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New Member

Frame Relay Traffic shaping - How Be really works ??

I am not sure how Be really work. I have a 2600 on which I enabled FR traffic shaping my values are CIR = 64, Bc=8 (1/8 of CIR), Tc=125 ms, Be=64k.

If I constantly send let say 85 kbps of traffic over the wan link in a lab, I sess the interface output only reaches 64 kbps. I never Be being sent. I have come to conclusion that Be is only sent if there is no traffic sent in the previous intervale, then Bc+Be can be send. I appreciate some insight from others on how Be really works ?

New Member

Re: Frame Relay Traffic shaping - How Be really works ??


the CIR value is the target value. The cisco Router don't send traffic above

that value. So this CIR value should be set ot the PIR value which is at least

by British Telecom the lowest access line speed from both ends of the PVC.

The real CIR value is named "mincir" in the cisco router. So the mincir

should be set to the real CIR value.

Set also the "frame-relay becn-response" if it is not set by default.

The Bc and Be value can adjust how fast a decreased traffic rate will increase

again after receiving BECNs. Cisco recommends to set the value BC = CIR/8

and Be = Bc * 7


map-class frame-relay shape32k256k

frame-relay adaptive-shaping becn

frame-relay cir out 256000

frame-relay bc out 32000

frame-relay be out 22400

frame-relay mincir out 32000

If you like you can also configure traffic-shaping with priority queueing.

Means, in case the router has to shape the traffic will be use the priority

queueing mechanism.

More infos can be found on CCO

Hope this helps

Ulrich Marzoli

New Member

Re: Frame Relay Traffic shaping - How Be really works ??

Thanks for your reply. However, it did not really answer the behavior that I have seen in testing with Be. I think the BT recommendation is right. In ur example. I dont fully agree with your example. If your circuit is clocked at 256 kbps. Then CIR of 256k set and bc=32000 , which means in one interval 32kbps of traffic, and 8 intervals (i.e 1 second), 256 kbps of traffic is send, which leaves no room for burst be.

New Member

Re: Frame Relay Traffic shaping - How Be really works ??

Hi, you are right about the Be value but if the target rate is the access-line speed the router tries always to reach the access-line rate. So, if you don't want this behavior you can set the values to the right values.

But consider, the values Bc and Be are also used to determine how fast the traffic-rate increases again after decreasing. If it takes too long, you can suffering from slow response even when the network is not congested.

regards Ulrich Marzoli

New Member

Re: Frame Relay Traffic shaping - How Be really works ??

You're right about the Bc and Be being used in the rate-increase. But still the example above in the URL provided is not a real-life example as it only is a point-2-point pvc. In a typical environment, you have a single T1 /T3 (frac t3)in the Hub and spokes are anywhere from 64k up to T1. In the example provides if the HUB was 1536 kbps and spoke was 64 k and if the cir was 32 k bi-directional. If the Cisco router is configured for 64k CIR, and if the Be is fairly large lets say 32 or 64 kbps, it could easily overflow the queues on the spoke frame relay switch going to the FR spoke router. My questions are 2-fold. If we are aggresive and set the Cisco CIR to the line-rate of the spoke, the should we really have Be, because Be is potentially being queued in the FR n/w because the spoke can only receive traffic at the line rate. When does Be really get transmitted. There is talk that it is only sent in the first interval (as per example) and also that depends on whether there are credits available (i.e that router did not send traffic up to CIR in the last interval etc).

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