HP StorageWorks 2/24 FW 07.00.00/HAFM SW 08.06.00 McDATA Products in a SAN Env - Page 147

Implementing SAN Internetworking Solutions 4 Large Fabric Problems • Inability to consistently schedule maintenance downtime for each SAN. • Stranded resources. Unused ports in one SAN cannot be used by applications in another (port limited) SAN, and expensive resources (such as tape backup elements) cannot be easily shared across SAN boundaries. Fibre Channel fabrics configure and manage themselves, and require operator intervention only upon failure. When a Fibre Channel device connects to a director or fabric switch, the device receives a unique 24-bit (three-byte) network address composed of domain, area, and port bytes. This address is used for routing data through the fabric. Domain identifiers (Domain_IDs) are typically reserved for fabric elements (directors and switches), and range between 1 and 31 for McDATA products. A fabric element with a unique Domain_ID then allocates the remaining two bytes (area and port) to provide network addresses for devices. The principal switch selection process ensures each fabric element has a unique Domain_ID. This process determines which director or fabric switch acts as the master in a newly-configured SAN and allocates unique Domain_IDs to the remaining elements. Without this process, two elements could have the same Domain_ID, resulting in duplicate addresses and misrouting of data. Principal switch selection is initiated by transmitting exchange fabric parameter (EFP) frames to expansion ports (E_Ports) of all connected switches until a user-defined fabric stability timeout value (F_S_TOV) is reached. The F_S_TOV is proportional to the number of fabric elements. If the fabric is large, the F_S_TOV is set higher and the selection process takes longer. During principal switch selection, a disruptive or non-disruptive build fabric event occurs. A non-disruptive event preserves fabric element Domain_IDs and does not require reassignment of network address blocks. However, a disruptive event may cause elements to acquire a different Domain_ID, which means each attached device must re-login to the fabric to acquire a new network address. Ironically, the self-configuring functionality provided by the Fibre Channel architecture makes it more problematic to build large fabrics or to extend fabrics over large distances. Fibre Channel fabrics tend to become unstable long before reaching the maximum theoretical switch count (239) because of ISL congestion, disruptive build fabric events, or sporadic disruptions. Implementing SAN Internetworking Solutions 4-3