

In a larger SAN there would typically be many more switches than what is shown in the diagram. That’s because you would typically need to use expansion ports to provide greater capacity, but also continue to provide redundancy. To see how this works check out Figure C.


Figure C: This is a redundant multi-switch fabric.

In the figure above I have omitted all but one node and one storage device for the sake of clarity. Even so, the diagram uses the same basic design as the diagram shown in Figure B. Each node and each storage device uses multiple host bus adapters to connect to two redundant switches.

The thing that makes this diagram different is that we have made use of the switch’s expansion ports and essentially formed two parallel, redundant networks. Each side of the network uses a series of switches that are linked together through their expansion ports to provide the needed capacity, but the two networks are not joined to one another at the switch level.

Conclusion

Although switched fabric is the most common Fibre Channel topology, it is not the only topology that can be used in a SAN. In the next article in this series, I will show you the point to point topology and the ring topology. As I do, I will discuss the unique port requirements for Fibre Channel rings.

If you would like to read the other parts in this article series please go to:

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