QoS Question (Policing & Strict Priority)


Just a quick clairifcation, im currently swatting for the 642-642 exam and was wondering if somebody could clarify the following code for me:

VOIP(config)#do show policy-map llq-policy
Policy Map llq-policy
Class voice
Strict Priority
Bandwidth 168 (kbps) Burst 4200 (Bytes)
police cir 192000 bc 6000
conform-action transmit
exceed-action set-dscp-transmit af11

Based on this config, its obvious that its using LLQ for the 'Class voice' as the strict priority has been set.
However the bandwidth has been set to 168kbps which I believe to be the maximum amount of bandwidth allocated to all priority voice traffic.
Why then does it allow you to police at a higher CIR level. If you are guranteeing no more than 168 bandwidth why bother allowing it to police more than 168kbps.

Any explination would be greatly appreciated. Its probably something obvious but my brain is now completely mush.



  • networker050184networker050184 Mod Posts: 11,962 Mod
    The class can go above the amount set with the priority command if there is no congestion. The class can not go over the policed rate regardless. So basically this policy would give the voice class 168k in times of congestion and limit it to 192k in times of no congestion.
    An expert is a man who has made all the mistakes which can be made.
  • surfthegeckosurfthegecko Member Posts: 149
    Oh right, the way I took it when watching the CBT Nuggets was that when you specify "priority xxx" within LLQ it acts as the maximum.

    Im sure it explained that it uses it as a maximum because it protects the voice traffic more. If it went above it when there was no congestion and then the traffic on the network changed and congetion occured, surely the voice data would be open to jitter/delay/disconnect etc?

    Thanks for your time so far.

  • networker050184networker050184 Mod Posts: 11,962 Mod
    When you use priority its only the maximum if there is congestion. If you want a maximum no matter what, and still give priority you need to do both.

    If you are using more than your priority and congestion kicks in, anything over gets strictly dropped. So, yes you would have some issues at that point.
    An expert is a man who has made all the mistakes which can be made.
  • surfthegeckosurfthegecko Member Posts: 149
    Thanks for the confirmation its much appreciated. Clears that one up anyway.
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