Not just a big switch


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ISL aggregation
ISLs connect different SAN fabrics and can be aggregated to increase throughput. Vendors offer different techniques beyond the standard route balancing via fabric shortest path first (FSPF) to improve ISL throughput, including:

  • Open trunking (McData)
  • Frame-based striping or advanced ISL trunking (Brocade)
  • Load balancing based on source, destination and exchange IDs (Brocade/Cisco)
McData's optional open-trunking technology on its 6140s provides automatic, dynamic and statistical load balancing across ISLs in a fabric. This feature monitors the FSPF routing database and when it detects a congested link, it tweaks the FSPF database so it reroutes the traffic to a less-trafficked link. Because McData allows the aggregation of 1Gb/sec, 2Gb/sec and 10Gb/sec ISLs between its directors, this prevents a 1Gb/sec link from becoming overloaded if it's the shortest path between two ports; in the event a 10Gb/sec ISL starts to fill up, it allows users to add any free 1Gb/sec or 2Gb/sec FC port on any line card to the aggregated ISL without requiring the need to consume another 10Gb/sec FC port.

Brocade offers two techniques that optimize throughput on ISL links between two of its FC directors. Its frame-based striping, or advanced ISL trunking, treats all ISLs between two FC directors as one logical ISL, allowing users to configure up to eight

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FC ports as one logical trunk between two SilkWorm 48000s. To overcome the designed intolerance of Fibre Channel for out-of-order FC frames, Brocade uses special ASICs on its line cards to allow frames to arrive out of order and to then be re-ordered to their original sequence. The primary drawback to this approach is the requirement that all FC ports must be on a single line card, which creates some availability concerns should an entire line card fail or need to be taken offline for maintenance.

The other technique, load balancing based on sender, destination and exchange ID, is supported by Brocade and Cisco, but is implemented in a manner that doesn't support interoperability between their directors. On Brocade's SilkWorm 48000, it's offered only on line cards that handle the 4Gb/sec protocol, while Cisco supports it on all of its 1Gb/sec and 2Gb/sec line cards.

The main differences between the two is that Cisco lets users aggregate any 16 ports on any line card in a chassis to form this logical ISL, while Brocade supports up to eight FC ports that must be on a single line card. Both configurations support a maximum throughput of 32Gb/sec, with Cisco using 2Gb/sec links and Brocade using 4Gb/sec links.

As organizations connect to independent and often rogue SANs through the use of WAN links and ISLs, new challenges arise. On the positive side, unused FC ports and additional storage capacity can often be harnessed in ways not previously considered. On the flip side, these connections bring the often unmanaged chaos that reigns in smaller SANs to the central data center: out-of-date microcode, lack of central supervision, little or no change control processes, personnel with varying degrees of expertise, and departments with their own ideas on how the SAN should be managed. These issues require a new set of services to be supported by the SAN fabric, and FC directors are where many organizations are initially turning to find answers.

This was first published in February 2006

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