Enterasys Networks X-PeditionTM manual Controlling Congestion in Frame Relay Networks

Controlling Congestion in Frame Relay Networks

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.

Note: BECN will not operate unless traffic shaping is enabled.

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.

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.

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.

9-6 Configuring Frame Relay