6509 Tail drop and WRED questions

[Difan Zhao]

Hi experts,

How do I tell if a traffic class (with specific cos value)  is using tail drop or WRED? When the queue reaches the random-detect-max-thresholds, will the traffic being tail dropped? Or will it still be randomly dropped until hit tail drop threshold? Thanks!

Interface GigabitEthernet2/24 queueing strategy:  Weighted Round-Robin

  Port QoS is enabled

  Port is untrusted

  Extend trust state: not trusted [COS = 0]

  Default COS is 0

    Queueing Mode In Tx direction: mode-cos

    Transmit queues [type = 1p3q8t]:

    Queue Id    Scheduling  Num of thresholds

    -----------------------------------------

       01         WRR                 08

       02         WRR                 08

       03         WRR                 08

       04         Priority            01

    WRR bandwidth ratios:    1[queue 1]  80[queue 2]  20[queue 3]

    queue-limit ratios:      1[queue 1]  80[queue 2]  20[queue 3]  15[Pri Queue]

    queue tail-drop-thresholds

    --------------------------

    1     70[1] 100[2] 100[3] 100[4] 100[5] 100[6] 100[7] 100[8]

    2     70[1] 100[2] 100[3] 100[4] 100[5] 100[6] 100[7] 100[8]

    3     100[1] 100[2] 100[3] 100[4] 100[5] 100[6] 100[7] 100[8]

    queue random-detect-min-thresholds

    ----------------------------------

      1    40[1] 70[2] 70[3] 70[4] 70[5] 70[6] 70[7] 70[8]

      2    40[1] 70[2] 70[3] 70[4] 70[5] 70[6] 70[7] 70[8]

      3    70[1] 70[2] 70[3] 70[4] 70[5] 70[6] 70[7] 70[8]

    queue random-detect-max-thresholds

    ----------------------------------

      1    70[1] 100[2] 100[3] 100[4] 100[5] 100[6] 100[7] 100[8]

      2    70[1] 100[2] 100[3] 100[4] 100[5] 100[6] 100[7] 100[8]

      3    100[1] 100[2] 100[3] 100[4] 100[5] 100[6] 100[7] 100[8]

    WRED disabled queues:

    queue thresh cos-map

    ---------------------------------------

    1     1

    1     2

    1     3

    1     4

    1     5

    1     6

    1     7

    1     8

    2     1

    2     2

    2     3

    2     4

    2     5

    2     6

    2     7

    2     8      0 1 2 3 4

    3     1

    3     2

    3     3

    3     4

    3     5

    3     6

    3     7

    3     8      6 7

    4     1      5

    Queueing Mode In Rx direction: mode-cos

    Receive queues [type = 1q8t]:

    Queue Id    Scheduling  Num of thresholds

    -----------------------------------------

       01         WRR                 08

    WRR bandwidth ratios:  100[queue 1]

    queue-limit ratios:    100[queue 1]

    queue tail-drop-thresholds

    --------------------------

    1     100[1] 100[2] 100[3] 100[4] 100[5] 100[6] 100[7] 100[8]

    queue thresh cos-map

    ---------------------------------------

    1     1      0 1 2 3 4 5 6 7

    1     2

    1     3

    1     4

    1     5

    1     6

    1     7

    1     8

  Packets dropped on Transmit:

    queue     dropped  [cos-map]

    ---------------------------------------------

    1                0  []

    2                0  [0 1 2 3 4 ]

    3                0  [6 7 ]

    4                0  [5 ]

  Packets dropped on Receive:

    queue     dropped  [cos-map]

    ---------------------------------------------

    1                0  [0 1 2 3 4 5 6 7 ]

[Joseph W. Doherty]

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Posting

Q.    What is the advantage of weighted random early detection (WRED), and how   do I know if my line card can support WRED?

A. For congestion avoidance on output scheduling, the Catalyst 6500       (Cat6K) Switch supports WRED on some egress queues. Each queue has a       configurable size and threshold. Some have WRED. WRED is a congestion-avoidance       mechanism whic...

Refer to the Understanding       the Queueing Capability of a Port section of       QoS       Output Scheduling on Catalyst 6500/6000 Series Switches Running CatOS System       Software in order to determine whether a line card or queue structure       can support WRED. You can also issue the show port       capabilities command in order to see the queue structure of your       line card.

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801091a5.shtml#queueingcap

[Difan Zhao]

Sorry Joseph I probably didn't make myself clear... I know that my line card does support the WRED because in the "show queueing" output it does include the min/max thresholds... The thing I don't know about is, how do I know if my traffic (e.g. with cos of 5 or voice traffic) will be WRED or just tail dropped?

Thanks!

[Adrian Coto]

Hello Difan,

Assuming you Have a LAN card:

I have no simple answer. We have to check at least two tables on the QoS config guide for the specific Hardware and Software.

  1.  Default Values with PFC QoS Enabled (aka what goes where by default)

  2.  Default Drop-Threshold Percentages and CoS Value Mappings (aka how queues behave by default)

For instance, on IOS 12.2SX:

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/qos.html#wp1480563

   1p7q8t Card, Transmit Queues

     CoS 5 --> Strict Priority Queue

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/qos.html#wp1478881

  1p7q8t Card, Transmit Queues

    Standard Transmit Queues 1-7 are WRED enabled by default

    Strict priority Queue is Tail drop (non configurable)    bingo!

Regards,

Adrian

[Difan Zhao]

Thanks Adrian... Is there no command to see what type of drop it is using...? Or are you saying that certain queues have to use WRED and rest tail drop and they are not configurable?

[Adrian Coto]

I understand that different cards have different defaults, and defaults may change for different IOS Versions.

Reading the tables for IOS 12.2SX the most common case i can see is:

  Standard TX queues are WRED by default, configurable to tail drop

  Strict priority TX Queue is Tail drop by default and not configurable to WRED.

But it is better to check on your specific IOS and Cards versions

[Adrian Coto]

Now I wonder if the line

"WRED disabled queues:"

for "show queueing interface" command will list any standard queue using tail drop...

I currently do not have access to a 65xx, may you please try it.

[Difan Zhao]

ummm that makes sense... How did I miss it... I thought I went through every single line very carefully

Anyway thanks very much. But do you have the commands to use tail drop instead of WRED for some queues just to test it?

Thanks,

[Adrian Coto]

How about disabling WRED:

no wrr-queue random-detect queue_id

Adrian