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Doubt about QoS queues

I´ve a doubt about QoS and how egress queues manages QoS markings. I know that cos-dscp mapping is used when inbound traffic has CoS QoS labels. So this traffic is then mapped to a DSCP value and.... what happens next? is it managed by a mls qos srr-queue output dscp-map or by

a mls qos srr-queue output cos-map???. What if I´m remarking traffic with a policy-map to change DSCP values, 
is this DSCP value converted to a CoS value before getting into the output cos-map queue or is it handled
  directly by the output dscp-map queue?. I´m confused about it (layer 2 QoS)and i´ve check Cisco doc already but
don´t know when each type of output queue is used. Any help will be appreciated. Thanks
   
3 REPLIES

Doubt about QoS queues

ok as you understood once the traffic entered the switch port (inbound ) and the port has trust of lets say cos then the DSCP will be re written based on the cos-to-dscp map ( unless this capability is disabled not recomended )

now the frame/packet when it gose to the output port/interface it will be treated based on the srr-queue set if configured ( interface level take precedence ) if not the global qose config will take place for dscp out-put

and if the priority queue is enabled the traffic marked with the priority always serviced first in the outbound direction

the re marking with the policy always take place after traffic being handled and before sent out the port

hope this help

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Community Member

Doubt about QoS queues

Thanks for your reply marwanshawi (you are likely to answer firts than everybody).

Ok, let me see if I understood your explanation. Are you telling me that the only way the switch looks for what is established in the priority queue is that there is no que set configured in the output interface? (by the way, i´ve seen queue set applied to ports that will not function as output interfaces within the flow of the QoS traffic, is that ok?). If so, and supposing that i´ve my DSCP value obtained from the cos-dscp mapping, will the srr output queue dscp-map be used instead of the cos-map one for that specific value? Thanks.

Doubt about QoS queues

with Queue set you can change threshold weight or bandwidth % allocation in other words you can apply these changes to certain interfaces to change it from the default or global settings

the priority queue is differnt

, each Cisco switch model has a queue can be used as PQ once enabled on the interface level and all what you need is to put the desired cos/dscp in that queue

hope this help

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