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Issue about QoS using "priority percent 50" and "police cir"

I have a question about the next configuration:


class-map match-any Precedencia3

match ip precedence 3


class Precedencia3

priority percent 50

police cir 64000 bc 2000 be 2000

conform-action transmit

exceed-action drop


As you can see iam using two command inside the class: "priority percent 50" and "police cir 64000 bc 2000 be 2000". The total bandwith of my link is 1024k, what is the bandwith that the precedence 3 is goint to takes?

Posible answers:

-512k -> because the command "priority percent 50"

-64k -> because the command "police cir 64000 bc 2000 be 2000"

-The commands are working together and i dont know what is it efect of this configuration

If do a sustraccion of (512k -> "priority percent 50") and (64k -> "police cir 64000 bc 2000 be 2000")= 448 k

The 448k are using?

Best regards.


Re: Issue about QoS using "priority percent 50" and "police cir"

I haven't tested this out, but an educated guess would be that the traffic in this class will only ever consume 64K of bandwidth. The "priority percent 50" command would allow up to 512K of traffic, but the "police" command then drops traffic above 64K. I would be interested to know the outcome if you test this out.

Hope this helps.


Super Bronze

Re: Issue about QoS using "priority percent 50" and "police cir"

A. "-64k -> because the command "police cir 64000 bc 2000 be 2000" "

NB: because it will engage before the LLQ policer (@ 512 Kbps).


There's a couple of possible reasons for a configuration like this:

a) The explict policer is 100% active, the LLQ policer (usually) only when there's interface congestion.

b) With the explict policer we can tune Bc and Be.

Similar configuration can be useful too. For example, you might want to use an embedded policer with less bandwidth, but always transmits to set a soft limit (i.e. records stats that you might be getting close to exceeding your hard cap). I've also use similar embedded policers to mark down (i.e. change) ToS. For example, over 64K mark as AF32, under mark as AF31 (both also seen as IP Prec 3, but we can have different drop probilities, because of the DSCP difference, depending on actual transmission rate).

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