Question

What does "random-detect" on a class map do?

Asked by: cpena123

I am trying to standardize QoS for VoIP on a Cisco 2610.

I have seen some of our equipment out there configured with policy maps like this:
policy-map voice-priority
  class voice
    priority 1152
  class class-default
   fair-queue
   random-detect

And some like this:
policy-map voice-priority
  class voice
    priority 1152
  class class-default
   fair-queue


What is the random-detect command doing here?

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Asked On
2008-04-02 at 11:19:01ID23290316
Tags

Cisco

,

2610

Topics

Voice Over IP

,

Network Routers

,

Telecommunications

Participating Experts
1
Points
500
Comments
1

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Answers

 

by: KlBePosted on 2008-04-02 at 12:17:18ID: 21266397

To understand what random early detect, or RED, does you need to know how TCP works.

TCP uses a window, x amount of packets it will send before confirming that they arrived well / retransmit, if a device was able to send i.e. 8 packets without any errors, it will increase it's window size to i.e. 16 etc etc.

Now you can image that there are several TCP streams going through an interface, and they all gradually increase their window sizes. At some point you will have reached the throughput that your link can handle, the buffer in the router will start filling and will 'overflow', new packets will just be dropped.
So all those TCP streams will have congestion at the same time, result ? all TCP connections will drastically reduce their TCP window size and the same thing starts all over again.

The result is inefficient BW usage with big peaks and then big drops in throughput. So what RED will do is just drop random packets here and there, even though the buffer isn't full yet, the result is that that TCP stream that lost a packet will drop it's window size and overall the bandwidth usage is much flatter and better controlled.

Remarks :
* only use RED on classes which have over 80% TCP traffic in it, it is utterly useless for UDP as it does not have this windowing scheme
* as a result, do NOT use RED on a voice class

20120131-EE-VQP-002

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