Tc in Frame-Relay Traffic Shaping

[amlendu_kumar]

In one Cisco document maximum value of Tc possible is 125 Msc "Time interval over which Bc or (Bc+ Be)

bits are transmitted. Tc is calculated as Tc =

Bc / CIR. The Tc value is not directly

configured on Cisco routers. It is calculated

after the Bc and CIR values are configured.

Tc cannot exceed 125 ms."

But another example has Tc of 250 Msc

"map-class frame-relay cisco

frame-relay cir 64000

frame-relay mincir 32000

frame-relay adaptive-shaping becn

frame-relay bc 8000

"

CAn any one identify the correct Tc value. Moreover with an MTU of 1500 bytes what is the min Tc value

[stephenshaw]

Hi Kumar,

use the Tc=125 ms value. Cisco divide a 1 second time period into 8 time periods, which is where the 125 ms value comes from.

Steve

[amlendu_kumar]

Thats correct but the second example also qouted from Cisco has a Tc of 250 Msc which doent seem quite right when the documentation says 125 Msc to be maximum tc once FRTS is turned on.

[stephenshaw]

Examples aren't always accurate - that's why I didn't answer it. Let Cisco answer that one.

Steve

[amlendu_kumar]

I wonder if any Cisco techie can answer this ?

Thanks in advance

[dgahm · Accepted solution]

I have a backup frame circuit so I tried it to see what the max Tc can be.

Here is the original map class:

map-class frame-relay 128CIR1

frame-relay adaptive-shaping becn

frame-relay cir 256000

frame-relay bc 32000

frame-relay mincir 128000

A 'show traffic shape' confirms an interval of 125ms.

I changed frame-relay bc to 8000 and the interval went to 31ms. I then changed it to 64000 and the interval was 125ms.

So a 7206 router running 12.2(15)T9 confirms that the max is 125ms.

[gscholz]

Tc of 125ms is optimum for data when you start looking at delay sensitive applications you obviously require something a lot less and also require fragmentation for large MTU packets