Cisco MDS 9000 Series Configuration Manual page 177

Congestion Detection, Avoidance, and Isolation
Figure 2: Traffic Flow Using Virtual Links
Table 19: Virtual Link-to-QoS Priority Mapping, on page 159
Use this information while setting a zone QoS priority in a zone where Congestion Isolation is enabled in
order to avoid QoS priority flow from being treated as slow flow.
Table 19: Virtual Link-to-QoS Priority Mapping
Virtual Link
VL0 (control traffic)
VL1 (not used for any
traffic)
VL2 (slow traffic)
VL3 (normal traffic)
Congestion Isolation
The Congestion Isolation feature uses VL capabilities to isolate the flows to the slow devices on an ISL to a
low-priority VL that has less buffer-to-buffer credits than the buffer-to-buffer credits used for the normal
traffic VL. Traffic in the direction of the slow device is routed to a low-priority VL. Normal devices continue
to use the normal VL that has more buffer-to-buffer credits. Slow devices can be marked as slow either via
the port monitor or manually, using the congestion-isolation include pwwn pwwn vsan vsan-id command.
Note When a device is manually marked as slow using the congestion-isolation include pwwn pwwn vsan vsan-id
command or automatically detected as slow via the port monitor, the Fibre Channel Name Server (FCNS)
database registers the slow-device attribute (slow-dev) for the device and distributes the information to the
entire fabric.
You must ensure that the following requirements are met before enabling the Congestion Isolation feature:
• Flows must traverse ISLs because Congestion Isolation functions only across Fibre Channel ISLs.
QoS
Priority
7
5, 6
2, 3, 4
0, 1
Cisco MDS 9000 Series Interfaces Configuration Guide, Release 8.x
provides VL-to-QoS priority mapping information.
Congestion Isolation
159