Re: WRED thresholds and queue-limit

Yuri,
Thank you for your response, this is very good information. This looks like
a well hidden detail that Cisco changed when they implemented HQF.
Basically HQF (any IOS higher than 12.4(22), WRED has an additional check,
which is the queue-limit of each class. If the queue's current size, not
the average queue depth, is above the queue-limit it will tail drop all
packets exceeding that class' queue-limit. This is independent of what
value wred max-threshold value you have.

One thing that I'm still little confused it the hold-queue and queue-limit.
Based on the document you provided:

"Class-Based shaping policy applied to physical interface in HQF code: 1000
packets, tunable with a combination of interface CLI hold-queue out and
child policy queue-limit where child policy queue-limit has an upper bound
of interface hold-queue out."

...does this mean that each child's class-map's queue-limit has to be add
up to the hold-queue of the physical interface?

Regard,

Tom Kacprzynski

On Thu, Jan 26, 2012 at 12:32 AM, Yuri Bank <yuribank_at_gmail.com> wrote:

> It depends on the version of IOS. Pre-HQF, queue-limit is ignored if
> random-detect is configured under a class. In HQF, queue-limit WILL take
> affect when configured with random-detect, but it simply sets a hard limit
> and must be higher than Max Threshold.
>
> Unfortunately I wasn't able to test this since I only had 12.4(15) in
> Dynamips, but according to Cisco this is the case:
>
>
> http://www.cisco.com/en/US/products/hw/routers/ps341/products_tech_note09186a0080af893d.shtml
>
> -Yuri
>
>
>
>
>
>
> On Tue, Jan 24, 2012 at 1:10 PM, Tom Kacprzynski <tom.kac_at_gmail.com>wrote:
>
>> Hello,
>> Is this a valid configuration of WRED, where the queue-limit is set to 64
>> packets by default, the WRED Min Threshold is 100 (packets) and Max
>> threshold is 400 (packets)? Would this configuration defeat the whole
>> purpose of using WRED, as the average queue depth will never reach 100
>> packets (min threshold)? Am I understanding this correctly?
>>
>>
>> <sample output>
>> R1#sh policy-map int gi0/0 out
>> GigabitEthernet0/0
>>
>> Service-policy output: test-pmap
>>
>> Class-map: test1 (match-all)
>> 0 packets, 0 bytes
>> 5 minute offered rate 0 bps, drop rate 0 bps
>> Match: none
>> Queueing
>> queue limit 64 packets
>> (queue depth/total drops/no-buffer drops) 0/0/0
>> (pkts output/bytes output) 0/0
>> bandwidth 30000 kbps
>> Exp-weight-constant: 9 (1/512)
>> Mean queue depth: 0 packets
>> dscp Transmitted Random drop Tail drop
>> Minimum Maximum Mark
>> pkts/bytes pkts/bytes pkts/bytes
>> thresh thresh prob
>>
>> cs3 0/0 0/0
>> 0/0 100 400 1/10
>>
>>
>> <sample config of wred>
>> policy-map test-pmap
>> class test1
>> bandwidth 30000
>> random-detect dscp-based
>> random-detect dscp 24 100 400
>>
>>
>> Thank you,
>>
>> Tom Kacprzynski
>>
>>
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Received on Thu Jan 26 2012 - 17:14:10 ART