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[ScreenOS] Behavior change in handling Flow Control PAUSE Frame

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Article ID: KB28081 KB Last Updated: 02 Oct 2013Version: 1.0
Summary:

This article documents a problem seen on the ScreenOS devices when it receives heavy traffic, which triggers to run flow control.

Symptoms:

If the traffic is dropping and OutPause is increasing, it is an indication that flow control is triggered.  OutPause counter can be checked using the command get gbe counter <slot number>.

1. Before PAUSE frame is sent out from SSG interface:

ssg550-> get gbe counter 5

Switch Port8 HW counters:
InUnicast              0  InBroadcast            0  InMulticast            0
InPause                0  InUndersize            0  InFragments            0
InOversize             0  InJabber               0  InFCSErr               0
OutUnicast             0  OutBroadcast           0  OutMulticast           0
OutPause               0  OutExcessive           0  OutCollisions          0
OutDeferred            0  OutSingle              0  OutMultiple            0
OutFCSErr              0  OutLate                0


2. After PAUSE frame is sent out from SSG interface:


ssg550-> get gbe counter 5

Switch Port8 HW counters:
InUnicast       43327747  InBroadcast       726791  InMulticast           14
InPause                0  InUndersize            0  InFragments            0
InOversize             0  InJabber               0  InFCSErr             365
OutUnicast      47386053  OutBroadcast      262350  OutMulticast      423496
OutPause             788  OutExcessive           0  OutCollisions          0
OutDeferred            0  OutSingle              0  OutMultiple            0
OutFCSErr              0  OutLate                0

When flow control is triggered, the ScreenOS device sends out PAUSE frame, which halts the transmission of the sender for a specified interval of time.  Theoretically flow control should be recover after traffic slow down.


	
	
Cause:

Solution:

The behavior of sending out PAUSE frame by flow control has been changed in PR607132.  This is only applicable to SSG platforms. In ScreenOS 6.3r10 and later, flow control is disabled so that the ScreenOS device will not send out a PAUSE frame in a congested network environment.  This flow control configuration is a hard-coded configuration, and cannot be changed by users.

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