Somes questions about WFQ and Per-Hop Behavior

Question

Hi !
I'm currently self studding for my CCNP ONT certification Exam.
I read the official certification guide for this exam from Cisco Press. But, some point are still confuse for me :
1- Why WRED is most deploy on cores devices then all devices in Enterprise Network ? (The author saied :"Because WRED is usually applied to network core router....") And which implementation of congestion management system I should deploy on none core device, because thoses can also experience congestion in some point of the day and tail drop should be avoid also on thoses (distribution and access devices) devices also if I understand correctly ?
2- I understand WRED is an evolution of RED. Is still exist some reason to choose to use RED over WRED ? If WRED is available in device, is the RED is also available ? In my book in the example it say to activate WRED the command is RANDOM-DETECT, what about to activate RED ?
3- I'm confused about "class selector Per-Hop Behavior", if I'm understand correctly, We have 8 sub-classes "class selector Per-Hop Behavior" for each : Default Per-Hop Behavior, Assured Forwarding 1 to 4, and Expedited Forwarding, where the lowest value have higher probability to be drop. The class "class selector Per-Hop Behavior" work in the inverse of there parent where the highest value have higher probability to be drop.
in the exemple (copied from the book) how the preference of dropping is selected :
random-detect dscp-based
random-detect dscp af21 32 40 10
random-detect dscp af22 28 40 10
random-detect dscp af23 24 40 10
random-detect dscp cs2 22 40 10
I'm correctly understand how I should interpret the "random-detect dscp af" in comparaison of each other but, I don't understand where the I should place the "random-detect dscp cs2" with regard to the others.
Thanks a lot in advanced for your help !
Excuse my Engliash, but I'm a French people !

Answer 1

Hi,
The main difference betwwen PQ and LLQ is this.
PQ: Exclusively starves other traffic while serving packets in its queue. Infact PQ is so greedy, other traffic may never get serviced as long as there are packets in the PQ.
LLQ: It combines the bandwidth reservation of CBWFQ with PQ.
LLQ uses Policing to ensure that Bandwidth configured for PQ is not exceeded and when exceeded, drops the packets and then service the non-LLQ queues.
Hence LLQ ensures guranteed badnwidth only to the contracted amount.
Eg if a LLQ has 30kbps of bandwidht reserved. This can take a single G.729 call. If a 2nd call comes in, the policer will discard packets and both calls will sound bad. Hence its important to use CAC to limit the number of calls allowed to that configured for LLQ bandwidth, so the policer does not drop packets.
LLQ traffic uses the EF Diffserv PHB.
The EF PHB has 2 components.
1. Queueing to provide low delay, jitter or loss plus a guranteed bandwidth.
2. Policing to prevent EF traffic/LLQ traffic from starving other traffic of bandwidth.
So in summary, PQ will always treat PQ traffic firts and will starve other traffic of bandwidth
LLQ on the other hand will serve LLQ queues to the tune of bandwidth configured for them and then service other queues too.
HTH, Please rate useful posts