Guidelines And Limitations For Congestion Isolation; Extended Receiver Ready - Cisco MDS 9000 Series Configuration Manual

Congestion Detection, Avoidance, and Isolation
a value equal to or less than slowport-monitor, there will be no frames queued for transmit and slowport-monitor
will not detect the slow port.

Guidelines and Limitations for Congestion Isolation

Extended Receiver Ready

• ER_RDY is supported only on Fibre Channel ports on Cisco MDS 9700 Series with Cisco MDS 9700
• Trunking must be enabled on all ISLs in the topology for ER_RDY flow-control mode to work.
• After the system fc fow-control er_rdy command is configured on both the local switch and its adjacent
• For migration purposes, port channels can have their member links in both R_RDY and ER_RDY
• VL1 is reserved for host bus adapter (HBA) and zone quality of service (QoS).
• Inter VSAN Routing (IVR), Fibre Channel Redirect (FCR), Fibre Channel Over TCP/IP (FCIP), and
• In-Order Delivery (IOD) may get affected when the flow-control mode is initially set to ER_RDY and
• Switches running releases prior to Cisco MDS NX-OS Release 8.1(1) in a fabric are unaware of slow
• If you have configured the buffer-to-buffer credits using the switchport fcrxbbcredit value command
16-Gbps Fibre Channel Switching Module (DS-X9448-768K9), Cisco MDS 9000 Series 24/10 SAN
Extension Module (DS-X9334-K9) (Fibre Channel ports only), Cisco MDS 9700 48-Port 32-Gbps Fibre
Channel Switching Module (DS-X9648-1536K9), and MDS 9396S switches. In a fabric consisting of
supported and unsupported switches (mixed fabric), this feature may not work effectively. In a mixed
fabric, ER_RDY flow-control mode is used only between supported switches and R_RDY flow-control
mode is used between unsupported switches.
switch, the ISLs connecting the switches should be flapped to put the ISLs in ER_RDY flow-control
mode. In port channels, these links can be flapped one at a time, preventing loss of connectivity.
flow-control modes. This is to facilitate nondisruptive conversion from R_RDY to ER_RDY flow-control
mode. Do not allow this inconsistent state to persist longer that it takes to perform the conversion from
R_RDY to ER_RDY flow-control mode.
Fibre Channel over Ethernet (FCoE) are not supported in ER_RDY flow-control mode.
when the device's flows are moved from one VL to another VL.
devices. Upon upgrading to Cisco MDS NX-OS Release 8.1(x) or later, these switches become aware
of the slow devices.
in the Cisco MDS NX-OS Release 7.3(x) or earlier, upgraded to Cisco MDS NX-OS Release 8.1(1), and
set flow-control mode to ER_RDY, the buffer-to-buffer credits that are already configured get distributed
to the VLs in the following manner:
• If the buffer-to-buffer credits value that is configured is 50, the default buffer-to-buffer credit values
5, 1, 4, and 40 are allocated to VL0, VL1, VL2, and VL3 respectively.
• If the buffer-to-buffer credits value that is configured is more than 34 and less than 50, the
buffer-to-buffer credits get distributed in the ratio 5:1:4:40.
• If the buffer-to-buffer credits value that is configured is more than 50, the default values 5, 1, 4,
and 40 are allocated to VL0, VL1, VL2, and VL3 respectively. The remaining buffer-to-buffer
credits get distributed in the ratio 15:15:40:430 (VL0:VL1:VL2:VL3).
• If you are upgrading or if you are in the Cisco MDS NX-OS Release 8.1(1), if ER_RDY was enabled,
and if the buffer-to-buffer credits value that is configured is less than 34, the VLs are stuck in the
Guidelines and Limitations for Congestion Isolation
Cisco MDS 9000 Series Interfaces Configuration Guide, Release 8.x
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