Explain Rule 17 in DMX?

You have a lot of quesions today lakshman.

The rule of 17 is simply a way to make sure that the paths you connect your host to are not running on the same director, but one physically far away from it.

Rule of 17 ensured that FAs being used for host connectivity were in different power zones.

And as dynamox says, different power zones

It wasn't power zones.  Rule of 17 came in due to the bus architecture of the early Symmetrix arrays (Symm3 -5's).  The arrays had an odd and even bus if the termination chips failed the bus would go down.  If your hosts were attached to directors on both buses you would maintain connectivity hence the rule of 17 gauranteed hosts were on an odd and an even director.  With the DMX (direct matrix) and the V-Max arrays its not a requriement as its no longer a bus architecture but people maintain it as a good practice.

correct me if i am wrong but DIR 3 (odd) and DIR 4 (even) reside on different buses yet in the same power zone, so even if you had your host connected to 3 and 4 ..if that power zone went down ..your hosts went down.

You are all correct

The original Rule of 17 was put into place to ensure that there was a path on each bus (odd and even).

The bus architecture went away in DMX-1 (aka Symm6).  But we had 2 power zones; one zone for directors 1-8, and another zone for directors 9-16.  So the Rule of 17 still had value.

There is no technical reason to use Rule of 17 on DMX-3, but many people still do just for the sake of knowing where things are, or they "grew up with it."

Same for VMAX, but depending on which Engines are configured, it may not be possible.

Following the rule of 17 you wouldn't attach a host to director 3 and 4.  Power zones were not in play when the "rule of 17" was created.  While the power zone feature was there if you lost an entire power zone a signal was sent over a legacy bus to shut the other power zone down.  Hardware could support it but the microcode did not as the only way a system would sustain is if it was 100% mirrored (raid-1).  If you had any raid configured you would lose 1/2 the raid group.