Router -can it dynamically alter CoS when rate-limiting?

Unanswered Question
Jun 3rd, 2009


I have a QoS policy which rate limits certain traffic...

using this command:

police 3000000 conform-action set-prec-transmit 4

exceed-action set-prec-transmit 0 violate-action set-prec-transmit 0

I want the router to also alter the CoS settings when this happens. Our service provider drops based on CoS, so if we can't set the CoS, we won't be able to utilize our MetroE link effectively.

Thanks for your help!

Lisa G

I have this problem too.
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Jon Marshall Wed, 06/03/2009 - 12:46


So your router is connected directly to the MetroE link ? or does the router handoff to a switch ?

What router model are you using and if there is a switch handoff which switch model ?


Giuseppe Larosa Thu, 06/04/2009 - 01:33

Hello Lisa,

default behaviour is to have CoS to be a copy of IP Prec bits.

So you should already be fine.

However, you need a 802.1Q trunk connected to the router.

To have multiple actions you should move to modular QoS and to combine settings with multiple actions in policing.

see for set cos


for multiple actions

Hope to help


lgontarski Thu, 06/04/2009 - 04:48

Hi Guiseppe,

Thanks for your response! I've checked the sniffer trace, and the CoS Priority bits are not being set, with either the pre-policing ToS values, or the post-policing values. They are always set to 0.

I'll look at the info you sent re multiple actions.

Thanks !!

Lisa G

Giuseppe Larosa Thu, 06/04/2009 - 04:54

Hello Lisa,

be aware that if you are using a SPAN session to sniff traffic this can have an effect on the CoS you see on captured traffic that is it can be put to zero on the destination/mirror port.

Hope to help


Joseph W. Doherty Fri, 06/05/2009 - 03:14

"default behaviour is to have CoS to be a copy of IP Prec bits. "

Just wondering, your first reference has:

"Command Default

No CoS value is set for the outgoing packet. "


"Usage Guidelines



The set cos command should be used by a router if a user wants to mark a packet that is being sent to a switch. Switches can leverage Layer 2 header information, including a CoS value marking. "

Would seem to imply explicit configuration is required. Could you provide an explicit reference of IP Prec to CoS as default?


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