Monday, November 17, 2008

Low Latency Queue (LLQ) Cheatsheet

  • LLQ = CBWFQ + “priority” keyword
  • You can have more than 1 queue at a time
  • PQ can consume 100% bandwidth and starve low priority queue
  • LLQ discard traffic when exceed configured bandwidth – Policing!

Example Configuration

R11(config)#class-map voip-class
R11(config-cmap)#match ip dscp ef
R11(config-cmap)#policy-map voip-map
R11(config-pmap)#class voip-class
R11(config-pmap-c)#priority percent 33
R11(config-pmap-c)#int fa0/0
R11(config-if)#service-policy output voip-map

Double LLQ – 2 policing in 1 queue


1 Video Conference session = 320kbps

3 G.729a voice calls = 80kbps

If you configure “priority 400”, and if you have more than 3 G.729a voice calls, it will affect the video conferencing quality as it will exceed the policed rate 400kbps.

If you have 2 separate class-maps, i.e.

R11(config-pmap)#policy-map 2llq-map
R11(config-pmap)#class voice-class
R11(config-pmap-c)#priority 80
R11(config-pmap)#class video-class
R11(config-pmap-c)#priority 320

You can’t have more than 1 video conference call or more than 3 G.729a calls as it is policed in 2 difference classes. 

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