EMC: SAN switch fabric topology mathΒΆ

Title:EMC: SAN switch fabric topology math
Author:Douglas O’Leary <dkoleary@olearycomputers.com>
Description:Math demonstrating port gains by adding switches
Date created:
Date updated:
Disclaimer:Standard: Use the information that follows at your own risk. If you screw up a system, don’t blame it on me...

EMC recommends no more than four connectrix switches per fabric based on the following formulae:

  One Switch   Two Switch
32 Total ports 64 Total ports
  • 4
ports reserved for card failure
  • 4
ports reserved for card failure
28 ports remaining.
  • 4
ports reserved for E_ports
  • (int(28/5))
No more than 4:1 ratio, hosts : fa 56 ports remaining.
23 Possible host connections
  • (int(56/5))
No more than 4:1 ratio, hosts : fa
/ 2 to support multi-pathing 45 Possible host connections
11 total host connections / 2 to support multi-pathing
    22 host connections (gain of 11)
  Three switches   Four switches
96 total ports 128 total ports
  • 4
ports reserved for card failure
  • 4
ports reserved for card failure
  • 12
ports reserved for E_ports
  • 24
ports reserved for E_ports
80 ports remaining 100 ports remaining
  • (int(80/5))
No more than 4:1 ratio, hosts : fa
  • (int(100/5))
No more than 4:1 ratio, hosts : fa
64 Possible host connections 80 Possible host connections
/ 2 to support multi-pathing / 2 to support multi-pathing
32 host connections (gain of 10) 40 host connections (gain of 8)

Putting in that fourth connectrix means that you gain only 8 host connections from a 32 port connectrix switch. At EMC prices, that’s going to be a hard sell to the resident bean counters