Congestion Isolation - Cisco MDS 9000 Series Configuration Manual

Congestion Detection, Avoidance, and Isolation
• ER_RDY is not supported on interfaces whose speed is set to 10-Gbps.

Congestion Isolation

• Congestion Isolation is disabled by default.
• Congestion Isolation and its configurations are applicable only to the switch being configured, and not
• If you enable the ER_RDY and Congestion Isolation features on a supported switch before adding it to
• In a fabric consisting of supported and unsupported switches, Congestion Isolation functions as desired
• After a device is detected as slow, only the traffic moving in the direction of the slow device is routed
• When a slow device is detected or a device is configured as slow, the switch sends an FCNS notification
•
to the entire fabric.
a fabric that is using ER_RDY flow-control mode, the ISLs that are connected between the supported
switch and its adjacent switch are automatically in the ER_RDY flow-control mode and you need not
flap the links on the switch for the links to use the ER_RDY flow-control mode.
only between supported switches. Congestion Isolation functionality between unsupported devices is
unpredictable.
to a low-priority VL (VL2). Traffic in the reverse direction is not classified as slow, and is unaffected.
to all the other switches that are capable of supporting the Congestion Isolation feature and also to the
switches that may not have this feature enabled. If the switch is capable of supporting this feature but
does not have it enabled, then the FCNS notification is rejected and the following messages are displayed
at the originating switch:
• %FCNS-2-CONGESTION_ISOLATION_FAILURE: %$VSAN vsan-id%$ SWILS-RJT received
from domain domain-id for congestion-isolation. Issue includes CLI/FCNS DB refresh on the remote
domain.
• %FCNS-2-CONGESTION_ISOLATION_INT_ERROR: %$VSAN 237%$ Error reason:
Congestion-Isolation disabled on the remote domain. Please enable the feature on the remote domain.
If the Congestion Isolation feature is configured on all the intended switches, these messages do not have
any negative effect and can be ignored. For example, if a Cisco MDS switch is connected via FCoE ISLs
then the Congestion Isolation feature does not apply to this switch and these messages can be ignored.
However, ER_RDY and Congestion Isolation features can be configured on an FCoE connected switch
preventing the messages from being displayed.
Figure 3: Traffic Flow When Multiple Targets are Connected
connected to switch SW1 and two hosts (Host H1 and Host H2) connected to switch SW2. Both hosts
H1 and H2 are zoned with all four targets T1 to T4. Host H2 is detected as a slow device. The traffic
from the targets to host H2 is marked as slow and is routed to VL2. Since VL2 has fewer buffer-to-buffer
credits and because host H2 is itself withholding buffer-to-buffer credits from SW2, traffic on VL2 from
SW1 to SW2 will be constrained by what host H2 can receive. This results in switch SW1 withholding
buffer-to-buffer credits from all four targets T1 to T4. This will affect all traffic being sent by the targets
to any destination. Consequently, other hosts zoned with the targets, like host H1, will also see their
traffic affected. This is an expected behavior. In such a situation, resolve the slow-drain condition for
the traffic to flow normally.
Cisco MDS 9000 Series Interfaces Configuration Guide, Release 8.x
Congestion Isolation
shows a fabric that has multiple targets
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