SAN Hardware building blogs- HUBs

Hubs only understand the arbitrated loop topology. When you connect a device to a hub, it will cause the arbitrated loop that the hub is managing to re-initialize. The device will be assigned an AL_PA (arbitrated loop physical address), and it will begin arbitration when it needs to communicate with another device on the loop.

Types of Hubs:-

There are managed (i.e., "smart") hubs and unmanaged (i.e., "dumb") hubs. An unmanaged hub is unable to close the loop when a device on the loop is malfunctioning; therefore, a single bad device can disable the entire loop. A managed hub could detect the bad device and remove it from the loop, allowing the rest of the loop to function normally.

Although there are plenty of unmanaged hubs available, the cost difference between managed and unmanaged hubs is minimal, and the functionality difference between them is quite great. When considering a new SAN, one should also consider whether a hub is even appropriate. The cost difference between hubs and switches gets smaller and smaller every day, and the functionality difference is even greater than the difference between managed and unmanaged hubs. Since arbitrated loop is cheaper than fabric, I have seen a number of sites build SANs based on hubs and arbitrated loop -- only to rip out the hubs and replace them with switches a year or two later.