This would be best described with a diagram but I'll try.
Hard zoning is created by using switch port numbers instead of World Wide Names in your zone configuration files (soft zoning). The good thing about hard zoning is that it is more secure than soft zoning and enables the replacement of host HBAs without changing zone information in the switches. The bad side is that if you lose a switch port and move to a different port, you also need to change the zone configuration to represent the connection to the new port.
You can have multiple zone configurations in a switch and multiple switches in a fabric. A zone configuration can include multiple "zone sets," that can include either port numbers or WWNs (or aliases) for individual servers and storage. As you connect the switches together using E-ports (or ISL ports), the zone data is merged if everything is unique or else the switches complain and do not let you connect them together. If you have a virgin switch with no zone configuration on it and connect it to another switch that does, the virgin switch will download the zone configuration and the configuration will now apply to both switches in the newly created fabric.
So now you have two switches in a single fabric with one zone configuration active for both switches. When using hard zoning you must specify the switch domain ID (which switch in the fabric) and the port number of the switch (which port number on the switch). An overlapping zone is created when a single physical port is included in multiple zone sets in the same active zone configuration.
Say you have an NT server connected to port 1 on switch 1 and two storage arrays connected to port 2 on switch 1, and port 2 on switch 2. You also have a Unix server on port 3 of switch 1 that has access to the storage on port 2 of switch 1.
The Unix server gets to data located on the array on port 2 of switch.
1. The NT server gets its data from both storage arrays. (Port 2 on switches 1 and,
2. In order to give the NT server access to both storage arrays, you would create an overlapping zone that included the storage array on port 2 on switch 1 into the Unix zone and the NT zone.
If the storage array does not also provide for LUN security, it may be possible for the NT servers to see the Unix storage on the array on switch 1, port 2. This may cause problems and therefore the need to watch for overlapping zones.
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This was first published in June 2002