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for all VCs to relieve congestion. Enabling the
throttle-transmit-on-fecn
parameter,
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 notification 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-fecn
is 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 notification to the source of the congestion, enable the
notify-fecn-source
parameter 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.
Note: If multiple DLCIs are configured 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 first DLCI is
corrected.
Similarly, if the network provider supports CLLM, you can configure 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 configured for each PVC contained in the CLLM message. When the device
receives a CLLM message that indicates:
v A short-term condition, and the configured CIR for the PVC is nonzero, the
Frame Relay protocol will throttle the transmit rate for the affected PVCs by the
configured IR decrement percentage.
Router
Router
A
XY
B
C
FECN = 1
BECN = 0
FECN = 0
BECN = 1
Congestion
Sender Receiver
time
sender
Maximum IR
CIR
Minimum IR
BECN - 1 BECN - 0
Figure 25. Congestion Notification and Throttle Down
Using Frame Relay
Chapter 30. Using Frame Relay Interfaces 401