User`s guide

126 XSR User’s Guide
Controlling Congestion in Frame Relay Networks Chapter 7
Configuring Frame Relay
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 should
transmit up to 16000 bits every 125 milliseconds.
NOTE
When adaptive shaping is enabled and BECNs are received, the XSR
becomes congested and lowers the output rate on the DLCI. Other DLCIs’
throughput is not affected.
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 3 seconds, 1/2 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 7/8ths of the current CIR. Every three seconds that BECNs are received,
the CIR will be reduced by an additional 7/8ths of the new CIR value, until
the new CIR value is 1/2 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.