Comparing Bandwidth and Priority in QoS

source:

http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a0080103eae.shtml


 Function

bandwidth Command

priority Command

Minimum
bandwidth guarantee

Yes

Yes

Maximum bandwidth guarantee

No

Yes

 

Q. does it mean that bandwidth can be allocated more (in the event of congestion) than the specified using bandwidth keyword, however bandwidth can not be given more to the traffic when used Priority key word ?

Comments

  • Not able to follow you link. However the priority command has an inbuilt policer, so yes with the priority command you will not get more than configured with the priority command. Using the bandwidth you specify the min, there is not max limit.

  • Robot,

    Thanks and appreciate the response. Can you support with some links, docs etc.

    Here is cisco link which forced me to take a deep dive:

    http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a0080103eae.shtml

  • Hi

    Can you post the sections of the doc that are of concern to you and that are the source of your questions? From the doc you referenced:

     

    The bandwidth command provides a minimum bandwidth guarantee during congestion.

     

    Then for priority it says.

     

    During congestion conditions, the traffic class is guaranteed bandwidth equal to the specified rate. (Recall that bandwidth guarantees are only an issue when an interface is congested.) In other words, the priority command provides a minimum bandwidth guarantee.

    In addition, the priority command implements a maximum bandwidth guarantee. Internally, the priority queue uses a token bucket that measures the offered load and ensures that the traffic stream conforms to the configured rate. Only traffic that conforms to the token bucket is guaranteed low latency. Any excess traffic is sent if the link is not congested or is dropped if the link is congested.

     

    From the 12.4 T command reference:

     

    When the device is not congested, the priority class traffic is allowed to exceed its allocated bandwidth. When the device is congested, the priority class traffic above the allocated bandwidth is discarded.

     

    HTH

  • This concept is fundamental to CBWFQ, and really shouldn't require a deep dive - it's explianed in that blurb from your first post.  Historically PQ was used for traffic you *REALLY* cared about, but the problem with it was that it would always give absolute priority to that traffic type, so if you only had a t1, and had a t1+ worth of that traffic, *nothing else* would ever be sent.  With CBWFQ, they added a policer to to PQ, which forces it to police traffic to the same level it guarantees.  That traffic recieves absolute priority, but it cannot saturate your entire pipe.

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