SECURING FIBRE CHANNEL FABRICS - Brocade

FC Fabric Features and ServicesAn FC switch will not transmit data to another switch port until thatport has advertised a BB credit. The credit is essentially a promise thatthe receiving port will be able to deliver the frame to its next destination,either by forwarding it immediately or by storing it and forwardingit later. When a port sends a frame, it cannot use that credit again untilthe receiving device returns it as an R_RDY call. Once a frame is sentto its next destination, the buffer is freed up. At that point, the associatedBB credit is released to notify the other switch that memory isonce again available on that port to receive another frame.FC Fabric Features and ServicesThe simple name server (SNS), a repository of all known devicesattached to the fabric, acts as the fabric’s “directory assistance.”When a device is connected to a switch, one of the steps it must gothrough is registering its WWN with the SNS. The SNS may also beresponsible for enforcing zoning rules via the WWN on some vendorswitches, but Brocade enforces all zoning using the ASIC componentof the hardware.ZoningFabrics can become quite large, span great distances, and consist ofthousands of nodes. To avoid one flat network allowing every device tobe aware of every other device, zoning is used to isolate groups ofdevices. Zoning is a fabric-based service that groups devices that needto communicate with each other. Once a device is assigned to a zone,it can communicate only with other devices in that zone.Zoning terminology can be confusing, mainly for historical reasons.The terms “hard and soft zoning,” “port and WWN zoning,” and “hardware-enforced”and “software-enforced zoning” are often usedinterchangeably. To be clear, there are two basic methods by whichzone members are identified and enforced within a fabric:• By the hardware via the ASIC: hardware-enforced zoning• By software as a service: software-enforced zoningSecuring Fibre Channel Fabrics 29