User`s guide

XSR User’s Guide 125
Chapter 7 Controlling Congestion in Frame Relay Networks
Configuring Frame Relay
Backward Explicit Congestion Notification (BECN)
Backward Explicit Congestion Notification (BECN) sets a bit in frames
traveling the opposite direction of frames encountering a congested path. A
DTE device receiving frames with the BECN bit set can request that higher-
level protocols take flow control action as appropriate. Frames received with
the BECN bit set indicates that the transmit path is congested.
Figure 15 Congestion Notification
Using BECN bits to control the outbound flow of data is known as adaptive
shaping. This feature is disabled by default on the XSR. To activate the
feature, you must first enable traffic shaping on the interface. Second, you
must associate a map class with this interface, sub-interface or DLCI which
has the adaptive shaping parameter enabled.
Be aware that unless traffic shaping is enabled, BECN will not operate.
The following sample configuration shows how to activate BECN support:
XSR(config)#map-class frame-relay STG
XSR(config-map-class<STG>)#frame-relay cir out 64000
XSR(config-map-class<STG>)#frame-relay bc out 8000
XSR(config-map-class<STG>)#frame-relay be out 8000
XSR(config-map-class<STG>)#frame-relay adaptive-shaping
XSR(config)#interface serial 1/0
FECN
BECN
Switch A Switch C
Switch B
Source DTE Destination DTE
Congestion
Reduce Sent Traffic
Ignored