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The primary benefit of port buffer credits is to keep data flowing across distances. The size of the buffer credit needed on each FC port will depend on four factors:
- The amount of data going through the port
- The speed of the port
- The distance between the FC ports
- If the WAN gateway devices used provide additional buffering
For distance replication, vendors generally recommend approximately one port buffer credit for every kilometer over a 1Gb/sec link. In most situations, you'll only need to devote a few FC ports for long-distance replication with the rest of the ports reserved for local connectivity. To provide as much flexibility as possible, vendors offer choices for how buffer credits can be configured and re-allocated among ports.
The ability to allocate buffer credits to FC ports lets users install higher port-count line cards and meet high-throughput replication and
Cisco is the only director vendor that allows users to increase the number of buffer credits, up to 3,500, with an optional license. The only other way to attain sufficient buffer for distance replication is to use a gateway device. Gateway appliances such as Ciena Corp.'s CN 2000 or Nortel Networks' Optical Metro 5000 series (formerly OPTera Metro 5000) offer additional buffering that enables the directors to do long-distance replication over FC when they're connected to SONET or dense wavelength division multiplexing (DWDM) networks.
This was first published in February 2006