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tim
New Member

tc bc be Frame-Relay traffic shapping for VOICE

Need to be clarified something with with frame-relay bc command.

If I want to be very strict with my traffic shapping which set of commands should I use and why, I have work out my oun calculations but need some advise.

Example A

map-class frame-relay VOICE

frame-relay cir 64000

frame-relay bc 300

frame-relay be 0

frame-relay mincir 64000

frame-relay voice bandwidth 64000

frame-relay fragment 80

no frame-relay adaptive-shaping

frame-relay fair-queue

EXAMPLE B

map-class frame-relay VOICE

frame-relay cir 64000

frame-relay bc 6400

frame-relay be 0

frame-relay mincir 64000

frame-relay voice bandwidth 64000

frame-relay fragment 80

no frame-relay adaptive-shaping

frame-relay fair-queue

frame relay bc tc and be have me confused

The amount of data to send per each Tc interval in bits. Ideally for data PVCs Bc = CIR/8 so that Tc = 125msec. If we are doing voice on the PVC, then Bc = CIR/100 is preferable, so that the interval Tc = 10msec (as voice packets cannot tolerate a longer delay). The value of Bc by default is the CIR in bits.

2 REPLIES
Cisco Employee

Re: tc bc be Frame-Relay traffic shapping for VOICE

The formula for the relation of bc to tc is this:

Tc = Bc/Cir ! time interval (the internalized version of Tc where the

time interval can't exceed 125ms)

CIR = Average rate you want to send out (This is generally not the

same as the CIR you get from your provider unless you aren't

allowed to send above CIR) This is measured in bits/second.

Bc = Amount of data to send per each Tc interval. This is

measured in bits.

(this also gets internalized and real amount of data sent per

interval is expressed in bytes by the "increment" variable).

Be = Amount of excess data allowed to be sent during first

interval once credit is built up. Also measured in bits.

Mincir = Minimum amount of data to be sent during periods of congestion.

This defaults to half of CIR.

Interval = Bc/CIR with the maximum size being 125ms.

byte increment = Bc/8 . Must be > 125. Upper side has no bound

or limitation except if interval is locked at 125ms.

limit = byte increment + Be/8 (measured in bytes)

so for Tc to be 10ms, Bc must be 1/10th of CIR.

also see here:

http://www.cisco.com/warp/public/788/voip/fr_traffic.html

tim
New Member

Re: tc bc be Frame-Relay traffic shapping for VOICE

The URL is saying BC=1000 is enough so way not the small amout allowed by the IOS which is BC=300?

Make Bc small so that Tc (shaping interval) is small (Tc = Bc/CIR)

The minimum Tc value is 10 ms, which is ideal for voice. With a small Tc value, there is no risk of large packets using all the shaping credits. Large Tc values can lead to large gaps between packets sent because the traffic shaper waits an entire Tc period to build up additional credits to send the next frame. Making Bc = 1000 bits is usually a low enough value to force the router to use the minimum Tc of 10ms. This setting should not affect data throughput.

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