IBM SC30-3681-08 manual Congestion Notication and Throttle Down

Using Frame Relay

for all VCs to relieve congestion. Enabling the throttle-transmit-on-fecnparameter, along with either the CIR or congestion monitoring options, causes the device to treat a FECN like a BECN thus reducing overall FR network congestion when any congestion noti®cation is received. Use the throttle-transmit-on-fecn parameter only in FR networks whose queuing methods do not provide dedicated buffers for both input and output. If the throttle-transmit-on-fecnis enabled, FR will decrease a VC's VIR at most once every second for each BECN or FECN received.

Some FR network switches set FECN to indicate congestion but do not set BECN. To provide congestion noti®cation to the source of the congestion, enable the notify-fecn-sourceparameter allowing the device to set BECN in frames that it transmits over a VC on which it has received a FECN. This action provides a signal to the device that is causing the network congestion to throttle down its VC's VIR.

Congestion

B

	FECN = 0
	BECN = 1
		Router
			A	C
Sender		X

sender

BECN - 1

FECN = 1

BECN = 0

Router

YReceiver

BECN - 0

Maximum IR

CIR

Minimum IR

time

Figure 25. Congestion Noti®cation and Throttle Down

Note: If multiple DLCIs are con®gured between two end-stations when congestion occurs, it is possible that a second DLCI may be used to transmit data at a higher throughput until the congestion condition on the ®rst DLCI is corrected.

Similarly, if the network provider supports CLLM, you can con®gure Frame Relay to throttle down its transmit rate for PVCs contained in a CLLM message. CLLM messages contain a cause code that indicates the type and severity of the problem being reported. The device reacts differently depending on the cause code and the CIR con®gured for each PVC contained in the CLLM message. When the device receives a CLLM message that indicates:

vA short-term condition, and the con®gured CIR for the PVC is nonzero, the Frame Relay protocol will throttle the transmit rate for the affected PVCs by the con®gured IR decrement percentage.

Chapter 30. Using Frame Relay Interfaces 401