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.
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.