QOS Drop question

MACattack

Hi, I am getting confused in Congestion Avoidance. I know the class and drop precedence but when using WRED I am now stock.


I used CBWFQ as my congestion management and used WRED as my Congestion Avoidance.


I issued on my cli :

random-detect DSCP-based
random-detect DSCP AF11 20 30 10 (DSCP value, min, max and MPD)
random-detect DSCP AF12 24 40 10
random-detect DSCP AF21 19 40 10

In DSCP values AF 12 got class Priority value with drop prec of 2 higher than AF11 so it will be drop first. BUT here's the confusing part as mentioned on the cisco press when using WRED it is now base it's drop in the max value. In above value AF12 before start droping arriving packets it must reach 40.

In Af11 before start dropping it must reach 30. Does it mean the value in the DSCP class AF 12 drop probability is ignored? We know that 12 has higher drop prec than 11.



The more deeper I am facing QOS the more complicated it is....

please help....

MACattack

I guess , I am only answering my own question, so basically as mentioned in the book random-detect basing its droping mechanism in the maximun threshold if it reach.

With WRED using normail TAIL drop as default then it will use the Drop precedence in the DSCP values....

Got it...

EdTheLad

Seems you are not quite understanding all the parameters.AF11 will randomly drop packets if the queue depth increases above 20 but is less than 30.If the queue depth goes above 30 it will taildrop. AF12 will randomly drop packets if the depth goes beyond 24 and taildrop when above 40.AF21 will randomly drop packets when the queue goes above 19 but will taildrop when it goes above 40.I havent looked at Qos in a while so i'm rusty but as i remember the default class in CBWFQ can use wfq instead of tail-drop, but if you are using a user defined class simple tail-drop will be used so the prec value wont matter.

Ferret999

Okay just to add to what EdTheLad has said,

Firstly you are getting confused between your queuing mechanism and your congestion avoidance mechanism.

The command random-detect DSCP-based tells the router to use WRED based on DSCP so this is the quick way to setup WRED using the default values. So when you issue this command the router will use the Cisco default drop setting based on DSCP values. So AF11 will have a lower drop probability than AF12 or AF13.

With the commands

random-detect DSCP AF11 20 30 10
random-detect DSCP AF12 24 40 10
random-detect DSCP AF21 19 40 10

you are actually changing the default values for AF11, AF12 and AF21 and setting your own values.

So as Ed says when the queue has more than 20 packets marked with a DSCP value of AF11 it will start dropping you have set the MPD to 10 so one out of every 10 packets will be dropped. Once you hit 30 packets in the queue all incoming packets in the queue will be tail dropped.

CBWFQ is your queing mechanism so you can setup class based queues. However all the individual queues will still operate on a FIFO basis. With CBWFQ you can setup the default queue to use WFQ instead of being a FIFO queue.

dtlokee

The MPD is an increasing value when the lower queue limit is hit it begins dropping packets until the upper bound is hit that is when it will drop at a rate equal to the MPD, in this case 1/10. Above the upper bound it will tail drop the packets.

Ferret999

The MPD is an increasing value when the lower queue limit is hit it begins dropping packets until the upper bound is hit that is when it will drop at a rate equal to the MPD, in this case 1/10. Above the upper bound it will tail drop the packets.

ohps forgot that detail thanks for reminding me about that dtlokee

MACattack

Sorry guys, I was so busy in my work.... anyway thanks for all the reply, really I am reading as fast as I can and put in my head as fast as possible...

MACattack

AHHHHHH, my head is turning upside down and my brain ig squeezzing........
Am I the only experiencing this........ as near as you are in having certificate the more cramming I am getting, like I want to learn GRE over IP sec and I am reviewing also frame relay for my home lab and also to test QOS .....


I am also doing my voice lab CCM 6....

My head is getting over heat...... any advice.......

by the way I really really appreciate your help guys...... If you are just near with me I will treat you .......




God bless me and all people taking Cisco stuff......ENJOY..