High Number of Out Discards on 2960x Switches

[Dave Cancela]

Hi. I first would like to point out that i did not configure these switches, i inherited them. So here goes.

We are getting a lot of out discard errors on all our edge switches. Our edge switches are all 2960x's and are stacked (there are about 3 or 4 per stack). We have 11 edge switch locations. I believe it has to do with our qos setup.

We currently have mls qos enabled and here is the config for that portion:

!
mls qos map cos-dscp 0 8 16 24 32 46 46 56
mls qos srr-queue output cos-map queue 1 threshold 3 4 5
mls qos srr-queue output cos-map queue 2 threshold 1 2
mls qos srr-queue output cos-map queue 2 threshold 2 3
mls qos srr-queue output cos-map queue 2 threshold 3 6 7
mls qos srr-queue output cos-map queue 3 threshold 3 0
mls qos srr-queue output cos-map queue 4 threshold 3 1
mls qos srr-queue output dscp-map queue 1 threshold 3 32 33 40 41 42 43 44 45
mls qos srr-queue output dscp-map queue 1 threshold 3 46 47
mls qos srr-queue output dscp-map queue 2 threshold 1 16 17 18 19 20 21 22 23
mls qos srr-queue output dscp-map queue 2 threshold 1 26 27 28 29 30 31 34 35
mls qos srr-queue output dscp-map queue 2 threshold 1 36 37 38 39
mls qos srr-queue output dscp-map queue 2 threshold 2 24
mls qos srr-queue output dscp-map queue 2 threshold 3 48 49 50 51 52 53 54 55
mls qos srr-queue output dscp-map queue 2 threshold 3 56 57 58 59 60 61 62 63
mls qos srr-queue output dscp-map queue 3 threshold 3 0 1 2 3 4 5 6 7
mls qos srr-queue output dscp-map queue 4 threshold 1 8 9 11 13 15
mls qos srr-queue output dscp-map queue 4 threshold 2 10 12 14
mls qos queue-set output 1 threshold 1 100 100 50 200
mls qos queue-set output 1 threshold 2 125 125 100 400
mls qos queue-set output 1 threshold 3 100 100 100 400
mls qos queue-set output 1 threshold 4 60 150 50 200
mls qos queue-set output 1 buffers 15 25 40 20
mls qos
!

Here is one of the ports that have the issue:

Port           Align-Err     FCS-Err    Xmit-Err     Rcv-Err  UnderSize  OutDiscards
Gi2/0/30    0                 0               0                0            0                 13005659

Port Configuration:

Current configuration : 284 bytes
!
interface GigabitEthernet2/0/30
 switchport trunk native vlan 120
 switchport mode trunk
 srr-queue bandwidth share 1 30 35 5
 priority-queue out
 mls qos trust dscp
 auto qos trust
 storm-control broadcast level 60.00
 spanning-tree portfast
 spanning-tree bpduguard enable
end                          

GigabitEthernet2/0/30 is down, line protocol is down (notconnect)
  Hardware is Gigabit Ethernet, address is 38ed.1824.e61e (bia 38ed.1824.e61e)
  MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Auto-duplex, Auto-speed, 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 1w0d, output hang never
  Last clearing of "show interface" counters 4w2d
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 13005659
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     12505 packets input, 2199739 bytes, 0 no buffer
     Received 1709 broadcasts (1008 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 1008 multicast, 0 pause input
     0 input packets with dribble condition detected
     12616125 packets output, 2153347204 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 unknown protocol drops
     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

Any ideas?

And yes, the workstation is currently unplugged from the port at the moment. Duplex and Speed are set to auto, although i have noticed the wrong port speed gets assigned sometimes. In this case, it is set to full duplex and 1000 speed.

[Robert Hillcoat]

Try hard coding at both sides. 

[Dave Cancela]

Selecting port speed and duplex manually, causing the port to go down.

[Robert Hillcoat]

both sides? 

[Dave Cancela]

Yes, sir.

Switching it back to auto will get it back on 1Gbps/Full.

[Robert Hillcoat]

What version of IOS are you using? 

[Dave Cancela]

Version 15.0(2)EX5

I know there is a higher version, but we had to downgrade due to an issue of the switch stacks rebooting when logging into them. (Known Issue)

[Robert Hillcoat]

Ok, so you've cleared the counters, removed QOS from the port, shut then no shut, hard coded etc. etc. Could this possibly be related to the PC at the other end and not the switch? Do any other ports show the same behavior? Generally the output discard means that your sending more traffic than the port can cope with. Looks like something funny is going on with your speed and duplex settings. 

[Dave Cancela]

Its the same behavior across multiple devices AND multiple switch stacks. The only thing that is constant is that they are all 2960x's.

[Dave Cancela]

Also, i meant to add that we happen to be testing on a port connected to a workstation, but its not limited to the workstations. VoIP phones, AppleTVs, and other network devices are show the similar behavior. (Some are 100Mb devices, and others are 1Gbps)

[Joseph W. Doherty]

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Posting

You might just be bumping into a not uncommon issue on the 2960 (and 3560/3750), insufficient egress interface buffer RAM to deal with bursting.

If you really don't need to use QoS, disabling it often avoids the issue because buffer RAM isn't reserved for every port's four egress queues.

If you do need QoS, then buffer tuning might decrease your drops too.  Unfortunately, buffer tuning isn't for the novice, and one-size doesn't fit all.

A good document: https://supportforums.cisco.com/document/31581/egress-qos