FW V06.XX/HAFM SW V08.02.00 HP StorageWorks SAN High Availability Planning Guide (AA-RS2DD-TE, July 2004)

Physical Planning Considerations
151SAN High Availability Planning Guide
Figure 58: PDCM Array - Example Problem
A PDCM array configured for Director A prohibits logical port address 05 from
communicating with logical port addresses 0A, 0B, and 0C. No PDCM array is
configured for Director B. The PDCM array configured for Director A prohibits
the source server from transmitting or receiving data across ISL 2. However,
internal route tables on both directors indicate a valid server-to-destination device
path across ISL 1.
A problem arises when the source server transmits Class 3 Fibre Channel data to
devices across ISL 1, consuming the ISL bandwidth. Destination devices are
unaware of the PDCM array configured at Director A and transmit frames back to
the server across ISL 2. Because the server is prohibited from communicating
across this ISL, Class 3 Fibre Channel frames are discarded without generating a
busy (BSY) frame, reject (RJT) frame, or otherwise notifying the destination
devices. The server receives no response from destination devices and times out.
Thus, a server or device failure is indicated when in fact the problem is a
user-defined prohibited connection.
Preferred Path
The preferred path option allows a user to specify and configure one or more ISL
data paths between multiple directors or switches in a fabric. At each fabric
element, a preferred path consists of a source port on the director or switch being