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MX5 port shaper/policer

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  • 1.  MX5 port shaper/policer

    Posted 10-20-2017 23:14

    Hi everyone,

    I have a question.


    Is port's shaper/policer applied after the traffic has been queued and scheduled and put on the FIFI queue of the port?   In other words, does port 's policer/shaper act on the traffic that has been placed on the FIFO queue of the port after it has been scheduled from egress queues?


    Thanks and have a nice weekend!!







  • 2.  RE: MX5 port shaper/policer

    Posted 10-21-2017 06:51

    Classiification does happen after the fifo queue on the port.


    Check out chapter 2 in the free Day one book on MX packet walkthrough for all the details.



  • 3.  RE: MX5 port shaper/policer

    Posted 10-21-2017 19:01

    Thanks for your response.

    I do not see any mention of port shaper or interface specific  policer ( unless I missed it) .

    I know filter speific policer is appled before the packet is placed in egress queues on the port based.

    This is my guess as  I could not find any JUNOS doc to corroborate it.

    Before the traffic can be placed on the egress port, Traffic first is subjected  to Interface specific policer, if the traffic conforms, it is then placed in egress queues awaiting transmission by schedulers. Key point the egress outbound interface policer acts on the traffic first before traffic is queued into egress queues. 

    Not sure if the same is also true for Port shaper.


    Appreciate if you can clarify the above.


    Have a nice weekend!!



  • 4.  RE: MX5 port shaper/policer

    Posted 10-23-2017 04:24

    Sorry, I guess I don't understand the questions.  The fifo queue is either the first contact with packets on ingress or the last on egress.  Thus on ingress all processing is after fifo and on egress all processing happens before hitting the fifo queue.


    Or is the question not where is the fifo queue in processing but something else?


    If the packet walk through is not what you are looking for maybe this desccription of the QoS functions is what would help.


  • 5.  RE: MX5 port shaper/policer

    Posted 10-23-2017 18:55

    Thanks for your response, my apology for not very clear.

    This is what I visualize:

     Egress traffic from fabric->     -------q1---scheduler---|

                                                                                               | To FIFO queue of the port--->Port shaper/Port policer--> WIRE

     Egress traffic from fabric->    --------q2 -scheduler---|



    Above traffic bound for egress port are received into egress queues  from the fabric, schedulers pick up the packet from the queue and place it on the FIFO queue of the port. 


    So when we use Port based shaper or interface specific stateless filter, are we applying at the position shown above i.e 

    these  port based policers/shaper are applied post classification and scheduling and right before the traffic is put on the wire?


    I know Filter based egress policer acts on the traffic as it about to enter egress queues from the fabric. Note that this filter is applied before  queueing and scheduling but I am not sure about Port based shaper and interface specific policer.





  • 6.  RE: MX5 port shaper/policer
    Best Answer

    Posted 10-24-2017 02:58

    I read the packet walk through as clearly showing the FIFO queue is the last element before the wire and that policers are the first element on ingress


     Egress traffic from fabric->     -------q1---scheduler---|   Port shaper/Port policer-->  | To FIFO queue of the port--->WIRE




    WIRE >  To FIFO queue of the port---> Port shaper/Port policer--> q1---scheduler---|  Egress traffic from fabric



  • 7.  RE: MX5 port shaper/policer

    Posted 10-24-2017 08:28

    Great link


    Thanks have a nice day!!