How is input rate calculated?

[niro]

I have two questions about how input rate is calculated:

On a switch when you do a show int gi x/x, how is the 30 second input/output rate calculated?  How often is the switch actually sampling the the input/output rates to come up with the 30 second average (with load-interval 30)?

Second question is, for the SNMP 5 minute average rate counter, how many samples are being used to calculate that value?  Would getting the current rate SNMP value every 30 seconds be more accurate over 5 minutes (10 samples) then just getting the 5 minute SNMP average count every 5 minutes?

Thanks!

[Souvik Ghosh · Accepted solution]

Hi Niro,

The I/O rate is counter which counts the number of packet averaged out over a period of one load interval. For example if the interface has received x no of bits in a load inteval of y ( in secs) then the input rate would be x/y bits per sec. As simple as this. However these counters are almost never accurate and should be not be relied upon for accounting purpose, as we do not want to interrupt the cpu to count the packet when the priority is to forward them as quickly as possible. If you need to count the actual data rate on a link then an inline sniffer should be your option.

i am sorry i do not know the answer for your 2nd question.

Regards,

Souvik

[niro]

Thanks that makes sense. 

You said it counts the number of packets...so does it assume some packet size to get the rate or is it sampling packets to get an average size?  Or does it also keep track of total bits received and just averages that out?

[Souvik Ghosh]

it keeps a track of number of bits received and averages it out over a period of load-interval.

Regards,
Souvik