# CIR triangle forumla for QoS calculations

Supposedly an easy way to remember equations in physics and maths, is only really useful at low levels.

It works like this – if I want to remember how to use the formula for density, Density = Mass / Volume , I’d put it into a triangle like so –

/
/
/ M
/——
/ D * V
/———-

Now, if I want the formula for Density, I just cover up the D with my finger, and all that’s left is the M/V bit. That’s my equation. The same works for working out all the letters.

The idea is that you just remember the order of letters in the triangle rather than the equation. Not that much use, though.

Tc = Bc/CIR (in seconds) is the formula.

The router internally calculates the value of Tc based on the configured CIR and Bc values.

If Bc/CIR is more than or equal to 125 ms, it uses an internal Tc value if the router determines that traffic flow will be more stable with a smaller interval.
You can use the show traffic-shape command to determine whether your router is using an internal value for Tc or the value that you configured.

In your example with CIR of 64K and Bc of 8000K, (8,000/64,000) = 0.125 or 125ms. If we wanted a Tc of 100ms, we would simply configure the Bc to be 6400. Then using the formula Tc=Bc/CIR it would be 6,400/64,000 = .1 or 100ms.

Best wishes,

Keith