Specifications
DATA CENTER BEST PRACTICES
SAN Design and Best Practices 48 of 84
state machines at any one time. A state machine lives for the life of an exchange, and then it is deleted. FICON
Emulation operates somewhat differently, and it is not discussed here in detail. State machines live in the
VE_Port and require the outbound and return of an exchange’s sequences to pass through the same VE_Port;
otherwise, the state of the protocol errors, and a failure occurs. A sequence that passes through one set of
VE_Ports on the way out sets up state on those specic VE_Ports. If the sequence returns on a different set of
VE_Ports, there is no state, and the VE_Ports cannot determine what that sequence is and which exchange it
belongs to. To prevent this problem from occurring, the outbound and return path should be deterministic. There
are a couple of methods for creating deterministic paths for protocol optimization.
First and most common, provide only one physical path. If a single VE_Port connects fabrics on the end points
of an FCIP ISL, then there is only one physical path, and protocol optimization will work well. With FCIP Trunking,
one VE_Port does not mean one FCIP connection, as there may be multiple FCIP circuits and Ethernet ports
used to connect up the VE_Ports. This provides the resiliency and redundancy.
Second, if more than one VE_Port and protocol optimization are required, then you can use VF Logical Switches
to provide a deterministic path. By dividing the Brocade 7800 or DCX Backbone into separate LSs and assigning
a single VE_Port to that LS, a deterministic path across the Logical Fabric can be formed. Using VE_Ports on
an FX blade can provide 10 Gbps of FCIP to one LS. Of course, the downside to this method is that there is
no ubiquitous connectivity across all ports on the Brocade 7800 or DCX Backbone. The FC/FICON/VE ports
become isolated, which is why there is a deterministic path.
Third, you can use TI zones to conne data to deterministic paths specic to ports from end-to-end.
Virtual Fabrics
The Brocade 8510 Backbone with the FX8-24 Extension Blade and the Brocade 7800 Extension Switch (4/2
and 16/6 models) all support VFs with no additional license. The Brocade 7800 supports a maximum of four
Logical Switches and does not support a Base Switch. Because there is no Base Switch, the Brocade 7800
cannot provide support for XISL or FCR (no EX_Ports and VEX_Ports). FICON CUP is supported by IBM on two
LSs only. VF on the Brocade 7800 must be disabled if a separate RDR network is not feasible and FCR is
required to connect to production edge fabrics.
VF on the Brocade 7800/FX8-24 plays a primary role in providing ways to achieve deterministic paths for
protocol optimization, or for the purposes of specic conguration and management requirements providing
unique environments for FICON and FCP. Virtual Fabrics is the preferred alternative over TI Zones to establish
deterministic paths necessary for protocol optimization (FCIP-FW, OSTP, and FICON Emulation). Protocol
optimization requires that an exchange and all its sequences and frames pass through the same VE_Port
for both outbound and return. This means that only a single VE_Port should exist within a VF LS. By putting
a single VE_Port in an LS there is only one physical path between the two LS that are connected via FCIP. A
single physical path provides a deterministic path. When a large number of devices or ports are connected for
transmission across FCIP, as would be the case with tape for example, it is difcult to congure and maintain TI
Zones, whereas it is operationally simplistic and more stable to use VF LS.
Conguring more than one VE_Port, one manually set with a higher FSPF cost, is referred to as a “lay in wait”
VE_Port and it is not supported for FCIP-FW, OSTP or FICON Emulation. A “lay in wait” VE_Port can be used
without protocol optimization and with RDR applications that can tolerate the topology change and some frame
loss. A small number of FC frames may be lost when using “lay in wait” VE_Ports. If there are multiple VE_Ports
within an LS, routing across those VE_Ports is performed according to the APTpolicy.
Virtual Fabrics are signicant in mixed mainframe and open system environments. Mainframe and Open System
environments are congured differently and only VFs can provide autonomous LSs accommodating the different
congurations. Keep in mind that RDR between storage arrays is open systems (EMC SRDF, HDS Universal
Replicator/TrueCopy, IBM Metro/Global Mirror, and HP Continuous Access), even when the volume is written by
FICON from the mainframe.