Basic SAN Questions

Hi Chan,

  Here are the short answers to your questions...

1.  Please download the Host Connectivity Guide for Windows which is specific to the HBA type you are using (i.e. QLogic or Emulex) from EMC Online Support for detailed information on configuring a Windows host into an EMC SAN/storage environment.  Also, have the documentation handy for the array you are connecting the host to.

2. Fibre Channel can be used in three main connectivity topologies.  These are Switched Fabric, Point-to-Point and Arbitrated Loop.  In the typical SAN today using FC switches, Fabric connectivity is used.  In this, there are two main types of ports, F_Ports which are on the switch and N_Ports which are end device (i.e. hosts and storage arrays) ports.  When N_Ports are connected to F_Ports, the ports communicate and the N_Port is logged into the SAN for communication with other end device N_Ports also connected in the SAN switched fabric.

In some cases, an FC SAN is built without using switches.  This is called Arbitrated Loop.  Instead of to a switch, all devices are connected one to another in a loop topology without the aid of a switch.  Also, an Arbitrated Loop SAN can  "proxy" into a switched SAN fabric by connecting one of the loop devices into a SAN switch port. This allows all loop devices on the non-switched SAN to join a switched SAN fabric.  Simply, when a switch port connects to an Arbitrated Loop end device (NL_Port), it turns that switch port personality from F_Port to FL_Port (adding the "L" designates "Loop") which allows the switch to "talk" Arbitrated Loop through that switch port.  Where there is no switch, end devices with NL-Ports daisy chain one-to-another with all other devices on the loop using NL_port protocol.

3. Simply just swap out the SFP with another one and see if the error moves or stays with that port.  Other than this basic start, realistically, you must approach this with deductive reasoning using switch and end device information and logs available to you.  First, you need to find out from SAN switch logs and other switch information how the error is being detected.  You also may need to interrogate the affected end device as well. Only through study of this information can you decide if the error is or is not a bad end device port.  Also, the problem could be a bad switch port or optical cable, patch panel or other FC path failure or other reason that could cause loss of light or other port error condition to be detected as a port problem.  What can make this even more difficult is if the problem is intermittent. So, if you have good reason to think the problem may be an SFP, simply just swap it out first!  This will go a long way in helping to decide the question without spending too much troubleshooting time up front.

4. Generally switches are very stable operationally.  So, I would say user error in some area of configuration such as Zoning would be classified as the most general switch problems seen in the field.

I hope this helps answer your questions!

Mike Mendola

Senior Technical Education Consultant - SAN Team