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Not just a big switch

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Core components
With core components such as passive backplanes, concurrent microcode upgrades, purpose-built ASICs and redundant hardware components essentially equal among FC directors, vendors are finding other ways to differentiate their products. And with the growing need for higher port counts, distance replication and connecting SAN islands, vendors are adding functionality to FC directors in the following key areas:

  • Line cards
  • 1Gb/sec, 2Gb/sec, 4Gb/sec and 10Gb/sec FC ports
  • FC port buffer credits
  • Inter-switch link (ISL) aggregation and connectivity options
Vendors offer FC director line cards that allow users to configure directors to support a variety of port speeds and counts. For instance, Brocade's SilkWorm 48000 offers three different line cards that each support a different number of ports and port speeds. Brocade's FC4-16 and FC2-16 line cards each provide 16 FC ports, with the FC2-16 supporting 2Gb/sec and the FC4-16 4Gb/sec. For users to achieve the maximum 768 port count on the SilkWorm 48000, they need to use Brocade's FC4-32 line cards.

There are tradeoffs when maximizing port capacity and using faster FC speeds. McData's i10K supports 10Gb/sec FC ports, but those ports can only be connected to other i10K directors with the same 10Gb/sec FC ports because 10Gb/sec FC ports are based

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on a different technology than those at 1Gb/sec, 2Gb/sec and 4Gb/sec speeds.

Users of Brocade's SilkWorm 48000 will encounter similar issues. A SilkWorm 48000 fully populated with its FC4-16 line cards is the company's only 4Gb/sec configuration that allows its FC directors to operate at 4Gb/sec without any blocking of bandwidth. Brocade's FC4-32 line cards allow scaling up to the maximum port count on its 48000, but that configuration can only operate at a maximum of 2Gb/sec without blocking.

Despite these concerns, lower per-port prices are driving the move to line cards with higher port counts. Vendors report that users should generally expect to pay a 10% to 25% premium for line cards that support a higher number of ports. And despite the potential of back-end bottlenecks using the higher FC port count cards, most users aren't at risk, say vendors, because few production environments are reaching throughput limits on their FC directors. Cisco recently checked the utilization rates in its own production environment and found that most of its FC ports were averaging only 13MB/sec. This prompted Cisco to see if it could lower its own internal costs by increasing the number of ports on its blades.

To find the right balance between low and high port-count line cards, you need to identify the specific configurations and applications that require high-throughput FC ports. Applications such as backup/recovery and data replication, and FC ports dedicated to ISLs require high port throughput. By taking advantage of the port buffer credit and ISL aggregation features on the director ports, and by balancing which application or configuration uses which FC ports, there may not be a need to purchase lower port-count line cards.

This was first published in February 2006

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