Storage Basics: Storage Hubs and Switches

Thursday May 22nd 2003 by Mike Harwood
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Storage Basics: Storage Hubs and Switches

Those of us who have spent time working with Ethernet networks are well aware of the role both hubs and switches play in network architecture. When it comes to Fibre Channel networks, we still need to use switches and hubs, but as with Ethernet networks, hubs have fallen out of favor for higher performance switches. In this article, we take a brief look at the types of hubs that can be used with Fibre Channel networks and then compare and review how these hubs stack up against switches in practical application.

Hubs are used with the Fibre Channel Arbitrated Loop (FC-AL) topology to increase server and storage connectivity. Analogous to Ethernet hubs, Fibre Channel hubs form a focal point for the FC-AL topology, allowing the loop to be centrally monitored and managed. By using a hub, multiple servers are able to access several different storage devices. Hubs also allow network devices to be added or removed from the loop while the loop is still operational.

While the functions of the various types of Fibre Channel hubs are basically the same on the network, not all hubs are created equal, and using the right one for a particular network is an important consideration. There are three basic types of Fibre Channel hubs: unmanaged, managed, and switched hubs.

Unmanaged Hubs

On the lower end of Fibre Channel hubs are the unmanaged hubs. Similar to unmanaged hubs used on LANs, the Fibre Channel counterparts are not shipped with any management software and are therefore, in terms of administration at least, very easy to manage. In terms of function, their job is fairly straightforward -- when nodes attached to an unmanaged hub communicate, the hub port receives a signal from a device, regenerates that signal, and forwards it. Unmanaged hubs do not concern themselves with selective data distribution; rather, network traffic is sent to all ports whether the ports have need of that information or not. As discussed later, this approach presents both performance and security problems.

Unlike many other connected network devices, such as Host Bus Adapters (HBAs) or switches, unmanaged hubs are not assigned a Fibre Channel address. This means that they are not involved in any of the network's protocol-related activities. Perhaps the only bright spot for unmanaged hubs is their cost. Fibre Channel hub prices start around $1000 to $1500 and go up from there, making them an affordable solution.

Page 2: Managed Hubs

Managed Hubs

Managed hubs are one step above unmanaged hubs, with the obvious difference being that managed hubs are, well, manageable. Managed hubs are SNMP controllable. As a quick refresher, SNMP is an application layer protocol that enables the exchange of management information between network devices. An SNMP manager resides somewhere on the network and collects management information sent by SNMP agents on the network. Managed hubs are able to be SNMP agents, whereas unmanaged hubs are not.

Some of the common features found with managed hubs include the following:

Hot-Pluggable Ports - Most managed hubs provide a certain number of hot pluggable ports. Like any other hot plug device, these ports allow on-the-fly attachment and detachment of network devices such as servers or storage devices.

Dedicated port speed - When using a managed hub, each of the hub's individual ports can provide 100 Mbps data transfer speeds. The speed of each port on the hub is not affected by the use of the other ports on the hub.

Error Detection - Many managed hubs offer some form of diagnostic capabilities, providing the administrator with a method to see if anything is amiss. Many of the managed hubs are also capable of performing a Power on Self Test (POST). The POST check is run when the hub is powered on and routinely checks the status of internal components and even the status of attached nodes. If a failed node is detected, the port will be excluded from the loop.

Management interface - A managed hub needs an interface, and many of the newer hubs provide easy to navigate graphical interfaces. At the very least, there will always be the traditional command-line interface.

Switched Hubs

Switched hubs are the most recent addition to the family of Fibre Channel hubs and provide a bridge between the world of hubs and switches. They are manageable and provide the function of a hub in the FC-AL topology, but they also have many of the performance capabilities of a switch. The ports on a switched hub are dedicated, allowing for maximum bandwidth of each port and, unlike a hub, will only direct a data stream to the port that requested that information. Switched hubs also receive a Fibre Channel address and can be involved in protocol activities such as the management of other devices on the network, event logging, and diagnostics.

Page 3: Hubs vs. Switches

Hubs vs. Switches

Having reviewed Fibre Channel hubs, let's take a look at how hubs stack up against switches.

Network size - While it's really becoming less of a concern, the network size can impact the decision to go with hubs or switches. Some administrators suggest that switches should be used on networks of all sizes; in reality, if a network is comprised of fewer than 50 nodes, then hubs should do the trick. If, however, some applications are heavily used, such as full motion video streaming, then switching hubs may be needed.

Once the number of nodes grows in the network, hubs may not be able to manage the workload. Though there are no strict guidelines, in practical application, once a network surpasses 100 nodes, switches are the name of the game. A combination of both switches and hubs is sometimes used together in the network to accommodate increasing nodes.

Bandwidth - The issue of bandwidth has sent more that a few Fibre Channel hubs to the storage closet. A Fibre Channel hub, with the exception of a switched hub, is a shared device. This means that all devices attached to the hub are forced to share the bandwidth of the hub. In this case, it is possible for two or three high-performance devices connected to the hub to flood all of the ports. Fibre Channel hubs are similar to Ethernet hubs in that all ports see traffic from all other ports. This approach often leads to bandwidth congestion and sometimes security concerns. A switch, however, has completely independent ports, and each port can simultaneously support the maximum bandwidth limits of the topology used.

Because of the bandwidth limitations of the traditional hub, hubs are not the best choice on storage networks where heavy audio and video data is regularly exchanged.

Security - In the IT world today, security is a hot topic. Security may also come into play when deciding between hubs and switches. Some hubs do not allow zoning on the Fibre Channel network. Zoning is much like creating virtual LANs on an Ethernet network, and it provides a way to allow only certain ports to communicate with other certain ports in the fabric. The issue is obviously not a concern where hubs allow for network zoning.

Scalability - When it comes to scalability, switches are the clear winner. Switches are very scalable and will accommodate future growth. Hubs are a different story. Because of their security concerns and the fact that they use bandwidth sharing, they are not scalable to larger networks. If a network is small but is expected to experience rapid growth, switches are again the answer.

Summary

While being somewhat inferior in terms of performance, hubs are still entrenched in many Fibre Channel networks. Knowing what types of hubs are available, what they are designed to do, and what limitations they present are all important considerations when managing a Fibre Channel network. While hubs and switches are an integral part of the Fibre Channel network, we still need a way to connect these devices. In the next Storage Basics article, we'll take a look at the Fibre cabling and connectors used to connect hubs, switches, and other devices to the network.


» See All Articles by Columnist Mike Harwood

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