Configuring Slow Drain Device Detection And Congestion Avoidance; About Slow Drain Device Detection And Congestion Avoidance; Configuring Stuck Frame Timeout Value - Cisco MDS 9000 Series Configuration Manual

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Configuring Slow Drain Device Detection and Congestion Avoidance

S e n d d o c u m e n t a t i o n c o m m e n t s t o m d s f e e d b a c k - d o c @ c i s c o . c o m
Configuring Slow Drain Device Detection and Congestion
Avoidance
This section includes the following topics:

About Slow Drain Device Detection and Congestion Avoidance

All data traffic between end devices in a SAN fabric is carried by Fibre Channel Class 3, and in some
cases, Class 2 services that use link-level, per-hop-based, and buffer-to-buffer flow control. These
classes of service do not support end-to-end flow control. When there are slow devices attached to the
fabric, the end devices do not accept the frames at the configured or negotiated rate. The slow devices
lead to ISL credit shortage in the traffic destined for these devices and they congest the links. The credit
shortage affects the unrelated flows in the fabric that use the same ISL link even though destination
devices do not experience slow drain.
This feature provides various enhancements to detect slow drain devices that are causing congestion in
the network and also provides a congestion avoidance function.
This feature is focused mainly on the edge ports that are connected to slow drain devices. The goal is to
avoid or minimize the frames stuck condition in the edge ports due to slow drain devices that are causing
ISL blockage. To avoid or minimize the stuck condition, configure lesser frame timeout for the ports.
No-credit timeout drops all packets once the slow drain is detected using the configured thresholds. The
lesser frame timeout value helps to alleviate the slow drain condition that affects the fabric by dropping
the packets on the edge ports sooner than the time they actually get timed out (500 ms). This function
frees the buffer space in ISL, which can be used by other unrelated flows that do not experience slow
drain condition.
This feature is used mainly for edge ports that are connected to slow edge devices. Even though this
Note
feature can be applied to ISLs as well, we recommend that you apply this feature only for edge F ports
and retain the default configuration for ISLs as E and TE ports. This feature is not supported on
Generation 1 modules.

Configuring Stuck Frame Timeout Value

The default stuck frame timeout value is 500 ms. We recommend that you retain the default configuration
for ISLs and configure a value not exceeding 500 ms (100 to 200 ms) for fabric F ports.
Cisco MDS 9000 Family NX-OS Interfaces Configuration Guide
2-44
About Slow Drain Device Detection and Congestion Avoidance, page 2-44
Configuring Stuck Frame Timeout Value, page 2-44
Configuring No-Credit Timeout Value, page 2-45
Configuring Credit Loss Recovery Threshold and Action, page 2-45
Configuring Average Credit Non-Available Duration Threshold and Action, page 2-47
Chapter 2
Configuring Interfaces
OL-19445-02, Cisco MDS NX-OS Release 4.2(x)

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