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Walkthrough Wednesdays

How to apply LLQ across DSL on an 800 series router


Core Issue

The Configuring Low-Latency Queuing and Link Fragmentation and Interleaving section of Advanced Router Configuration correctly shows the service-policy command being issued on the dialer interface.

On a Cisco 827 router, Low Latency Queuing (LLQ) is functional on a dialer interface only with the ppp multilink command issued. Enable the ppp multilink command if a service provider does not support it.

In this case, the service-policy command can be issued but it has no affect. Issue the show policy interface dialer command to show all traffic passing through the default queue.


Issue the service-policy command to the ATM Virtual Circuit (VC) as shown in this configuration example:

interface ATM0.4 point-to-point 
 bandwidth 128 
 pvc 1/35 
  vbr-nrt 128 128
  tx-ring-limit 2              
!--- Optimise the hardware transmit (Tx) buffer for LLQ.For more information,
!--- refer to the Impact of Very Small tx-ring-limit Values 
section of Understanding and Tuning the tx-ring-limit Value.  
  service-policy output voice 
  pppoe-client dial-pool-number 1

Note: Due to the nature of the LLQ implementation on ATM, if an Unspecified Bit Rate (UBR) class is required then the bandwidth (policy-map-class) command must specify a percentage of the bandwidth. If a bandwidth value is preferred or required, then another VC class, such as VBR-nrt, must be chosen. In this case, arbitrarily large Peak Cell Rate (PCR) and Sustainable Cell Rate (SCR) values can be chosen to emulate the UBR class.

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