CIR in QoS configuration

[D@1984]

Hi,

Im studying QoS and trying to get my head around some of the terminology like CIR, Bc and Tc.

my question is how can we figure out the CIR when configuring the policing? I understand we can set the exceed action to transmit and tweak the policy later but how do we know the Bc and Tc to do some calculations?

thanks 

[David Ruess]

Hello,

I know the CIR is something you are guaranteed by your ISP so to calculate that would have to be discussed between you and them If its a lab its something you can configure different values with.

This article has a good explanation of Tc/Bc. If you want to read the whole article you'd need a subscription (its worth it for studies IMO), but what you can read in the preview might be enough.

https://networklessons.com/quality-of-service/qos-traffic-shaping-explained#:~:text=To%20sum%20things%20up%2C%20this,and%20is%20measured%20in%20bits.

-David

[Joseph W. Doherty]

@David Ruess's reference is still generally good, although I recall (?) Cisco has later changed the default Tc to less than 125 ms.

Anyway, think of the CIR as bandwidth rate we're trying to emulate.  For example, if we wanted a FE (100 Mbps) port to emulate an original Ethernet (10 Mbps) port, we would set the CIR, on the FE port, to 10 Mbps.

As David's reference explains, we cannot (often) physically slow a port's physical transmission rate (exceptions include multi speed Ethernet ports), so if we cannot, what do we do? That's where Tc and Bc (and Be) come in.

During a time interval, of our choice/need, we "count" volume of bits and limit actual transmission such that the CIR bit volume is not exceeded for the Tc.

BTW, as long as CIR bit volume is not exceeded within a Tc actual transmission can start/stop within the Tc.

So, CIR is simply the transmission rate you're trying to emulate.  BTW, it usually is always less than port's physically bandwidth.

Selecting the Bc(/Be) and Tc is rather technical but Cisco defaults are often acceptable; how they impact shaping vs. policing differ too.