Enterasys Networks Security Router X-PeditionTM - Manual (Page 214)

Controlling Congestion in Frame Relay Networks
9-6 Configuring Frame Relay
Using BECN bits to control the outbound dataflow is known as adaptive shaping. It is disabled by
default on the XSR. To activate it, you must first enable traffic shaping on the port then associate a
map class with this interface, sub-interface or DLCI which has the adaptive shaping value set.
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
XSR(config-if<S1/0>)#no shutdown
XSR(config-if<S1/0>)#media-type V35
XSR(config-if<S1/0>)#encapsulation frame-relay
XSR(config-if<S1/0>)#frame-relay lmi-type ansi
XSR(config-if<S1/0>)#frame-relay traffic-shaping
XSR(config)#interface serial 1/0.1 multi-point
XSR(config-subif<S1/0.1>)#frame-relay interface-dlci 16
XSR(config-fr-dlci<S1/0.1-16>)#class STG
XSR(config-fr-dlci<S1/0.1-16>)#no shutdown
XSR(config-fr-dlci<S1/0.1-16>#ip address 210.16.0.1 255.255.0.0
Under normal circumstances, a DLCI is authorized to transmit a number of bits per an interval of
time. The number of bits is composed of adding Bc and Be values (8000, 8000 = 16000 bits). The
interval allowed to transmit this quantity of bits is based on the formula: Bc/CIR (8000/64000 = 125
milliseconds). So, under normal non-congested conditions, this DLCI can transmit up to 16000 bits
every 125 milliseconds.
Upon receiving the first BECN, the Be amount is removed from the equation. Now the DLCI can
transmit 8000 bits every 125 milliseconds. If no more BECNs are received within three seconds,
one-half of the Be amount is added back each 3-second interval until the Be is fully restored.
Upon receiving additional BECNs within three seconds, the CIR is reduced by 1/8th of the current
CIR. Every three seconds that BECNs are received, the CIR will be reduced by an additional 1/8th
of the new CIR value, until the new CIR value is one-half of the original CIR value. One-half of the
original CIR is called the minimum CIR, a non-configurable parameter.
Once BECNs stop being received, the current CIR begins to recover the original CIR at a rate of 1/
16th of the original CIR every 3 seconds: 1/16th facilitates a graceful, slower recovery in the hope
of preventing network thrashing when all devices start recovering from congestion. After CIR is
fully recovered, Be is reintroduced at a rate of 1/2 of Be every 3 seconds.
Note: BECN will not operate unless traffic shaping is enabled.
Note: If adaptive shaping is on and BECNs are received, the XSR becomes congested and cuts the
DLCI output rate. Other DLCIs’ throughput is unaffected.