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Mls qos protocol arp: try to understand CIR and Bc values

Hi,

in order to mitigate layer 2 loops on my C6500, I'd like to implement ARP rate limiter.

Doing a span of arp traffic on the Sup720 rp & sp, I notice an average arp traffic of 10kb/s. I don't notice traffic peak, event with a 100ms step.

So I configured my rate limiter as following:

mls qos protocol arp police 64000 1000.

64000bps is far from the 10000bps I observed with span

1000 byte of Bc means, if I am correct, means burst of 1000bytes on a 125ms interval.

Looking at span result, my burst value shloud be enough to avoid policed packets. But is is not the case:

#sh mls qos protoc

Modes: P - police, M - marking, * - passthrough

Module: All - all EARL slots;    Dir: I&O - In & Out;   F - Fail

Proto Mode Sid Mod Dir AgId Prec         Cir    Burst   AgForward-By   AgPoliced-By

------------------------------------------------------------------------------------

     ARP    P   1   1  In    4    -       64000     1000          42908           3008

     ARP    P   1   2  In    4    -       64000     1000          19652              0

     ARP    P   1   5  In    4    -       64000     1000          16764              0

     ARP    P   1   6  In    4    -       64000     1000          61872          20928

     ARP    P   1   8  In    4    -       64000     1000              0              0

     ARP    P   2   1  In    4    -       64000     1000          20996            836

     ARP    P   2   4  In    4    -       64000     1000           3488              0

     ARP    P   2   5  In    4    -       64000     1000          12834            256

     ARP    P   2   6  In    4    -       64000     1000          42738          10304

     ARP    P   2   7  In    4    -       64000     1000          17430            640

     ARP    P   2   8  In    4    -       64000     1000              0              0

     ARP    P   2   9  In    4    -       64000     1000           5632              0

So I change Bc value for 4000 (Tc=500ms). There is still policed packets, but result is better (in the following outpur, all policed packet appeard in one time, so I guess during a burst):

Proto Mode Sid Mod Dir AgId Prec         Cir    Burst   AgForward-By   AgPoliced-By

------------------------------------------------------------------------------------

     ARP    P   1   1  In    4    -       64000     4000          53100           1088

     ARP    P   1   2  In    4    -       64000     4000          19912              0

     ARP    P   1   5  In    4    -       64000     4000          20446              0

     ARP    P   1   6  In    4    -       64000     4000          70236          20096

     ARP    P   1   8  In    4    -       64000     4000              0              0

     ARP    P   2   1  In    4    -       64000     4000          25728              0

     ARP    P   2   4  In    4    -       64000     4000           3344              0

     ARP    P   2   5  In    4    -       64000     4000          15468              0

     ARP    P   2   6  In    4    -       64000     4000          53796           6210

     ARP    P   2   7  In    4    -       64000     4000          23222              0

     ARP    P   2   8  In    4    -       64000     4000              0              0

     ARP    P   2   9  In    4    -       64000     4000           4928              0

Then I tried Bc = 8000 (Tc = 1s!). Quite no more policed packets.

[Edit] And finally, Bc = 16000 (Tc = 2s, which has no sense!!) => still packets policed (span indicate no peaks!):

Proto Mode Sid Mod Dir AgId Prec         Cir    Burst   AgForward-By   AgPoliced-By

------------------------------------------------------------------------------------

     ARP    P   1   1  In    4    -       64000    16000        1355426          12032

     ARP    P   1   2  In    4    -       64000    16000         553734              0

     ARP    P   1   5  In    4    -       64000    16000         480478              0

     ARP    P   1   6  In    4    -       64000    16000        2071666          33472

     ARP    P   1   8  In    4    -       64000    16000              0              0

     ARP    P   2   1  In    4    -       64000    16000         657224           3976

     ARP    P   2   4  In    4    -       64000    16000          94732              0

     ARP    P   2   5  In    4    -       64000    16000         404400              0

     ARP    P   2   6  In    4    -       64000    16000        1412826          35584

     ARP    P   2   7  In    4    -       64000    16000         518716              0

     ARP    P   2   8  In    4    -       64000    16000              0              0

     ARP    P   2   9  In    4    -       64000    16000         149308              0

I guess there is a misunderstanding of me with this burst value, or with the span interpretation.

Has someone any idea ?

Thanks!

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