policer 1 mbps = 1,000,000 bps, 1,024,000 bps or 1,048,576 bps?

In the C3550 QoS reference

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refer to "1 mbps" using "police 1000000":

!--- Polices down to 1 Mbps and marks down according to the QoS map. > police 1000000 8000 exceed-action policed-dscp-transmit

If the "police 1000 kbps" syntax is used, would that be the same rate as above, or would "police 1000 kbps" mean 1024000 bps? And similarly, is "police 1 mbps" going to police at 1000000, 1024000 or 1048576 bits/sec?

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In networking, when talking about line rates, the metric prefixes apply. 1 kbps is 1000 bits/second, 1 mbps is 1000000 bits/second.

Reply to
Walter Roberson

more importantly - there are some uncertainties about policing that will swamp any difference.

i hit some nasties last time i had to decide exactly what to do with some policing requirements (on a Cat6k).

To be fair the issues were more about deciding what you actually mean by vague terms like "police to 30 Mbps".

biggest one is exactly what are you measuring?

eg different routers / switches etc will use the L1, L2 or L3 packet size to "count" how much bandwidth is used - and it never seems to be documented........

We had size constraints defined for L3 traffic, but the Sup 720 only seems to police @ L2. So - what size frames do we use to calculate policer settings?

this can make a big difference with smaller packets - L2 size on Ethernet is around 20% more than L3 for a minimum sized Ethernet frame......

the other thing likely to hit you is that the policing you get will depend on granularity - the policer might try to measure bytes or bandwidth, but it can only throw away complete frames.

so the device needs to keep track of fractions of packets, or you get cumulative errors.

finally - the devices, apps etc will react to packet loss, so exactly what happens is dependent on how they behave under loss.

and then the next issue - congestion control - which has to happen at layer

1, so you deal with frame preambles, inter frame gaps. Yet another set of fuzzy additions to the answers.

dont get me wrong - it works well.

it is just so traffic / frame size dependent that worrying about fractions of 1% of load is pretty much irrelevant.......

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Not exactly true. LAN speeds tend to be powers of 10 or integer multiples. WAN speeds are usually based on multiples of 64000 bps. That means that what's commonly called a 2Mbps line (E1) is actually

2048000 bps, 30x64000 bps channels plus management overhead, and 1.5Mbps (T1) is actually 1.544Mbps, 24x64kbps plus some stuff. Higher rates pack multiple E1s or T1s into further bundles.

That "multiples + overhead" stuff is why the nominal 10Gbps WAN standard transmission rate isn't quite the same as the (exactly) 10Gbps ethernet rate and also why you sometimes see the same service quoted at different rates - it's the difference between the raw bit rate and the user data that can be packed in between the overhead bits.

There's lots of plausible looking stuff in Wikipedia about this - try and then the "List of device bandwidths" link at the bottom of the page and head for the Wide area network and Local area network sections.


Reply to
Sam Wilson

Thanks, that looks useful.

My view of this as a customer is - who cares?

Well I don't really care - I would rather just knock the policer rate down a bit to be sure that I was on the right side.

On the other hand a supplier of services would be expected to provide accurate information and there is the famous case of the NM-4E1 (at a guess but possibly correct) that provided

4 E1 interfaces with a total bandwidth of 8,000,000 bps.

Unfortunately an E1 is a bit more than 2,000,000 bps maybe 1000 x 1024 or in other words n x 64000.

This of course meant that 4 x E1 did not work.

It was swiftly followed by the NM-4E1+ :-))

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