frame-relay traffic shaping

Unanswered Question
Feb 14th, 2009

hi guys i have the following question whihc i need to know how we can calculate Be and Bc with GTS

normally Bc = CIR *Tc/1000

while Be= (AIR-CIR)*Tc/1000

now i wonderring about tow things firs when i have the question say the burst allowed up to for example 30 second

useing up to line speed the line is T1

do i consider the T1 speed as 1544 or 1536? |( i have seen in some config guids used as 1536!!)

if i have a line T1 CIR 512 and allwoed to busrt up line speed for 30 second how i can calculate the Be and Bc

Thank you

I have this problem too.
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Giuseppe Larosa Sun, 02/15/2009 - 00:07

Hello Marwan,

T1 has 8kbps reserved for signalling so the traffic rate available to user traffic is 1536 kbps.

Think of Bc and Be as buffers.

You need to accomodate 30 seconds of traffic at 1536 kbps.

So Bc expressed in bits can be:

Bc = (15360000 -512000) bps *30 seconds

So Bc needs to be set to 30720000 bits to accomodate 30 seconds of traffic in excess of the CIR.

Check with command reference if Bc has to be expressed in bytes you need to divide by 8 (bits/byte)

Hope to help


Giuseppe Larosa Sun, 02/15/2009 - 02:57

Hello Marwan,

the question of being able to burst for 30 seconds can be a very indirect way to ask for a Tc that is different from the default value.

Because at every Tc only Bc bits token are used and Be is refilled using unused tokens of previous Tc intervals the request of an initial burst capacity at Access Rate for 30 seconds requires to have tokens in Be for all this excess traffic.

Hope to help


Marwan ALshawi Sun, 02/15/2009 - 13:44

Hi Giuseppe

thanks for ur answer

first the calculation above i think was for Be no Bc

which is AIR-CIR)*Tc/1000 !!

Also the question mentioned that the traffic use default interval which is 125ms !!!

Giuseppe Larosa Sun, 02/15/2009 - 14:16

Hello Marwan,

yes you're right I was thinking of Be not Bc.

The formula was for that.

I see that Joseph has provided a confirmation of the method.

Every Tc seconds the shaper gains tokens for the the equivalent of cir * Tc.

So defining Bc and Be using the 30 seconds time reference leads to these numbers.

Tc can be seen as a sampling rate: the lowest is Tc the less the instantaneous speed is free to peak over the average: the service provider exercises a stricter control.

If Tc is great the averaging effect is greater and even if the CIR is only 0.10*AR then instaneous rate can have high peaks up to AR.

Hope to help


Marwan ALshawi Sun, 02/15/2009 - 14:48

Thank you Guays

i have one more Issue as long as we are in Frame-relay but this time is Multicasting

i have simple hub and spoke tpology

the hub configured with to map statments of frame-relay point to both spoks with broadcast word

spoks has one with broadcast word for the hub other one only map the other spok and point it to the local DLCI

simple and everyone ping each other

now i run ospf over this network

and we know by defualt it is non-broadcast then i add the neighbor command on the hub point to each spoke

now evrthing works fine as well


the hub is the RP and discovery comand entered as well

one router behind the sopks joined an igmp group

each router run pim sapre-mode and the hub in addition to that has pim nbma-mode

the problem is

everything OK every on ping the joined igmp group

ecept from the other spoke

when i ping forst packet get replay and anything after first packet NO

in the hum and the RP as well what i noticed

first ping there is outgoing interface for that spoke for the subsequent the outgoing interface shows as NULL !!!!

however when i run ospf as poin-to-multipoint works fine

with EIGRP the same issue (split-horizon disabled)

any Idea Guys !!!

Thanks again

Joseph W. Doherty Sun, 02/15/2009 - 05:39

"normally Bc = CIR *Tc/1000 "

Correct, still holds true although you don't need the 1K divisior, which is for milliseconds.

So, 30 second (Tc) * 512000 bps (CIR) = 15360000 bits (Bc)

The thing to keep in mind, a shaper is independent of the link's bandwidth. Long term average transmission rate should correspond to CIR. How closely a shaper mimics a physical interface bandwidth with same bandwidth as CIR depends on Tc. A smaller Tc would more closely mimic a physical interface of the same bandwidth, i.e. less bursty.

On the question of 1544 vs. 1536, this WIKI article might further clarify:


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