Output errors for no reason ?

[tedauction]

Hello, I am running a stack of WS-C2960X-48FPD-L  15.0(2a)EX5  switches.

I have noticed that on some ports I am getting continual 'output drops'. These interfaces are only workstations with phones so will not be doing heavy traffic. I have confirmed this via traffic SNMP sensors. There throughput is far below the 100Mbps negotiated speed of the port e.g. average 1-3Mbps only.

In the output of 'sh int gi4/0/26' I do not see any output errors.

My question is why am I seeing output drops here - is it perhaps a bug with the firmware ?

Thank you kindly for any help

sh int gi4/0/26:

GigabitEthernet4/0/26 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is 046c.9dc0.079a (bia 046c.9dc0.079a)
Description: User Desktop/VoIP Phone
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mb/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 00:00:59, output 00:00:58, output hang never
Last clearing of "show interface" counters 18:30:58
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 150717
Queueing strategy: fifo
Output queue: 0/40 (size/max)
30 second input rate 256000 bits/sec, 57 packets/sec
30 second output rate 568000 bits/sec, 92 packets/sec
3384585 packets input, 1196257047 bytes, 0 no buffer
Received 6953 broadcasts (6717 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 6717 multicast, 0 pause input
0 input packets with dribble condition detected
5671101 packets output, 5788638630 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

[Ganesh Hariharan · Accepted solution]

Hi,

Typically, the output drops will occur if QoS is configured and it is not providing enough bandwidth to certain class of packets. It also occurs when we are hitting oversubscription.

Generally in Switches output queues to provide some QoS functionality in hardware at wire-speed. If you have 'mls qos' configured without any further port level config, you will find the available buffer space will be split between these 4 queues. Each queue will receive certain DSCP/CoS values and the queues will transmit in a shared round robin fashion.

Hope it Helps..

-GI

[Joseph W. Doherty]

Is there a gig path from the source to the switch with the 100 Mbps egress, or could there be more than one 100 Mbps concurrent source?  If either is true, for short periods, you can overrun the egress port capacity which can cause egress drops.  As noted by Ganesh, the issue is often more of problem if a 2960 has QoS enabled.

However, with QoS enable, you can "tune" QoS, and possibly mitigate burst egress drops.