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49817 Views 11 Replies Latest reply: Jul 20, 2011 8:13 AM by StuO RSS

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Total Output Drops

Jul 11, 2011 3:30 AM

StuO 29 posts since
Sep 21, 2008

Hi All,

Just trying to find out why I'm getting discards on a interface. Basically we have a remote office that connects to another office before connecting to the main backhaul.




The Link between Switch A and B is 100Mbps

The Link between Switch B and C is 1000Mbps (1GB).


Looking into the history of the link between A and B we can see that majority of the time the traffic is at 60-65% but will spike upto 85% 1-2 times a week.


GigabitEthernet1/0/2 is up, line protocol is up (connected)

  Hardware is Gigabit Ethernet, address is 001d.70b8.e282 (bia 001d.70b8.e282)

  Description: to SWITCH-A TO SWITCH-B

  MTU 1998 bytes, BW 100000 Kbit, DLY 100 usec,

     reliability 255/255, txload 48/255, rxload 13/255

  Encapsulation ARPA, loopback not set

  Keepalive not set

  Full-duplex, 100Mb/s, link type is auto, media type is 10/100/1000BaseTX SFP

  input flow-control is off, output flow-control is unsupported

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input 00:00:00, output 00:00:00, output hang never

  Last clearing of "show interface" counters never

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 1851383

  Queueing strategy: fifo

  Output queue: 0/40 (size/max)

  5 minute input rate 5293000 bits/sec, 2799 packets/sec

  5 minute output rate 19054000 bits/sec, 3739 packets/sec

     91760131827 packets input, 21396599373912 bytes, 0 no buffer

     Received 50352741 broadcasts (33957950 multicasts)

     36 runts, 0 giants, 0 throttles

     36 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

     0 watchdog, 33957950 multicast, 0 pause input

     0 input packets with dribble condition detected

     123201990031 packets output, 112703016614681 bytes, 0 underruns

     0 output errors, 0 collisions, 1 interface resets

     0 babbles, 0 late collision, 0 deferred

     0 lost carrier, 0 no carrier, 0 PAUSE output

     0 output buffer failures, 0 output buffers swapped out




All switches are Cisco 3750. What would you recommend to resolve this issue as I've been advised that the QOS is fine and link will not be upgraded to a 1GB link?


Now looking into this I've read that either of the following can help:


- Increasing queue-limit

- Increasing hold queue

Cisco advising against increasing in hold queue as TCP timer can expire whilst the packet is still in the queue.

Anyone out there that can provide further input?

  • Brian 2,971 posts since
    Aug 17, 2009
    Currently Being Moderated
    1. Jul 11, 2011 11:32 AM (in response to StuO)
    Re: Total Output Drops

    Aloha StuO,


    Output drops typically indicate a congested interface.  This can occur when going from a high speed interface (such as the SW B to SW C link at 1000Mbps) to a lower speed link (SW A to SW B at 100Mbps).  You mention the traffic on the SW A to SW B link is at 60% - 65%, but what is it on the SW B to SW C link?  Also, I notice the MTU on the GE interface is at 1998 bytes, why?  What is the MTU on the FE of SW A?


    Output drops are not all bad, as the drops indicate the flow control mechanisms of the upper layer protocols are working to decrease the TCP window size.  Also, if the drops are due to a constant high-rate flow of traffic you cannot prevent them, but you can control them.


    I would like to see both the SW A interface and the SW B interface when the load is at 60% - 65% on SW A.  This I believe will give you a much better picture of what is going on.  So, outputs of the "sh run", "sh int x/x", "sh proc cpu" to name a few.



  • smsnaqvi 532 posts since
    Feb 6, 2010
    Currently Being Moderated
    5. Jul 12, 2011 12:42 AM (in response to StuO)
    Re: Total Output Drops

    Hi dude


    Please read below. This could be related to the QOS parameters.


  • smsnaqvi 532 posts since
    Feb 6, 2010
    Currently Being Moderated
    6. Jul 12, 2011 12:45 AM (in response to smsnaqvi)
    Re: Total Output Drops

    Please also correct me if I am wrong but wont this

    priority-queue out command prioritize the traffic for the 4th queue thus causing congestion issues????

  • Brian 2,971 posts since
    Aug 17, 2009
    Currently Being Moderated
    7. Jul 12, 2011 11:36 AM (in response to smsnaqvi)
    Re: Total Output Drops

    I think smsnaqvi may be on to something.  You may have a problem with the QoS settings.  Have you changed any of the default parameters other than what is on the interface?


    Here is a picture showing the default CoS and DSCP mappings regarding which queue services that particular CoS or DSCP value.




    Also, I would check the current CoS to Egress queue mappins


    sh mls qos maps cos-output-q


    and the DSCP to Egress queue mappings


    sh mls qos maps dscp-output-q



  • Brian 2,971 posts since
    Aug 17, 2009
    Currently Being Moderated
    9. Jul 17, 2011 8:17 AM (in response to StuO)
    Re: Total Output Drops

    Hey StuO,


    Sorry for the late reply.  I see you have changed these from the defaults.  Is it possible to post the switch configs  you can hide any proprietary information.




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