Information About Congestion Isolation - Cisco MDS 9000 Series Configuration Manual

Information About Congestion Isolation

• Port monitor with portguard actions of flap and error disable: For more information, see the
Note The no-credit timeout functionality is only used for edge ports because these ports are directly connected to
the slow-drain devices. The no-credit timeout functionality is not supported on Generation 1 modules.
Information About Congestion Isolation
The Congestion Isolation feature can detect a slow-drain device via port monitor or manual configuration and
isolate the slow-drain device from other normally performing devices on an ISL. After the traffic to the
slow-drain device is isolated, the traffic to the rest of the normally behaving devices will be unaffected. Traffic
isolation is accomplished using the following three features:
• Extended Receiver Ready—This feature allows each ISL between supporting switches to be split into
• Congestion Isolation—This feature allows devices to be categorized as slow by either configuration
• Port monitor portguard action for Congestion Isolation—Port monitor has a new portguard option to
Extended Receiver Ready
Note Extended Receiver Ready (ER_RDY) feature functions only on Fibre Channel Inter-Switch Links (ISL) and
only between switches that support this feature.
ER_RDY primitives are used as an alternative to Receiver Ready (R_RDY). ER_RDY primitives virtualize
a physical link into multiple virtual links (VLs) that are assigned individual buffer-to-buffer credits, thereby
controlling the flow to the physical link. The ER_RDY feature is used by Congestion Isolation to route slow
flows to a specific VL, called a low-priority VL (VL2), so that all the normal flows are unaffected. ER_RDY
supports up to four VLs.
Figure 2: Traffic Flow Using Virtual Links, on page 159
VL0 (red link) is used for control traffic, VL1 (orange link) is used for high-priority traffic, VL2 (blue link)
is used for slow traffic, and VL3 (green link) is used for normal-data traffic. Slow flow detected at Host H2
is automatically assigned to VL2, which prevents the congestion of the link and allows the good flow from
Host H1 to use either the VL1 or VL3 depending on the flow priority.
Cisco MDS 9000 Series Interfaces Configuration Guide, Release 8.x
158
can have a dramatic effect on relieving congestion especially on the upstream ISLs. This allows unrelated
flows to move continuously. This is on by default with a value of 500 ms. If configured, it should be set
to a value that is lower than the configured (or defaulted) FCoE congestion-drop timeout. It is configured
via the system timeout fcoe pause-drop commands (available from Cisco MDS NX-OS Release 8.2(1)
onwards). The FCoE pause-drop timeout functionality is only used for edge ports, because these ports
are directly connected to the slow-drain devices.
on page 29 section.
four separate virtual links, with each virtual link assigned its own buffer-to-buffer credits. Virtual link
0 used to carry control traffic, virtual link 1 is used to carry high-priority traffic, virtual link 2 is used to
carry slow devices, and virtual link 3 is used to carry normal traffic.
command or by port monitor.
allow the categorization of a device as slow so that it can have all traffic flowing to the device routed to
the slow virtual link.
Congestion Detection, Avoidance, and Isolation
shows VLs managing the good flow and slow flow.
Port Monitor,