SAN Design: March 29, 2001 3:18 pm
53-0001575-01 BROCADE Technical Note Page: 23 of 31
for wiring components that do not need to be a long distance from the switch. Copper connections tend to be less expensive. If cost
is not a driver, designing a solution with all optical media will provide for greater flexibility in future system upgrades and expansion
and allow devices to be extended beyond the limits of one rack without having to replace any GBICs.
3.4 ZONING/NAME SPACE
Other considerations should also be weighed when designing a large switch count fabric. Some of these are:
•Total number of zones and zoned devices. The larger the zoning information, the more information content needs to be passed
between switches when the fabric re-configures. CPU cycles in each switch are used to accept, resolve, and re-set zone data. A
busy CPU can take too long to recognize new devices being attached to the fabric. Future versions of this document will contain
more specific data based on empirical testing, however the general rule is try and keep the number of zones down as the total switch
count increases. Zoning tables of several hundred entries have been tested and validated in the fabric sizes detailed in this
document.
•All switches need to have the same zoning information in order to be merged in a fabric. See the large port count fabric bring up
document referenced at the start of this section for details on adding switches with zoning to an existing fabric.
•Name Space -- name selection in zoning can be based on World Wide Name or on Domain/Port ID. Using the WWN provides for
more flexibility in moving devices in the fabric, the zoning follows the WWN no matter what port the device is plugged into the fabric.
However, CPU translation of the WWN to domain/port ID must take place imposing an additional processing load on the switches
in the fabric. Domain/PortID zoning will not require CPU translation and will also allow the hardware to enforce zoning. There is no
one right answer, just two options to choose from and each has benefits and drawbacks that need to be considered when designing
a zoning scheme.
•WWN based zoning: zoning follows device no matter what switch port; allows for moving cables and maintain-
ing zones. Drawback--requires CPU to translate WWN to port ID for zone checks
•Domain ID/Port ID based zoning: allows for hardware enforced zoning instead of software, no CPU translation
required, minimizes CPU load. Drawback -- if you move a cable the zoning table must be re-configured for the new
port.
•GUI Zoning Definition -- a zoning GUI tool that allows for specifications of zones using our Web Tools GUI is available in firmware
version 2.1 and later. This tool does require licensing of Web Tools as well as Zoning as added switch software options.
3.5 TOTAL SWITCH COUNT/PORT COUNT
The ultimate limitation in fabric design is a maximum of 239 physical switches, be they 8 or 16 port versions. This limit is imposed by the
actual number of domain IDs that can be uniquely established in the Fibre Channel Device ID header on the frames. The practical limit and
what has been tested is much fewer switches. BROCADE is currently testing and validating fabrics greater than 32 switches with as many
as 68 switches in the configuration. Table 1 in this document details the current tested switch/port count limits for fabrics. Future updates of
this table will result from testing taking place now with updated FOS releases and with testing performed on later iterations of the Fabric
OS. In addition, switch port count support will increase with next generation larger port count switches.