Buffer Credit Recovery; Fabric Considerations; Long Distance Link Initialization Activation; Extended Fabrics Device Limitations - HP A7533A - Brocade 4Gb SAN Switch Base Administrator's Manual

Buffer credit recovery

Buffer recovery credit allows links to recover after frames and R_RDYs are lost when the credit recovery
logic is enabled. Buffer recovery credit maintains performance; as soon as one credit is lost, it attempts to
recover. During link reset, the frame and credit loss counters are reset without performance degradation.
This feature is only supported on long distance E_Ports connected between GoldenEye2 and
condor2-based ports. Buffer recovery credit does not require any configuration.
If a long distance E_Port is connected to any other type of ASIC, the buffer credit recovery feature is
disabled. Virtual E_Ports and Virtual Ex_Ports do not support long distance. The buffer credit recovery
feature is enabled for the following flow control modes: Normal mode, Virtual Channel (VC) mode, and
Extended VC mode.
An FC_Port that supports BB_Credit Recovery, maintains the following BB_Credit recovery values:
• BB_SC_N is the log2 of BB_Credit Recovery modulus
• BB_RDY_N counts the number of R_RDY primitives received modulo 2BB_SC_N
• BB_FRM_N counts the number of frames received modulo 2BB_SC_N

Fabric considerations

Because the number of buffer credits available for use within each port group is limited, configuring buffer
credits for extended links may impact the performance of the other ports in the group used for core-to-edge
connections. Balance the number of long-distance ISL connections and core-to-edge ISL connections within
a switch. Configuring long-distance ISLs between core and edge switches is possible, but is not a
recommended practice.
With the exception of older switches and blades that use the Bloom ASIC, all switch ports provide
protection against buffer depletion through buffer limiting. A buffer-limited port reserves a minimum of
eight buffer credits, allowing the port to continue to operate rather than being disabled due to a lack of
buffers. The 8 buffer minimum allows 4 Gbps speeds over distances within most data centers.
Buffer-limited operation is supported for the L0 and LD extended ISL modes only, and is persistent across
reboots, switch disabling and enabling, and port disabling and enabling.

Long distance link initialization activation

VC translation link initialization (vc_translation_link_init), a parameter of the
portCfgLongDistance command, is enabled by default for long-distance links. To avoid inconsistency
in the fabric, make sure that this parameter is enabled on both ends of the link by entering the
portCfgLongDistance --vc_translation_link_init command. Specify 1 to activate long
distance link initialization sequence; specify 0 to deactivate this mode. When the command is run without
specifying a value, 1 is assigned automatically for the long-distance link in VC_RDY flow control;
otherwise, 0 is assigned. For a long-distance link not configured for ISL R_RDY mode, this parameter must
be set to 1; otherwise, it must be reset to 0.

Extended Fabrics device limitations

Extended Fabrics is normally not implemented on the following devices.
The
•
400 Multi-protocol (MP) Router
FC ports. The two Gigabit Ethernet ports provide SAN extension over IP networks using FCIP.
• The FC4- 1 6IP blade integrates eight Gigabit Ethernet ports and eight FC ports. It is used to implement
the iSCSI Gateway Service. The Gigabit Ethernet ports are used to connect iSCSI initiators, and the FC
ports are used to connect to any device.
and the FR4- 1 8i blade integrate two Gigabit Ethernet ports and sixteen
Fabric OS 6.1.x administrator guide 353