Burst period limits

[hotpackets]

Does Cisco implement certain limits to the burst size in relation to the CIR. I'm looking at a config for a customer and they basically have a burst-size value that's based on a burst period longer than an hour.

For example, having a policer with a 10K CIR on a 100Mbps interface and the Bc is 5MBytes. Will the router actually allow sending 5MBytes of data at once across that interface? I could saturate the 100Mbps interface for 400 ms with that much data. Or are there limits built in to prevent this regardless of what's configured?

[Joseph W. Doherty]

"Does Cisco implement certain limits to the burst size in relation to the CIR."

By default they do, i.e. if you only set the CIR value the IOS will default a Bc value based on CIR (which is really a default Tc value, which default varies based on the age of the IOS [I think IOS used to default to 25ms but later default might be 10ms.]).

"For example, having a policer with a 10K CIR on a 100Mbps interface and the Bc is 5MBytes. Will the router actually allow sending 5MBytes of data at once across that interface?"

It should.

"I could saturate the 100Mbps interface for 400 ms with that much data. Or are there limits built in to prevent this regardless of what's configured?"

Not that I'm aware of beyond whatever the maximum Bc value you're allowed to specify. (BTW, if you don't want to saturate a 100 Mbps interface for 400 ms, set a correctly sized Bc.

[hotpackets]

Thanks! I'm overseeing the network for a customer and I point out issues with a bunch of their QOS configuration. Their engineers are saying it works fine. I've worked with other vendor technology where they limit the burst period to 600ms regardless of configuration. I just wanted to see if Cisco does something similar before pushing back that they could potentially be screwing themselves over.

[Joseph W. Doherty]

Well, I will say, if a Cisco supports an hour Bc, that really can be configured, unclear why they police at all.

IMO, often using a policer is like 19th century surgery, i.e. let's amputate a limb (to save your life). (I often find shaping a better solution, but then not all Cisco devices support shaping [e.g. most switches].)

[hotpackets]

They needed to do it on ingress so a policer was our only option.