# question about RED

Hi !

I currently prepared my ONT certification Exam.

In the preparation of my exam certification I but ONT HowtoPass pratice Exam to make sure I'm ready to go at test center.

One question in the practice Exam is :

A RED profile is configured with a mark probability denominator of 1. What will be the result about this configuration on the packet lost as the average queue length reaches the maximum threshold ?

Base on my understand is since the average queue size is lower than minimum threshold, no drop occur, when minimum treshold is reach the mark probablylity denominator say drop 1 paquet on 1 paquet which is mean 100%. How can maximum threshold be be reach ?

Because the answer was : When the average queue lenght is at the maximum threshold 100% are drop.

**Correct Answer**by Giuseppe Larosa about

*8 years 6 months*ago

Hello Christian,

sorry If I've confused you.

However, the p of the mark denominator tells what is the arrival point of drop probability when average queue size is reaching max threshold.

x= average queue size

D= probability of be dropped

thr1= min threshold

thr2= max threshold

if x < thr1 D= 0 no packets dropped

if x > thr2 D=1 packets are dropped

if thr1 < x < thr2

D= (x - thr1) * 1 / [(thr2-thr1)*p]

if p=1

D=1 as x -> thr2-

if p>1 the arrival point of D function is not 1 when x -> thr2 but 1/p

For this reason I say that using a p>1 when permitted provides a better treatment for packets when x average queue size is between the two thresolds

see

For mark-prob-denominator, enter the denominator for the fraction of packets dropped when the average queue size is at the >>maximum threshold<<. The range is 1 to 65535. The default is 10. For example, if the denominator is 512, one out of every 512 packets is dropped when the queue is at the maximum threshold.

http://www.cisco.com/en/US/docs/switches/metro/catalyst3750m/software/release/12.2_35_se/configuration/guide/swqos.html#wp1281938

there were some nice graphs about this.

From a maths point of view the D function has a discontinuity point in x=thr2 when p>1

1/p for x= thr2-

1 for x= thr2+

Hope to help

Giuseppe