Qos Bandwidth Vs Prioirty

nangananga Member Posts: 201
Can you please help me understand with a working example what is the difference between Bandwidth Vs Priority when it comes to QoS implementation

This is confusing

Router(config)# policy-map policy-name

Router(config-pmap)# class class-name

Router(config-pmap-c)# priority kpbs [bytes]

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.

n 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. For more information, refer to What Is a Token Bucket?.


  • networker050184networker050184 Mod Posts: 11,962 Mod
    Priority is always sent first, but is also policed so that it does not starve the other queues. Bandwidth is guaranteed a minimum bandwidth, but has no maximum policer since it's queue is not always serviced first.
    An expert is a man who has made all the mistakes which can be made.
  • nangananga Member Posts: 201
    thank you , meaning during congestion, the priority traffic will be send first and will be guaranteed bit ( thats why it is mini guaranteed)
  • networker050184networker050184 Mod Posts: 11,962 Mod
    Yes. Basically imagine four lanes of traffic (the queues). There is a traffic cop (scheduler) telling when a car (packet) from the lanes can go. The first lane is the priority lane. Any cars in that lane go first no matter what. So as long as there are cars in the priority lane no one in the other lanes ever get to go. To avoid this the scheduler says you get priority, but only up to 10 cars a minute (policer). That way some cars are allowed to go from other lanes too. The other lanes have bandwidth reservations saying they get 2 cars a minutes, 1, 5 etc but the scheduler can't guarantee when those will go. Just that is the minimum cars that will get through within a minute.

    Hope that makes sense. That's kind of how I think about it in my head.
    An expert is a man who has made all the mistakes which can be made.
  • james43026james43026 Member Posts: 303 ■■□□□□□□□□
    Just to add a bit to this convo. The bandwidth option is commonly call traffic shaping, and the priority queue is commonly referred to as policing (although I would argue against it, as normally policing doesn't guarantee priority, which is what the priority option is for). One of the major differences, is in the way traffic looks when graphed. With traffic shaping / the bandwidth option, you end up with a nice smooth bandwidth graph, because packets will wait for tokens to refresh as long as there is enough buffer space. With traffic policing you end up with a rather rigid graph, because with policing, if there aren't enough tokens when a packet arrives at the head of the queue, then it is simply dropped, this continues until the scheduler see's a packet arrive at the head of the queue, and there are enough tokens to transmit the packet. I think Cisco has a really good article that helps explain it.
  • networker050184networker050184 Mod Posts: 11,962 Mod
    A bandwidth reservation is not the same as shaping traffic. Shaping is a way To set a maximum bandwidth while conversely a bandwidth statement is to reserve a minimum bandwidth.
    An expert is a man who has made all the mistakes which can be made.
  • james43026james43026 Member Posts: 303 ■■□□□□□□□□
    Thanks for pointing that out networker, I was also incorrect on the priority command as well. It appears shaping and policing are both ways to set a maximum bandwidth, and bandwidth and priority are used for setting minimum bandwidths. Cisco does a pretty good job summarizing that in this quote.

    [h=2]Minimum Versus Maximum Bandwidth Controls[/h]Both the shape and police commands restrict the output rate to a maximum kbps value. Importantly, neither mechanism provides a minimum bandwidth guarantee during periods of congestion. Use the bandwidth or priority command to provide such guarantees.
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