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New Member

Output Queue Drops question from a newbie

Greetings,

I have spent some time looking online and the forums and documents but have a few questions.  We are in the middle of a migration of in-house software and have had some performance bottlenecks in HTTP requests between clients and hosts on different servers that route through a 2960g switch in a bridge with another an RSTP setup.  While most of the issues have been resolved by repartitioning on the linux hosts and filers, I spent some time early on looking at the switch.  I am a Gnu/Linux guy, and only recently got thrown into the switching when our comms group got retasked so bare with me.

The switchport is connected to a linux host running RHEL5u5 with a bonded interface in transmit load-balance, only on MAC is visible to the switch at any time, the switchport config is pretty basic, as the switch config:

!

interface GigabitEthernet0/28

description dx1

switchport mode access

spanning-tree portfast

I noticed that the txload and rxload are relatively small, and remain relatively small, however the Total output drops increments pretty fast and with regularity, I read most times this can be just fine for various reasons, but was wondering if there is a way to sniff which packets are dropped, or if that is what that stat is telling me.  Is this something I should be concerned about, on the same hardware and setup running our legacy software the total output drops are 0-5, as opposed to across-the-board similar to the below on the new software, which does however use a lot more network traffic as it is more ESB/SOA based than the legacy software.

hsw1-awcn#show interfaces g0/28  

GigabitEthernet0/28 is up, line protocol is up (connected)

  Hardware is Gigabit Ethernet, address is e804.62b3.4c1c (bia e804.62b3.4c1c)

  Description: dx1

  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,

     reliability 255/255, txload 7/255, rxload 1/255

  Encapsulation ARPA, loopback not set

  Keepalive set (10 sec)

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

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

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

  Last input never, output 00:00:01, output hang never

  Last clearing of "show interface" counters never

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 163167301

  Queueing strategy: fifo

  Output queue: 0/40 (size/max)

  5 minute input rate 4670000 bits/sec, 1793 packets/sec

  5 minute output rate 29639000 bits/sec, 2992 packets/sec

     40329003402 packets input, 19801050808406 bytes, 0 no buffer

     Received 1330378 broadcasts (79 multicasts)

     0 runts, 0 giants, 0 throttles

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

     0 watchdog, 79 multicast, 0 pause input

     0 input packets with dribble condition detected

     54443241034 packets output, 55818849960705 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

We don't have any QoS setup, but was wondering also what the below is telling me, or if it is an issue;

hsw1-awcn#show platform port-asic stats drop GigabitEthernet0/28

  Interface Gi0/28 TxQueue Drop Statistics

    Queue 0

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 1

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 2

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 3

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 115263540

Here is some other output from the switchport that might be of use to someone with more knowledge than I.

hsw1-awcn#show interfaces g0/28 counters

Port            InOctets    InUcastPkts    InMcastPkts    InBcastPkts

Gi0/28    19801847031738    40331456565             79        1330324

Port           OutOctets   OutUcastPkts   OutMcastPkts   OutBcastPkts

Gi0/28    55820903755360    54134963247      298941448       13486895

hsw1-awcn#show interfaces g0/28 counters err

Port        Align-Err     FCS-Err    Xmit-Err     Rcv-Err  UnderSize  OutDiscards

Gi0/28              0           0           0           0          0    163167306

Port      Single-Col  Multi-Col   Late-Col  Excess-Col  Carri-Sen      Runts     Giants

Gi0/28             0          0          0           0          0          0          0

hsw1-awcn#sh mls qos maps cos-output-q

   Cos-outputq-threshold map:

              cos:  0   1   2   3   4   5   6   7 

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

  queue-threshold: 2-1 2-1 3-1 3-1 4-1 1-1 4-1 4-1

hsw1-awcn#show mls qos maps dscp-output-q

   Dscp-outputq-threshold map:

     d1 :d2    0     1     2     3     4     5     6     7     8     9

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

      0 :    02-01 02-01 02-01 02-01 02-01 02-01 02-01 02-01 02-01 02-01

      1 :    02-01 02-01 02-01 02-01 02-01 02-01 03-01 03-01 03-01 03-01

      2 :    03-01 03-01 03-01 03-01 03-01 03-01 03-01 03-01 03-01 03-01

      3 :    03-01 03-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01

      4 :    01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 04-01 04-01

      5 :    04-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01

      6 :    04-01 04-01 04-01 04-01

hsw1-awcn#show interfaces g0/28 switching 

GigabitEthernet0/28 dx1

          Throttle count          0

        Drops         RP          0         SP          0

  SPD Flushes       Fast          0        SSE          0

  SPD Aggress       Fast          0

SPD Priority     Inputs          0      Drops          0

     Protocol       Path    Pkts In   Chars In   Pkts Out  Chars Out

        Other    Process          0          0    2132504  128043660

            Cache misses          0

                    Fast          0          0          0          0

               Auton/SSE          0          0          0          0

Spanning Tree    Process          0          0   10465630  627937800

            Cache misses          0

                    Fast          0          0          0          0

               Auton/SSE          0          0          0          0

          CDP    Process          0          0     354589  146047840

            Cache misses          0

                    Fast          0          0          0          0

               Auton/SSE          0          0          0          0

hsw1-awcn#show interfaces g0/28 stats

GigabitEthernet0/28

          Switching path    Pkts In   Chars In   Pkts Out  Chars Out

               Processor          0          0   12999502  956834048

             Route cache          0          0          0          0

                   Total          0          0   12999502  956834048

hsw1-awcn#show queueing interface g0/28

Interface GigabitEthernet0/28 queueing strategy: none

     Transmit GigabitEthernet0/28             Receive

   1763188559 Bytes                       3735544227 Bytes                   

   3812006753 Unicast frames              3727259303 Unicast frames          

    406848156 Multicast frames                 32316 Multicast frames        

     43306196 Broadcast frames              27011280 Broadcast frames        

            0 Too old frames              1844580102 Unicast bytes           

            0 Deferred frames                6268671 Multicast bytes         

            0 MTU exceeded frames         1884695228 Broadcast bytes         

            0 1 collision frames                   0 Alignment errors        

            0 2 collision frames                   0 FCS errors              

            0 3 collision frames                   0 Oversize frames         

            0 4 collision frames                   0 Undersize frames        

            0 5 collision frames                   0 Collision fragments     

            0 6 collision frames      

            0 7 collision frames          2440117432 Minimum size frames     

            0 8 collision frames          2187857298 65 to 127 byte frames   

            0 9 collision frames          3488687700 128 to 255 byte frames  

            0 10 collision frames         1591178981 256 to 511 byte frames  

            0 11 collision frames         2555603644 512 to 1023 byte frames 

            0 12 collision frames           80792437 1024 to 1518 byte frames

            0 13 collision frames                  0 Overrun frames          

            0 14 collision frames                  0 Pause frames            

            0 15 collision frames     

            0 Excessive collisions                 1 Symbol error frames     

            0 Late collisions                      0 Invalid frames, too large

            0 VLAN discard frames                  0 Valid frames, too large 

            0 Excess defer frames                  0 Invalid frames, too small

   1181878413 64 byte frames                       0 Valid frames, too small 

   3009065386 127 byte frames         

   3273911113 255 byte frames                      0 Too old frames          

   1981929200 511 byte frames                      0 Valid oversize frames   

   3554245931 1023 byte frames                     0 System FCS error frames 

   4146032950 1518 byte frames                     0 RxPortFifoFull drop frame

            0 Too large frames        

            0 Good (1 coll) frames    

            0 Good (>1 coll) frames   

hsw1-awcn#show mls qos interface g0/28

GigabitEthernet0/28

QoS is disabled. When QoS is enabled, following settings will be applied

trust state: not trusted

trust mode: not trusted

trust enabled flag: ena

COS override: dis

default COS: 0

DSCP Mutation Map: Default DSCP Mutation Map

Trust device: none

qos mode: port-based

  • LAN Switching and Routing
7 REPLIES
Cisco Employee

Output Queue Drops question from a newbie

Hello,

I see all drops happening due to OutDiscards. Those drops happen when portTX port buffer is full and switch can't send it out. Other thing is that all your traffic is coming out through queue3. Even without QoS configured switch apply some default qos if configured globally. SO that queue may have not the whole port buffers and with a big number of small packets. Possibly QoS should be applied to solve it. Can you please also capture these commands to take a quick look.

show platform port-asic stATS Drop

show queueing interface g0/28  --- > the one above is not show queueing - it was show controller

Nik

New Member

Output Queue Drops question from a newbie

Thank you for the reply ... I have cleared the counters on the switchport to monitor the situation a little more and see what the ratio of drops to total packets are/have been over time.

There is no QoS configured on the switchport either, thanks for clarifying those two commands.  Most of the literature I have read online from Cisco staff replies in forums and whitepapers appear to have, as you noted, two solutions [1] increase bandwidth on the port and/or [2] implement QoS ... I have also read a lot about how drop queue counts are normal sometimes, and the ratio is what is more important, while ratios seems to be in the low single digits or lower, I am still slightly concerned as more traffic will be hitting the end device on the switchport in the future.

Thanks for any further information and that which you have provided thus far!

hsw1-tbw3#show queueing interface g0/28

Interface GigabitEthernet0/28 queueing strategy: none

And the below is the output of the drop statistics:

hsw1-tbw3#show platform port-asic stATS Drop

Port-asic Port Drop Statistics - Summary

========================================

  RxQueue 0 Drop Stats: 0

  RxQueue 1 Drop Stats: 0

  RxQueue 2 Drop Stats: 0

  RxQueue 3 Drop Stats: 0

  Port  0 TxQueue Drop Stats: 0

  Port  1 TxQueue Drop Stats: 0

  Port  2 TxQueue Drop Stats: 0

  Port  3 TxQueue Drop Stats: 1630

  Supervisor TxQueue Drop Statistics

    Queue  0: 0

    Queue  1: 0

    Queue  2: 0

    Queue  3: 0

    Queue  4: 0

    Queue  5: 0

    Queue  6: 0

    Queue  7: 16

    Queue  8: 252025

    Queue  9: 0

    Queue 10: 0

    Queue 11: 0

    Queue 12: 0

    Queue 13: 0

    Queue 14: 0

    Queue 15: 0

Port-asic Port Drop Statistics - Details

========================================

  RxQueue Drop Statistics

    Queue 0

    Weight 0 Frames: 0

    Weight 1 Frames: 0

    Weight 2 Frames: 0

    Queue 1

    Weight 0 Frames: 0

    Weight 1 Frames: 0

    Weight 2 Frames: 0

    Queue 2

    Weight 0 Frames: 0

    Weight 1 Frames: 0

    Weight 2 Frames: 0

    Queue 3

    Weight 0 Frames: 0

    Weight 1 Frames: 0

    Weight 2 Frames: 0

  Port 0 TxQueue Drop Statistics

    Queue 0

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 1

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 2

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 3

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

  Port 1 TxQueue Drop Statistics

    Queue 0

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 1

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 2

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 3

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

  Port 2 TxQueue Drop Statistics

    Queue 0

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 1

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 2

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 3

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

  Port 3 TxQueue Drop Statistics

    Queue 0

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 1

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 2

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 0

    Queue 3

      Weight 0 Frames 0

      Weight 1 Frames 0

      Weight 2 Frames 1630

  Supervisor TxQueue Drop Statistics

    Queue  0: 0

    Queue  1: 0

    Queue  2: 0

    Queue  3: 0

    Queue  4: 0

    Queue  5: 0

    Queue  6: 0

    Queue  7: 16

    Queue  8: 252025

    Queue  9: 0

    Queue 10: 0

    Queue 11: 0

    Queue 12: 0

    Queue 13: 0

    Queue 14: 0

    Queue 15: 0

Cisco Employee

Output Queue Drops question from a newbie

ok,

This can be really traffic specific. As port has no QoS on it - that output queue is FIFO. But buffer for it is still limited. Outdiscards we see - mean there is no egress buffers available so the packet will be dropped. If we have bursty traffic - that can overwhelm queue causing those drops. If this reallydependant of type of traffic - QoS can be a solution as we can put some thresholds making port to request more buffers from common pool in case of starvation.

Other thing to try is just to change the port (better on the other side of switch). Some time ASIC level problems/limitations can affect particular ports.

Nik

Output Queue Drops question from a newbie

Hi,

Try to configure the below command under the interface. This will set the queue size to max and therefore no drops on the interface.

  hold-queue 4096 in

Please rate the helpfull posts.

Regards,

Naidu.

New Member

Output Queue Drops question from a newbie

I assume you mean hold-queue 4096 out since my drops above are with the output queue?  If you mean to increase the size of the input queue then can you explain why that would help output drops?

I am also a little hesitant to up the queue sizes as I don't want to introduce further complications with retransmission if packets stay too long in the output queue.  But, the queue size here is only at 40, which seems a little low and most things I have looked at seem to suggest 1024 for GB connections so perhaps that is a simple solution.

Hall of Fame Super Gold

Output Queue Drops question from a newbie

Try this:

1.  Hardcode the interface to "speed 100";

2.  Clear the counters; and

3.  Observe if the drops are evident.

I know it's not something anyone likes to do but there are some high-end servers, when mis-configured, can't really take 1Gb traffic.

But once configured correctly, the output drops go away.  We have several cases like that and we get into heated arguments with the server guys about this.  So this is what we did and the server people "get the message". 

Output Queue Drops question from a newbie

Hi,

We have found that adding the following configuration to the interfaces on the router helps a lot: "hold-queue 4096 out 1G interfaces can benefit from “hold-queue 1024 out” as well. This can be configured on input as well, e.g. “hold-queue 1024 in” in the same place in the configuration. Looking at the output of “show interface” can tell you the size of the interface queues. Check before you make changes, since some interfaces default to a 2000-packet input queue.

Please rate the helpfull posts.
Regards,
Naidu.

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