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I have a access switch with a 10gb upstream interface and a 1gb downstream interface, i can see from the show interfaces [int] that traffic is being dropped on the upstream downstream direction.

TenGigabitEthernet1/1 is up, line protocol is up (connected)
Hardware is Ten Gigabit Ethernet Port, address is f4cf.e213.7e30 (bia f4cf.e213.7e30)
Description: MATE.A01
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
 reliability 255/255, txload 5/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 1000Mb/s, link type is auto, media type is 1000BaseSX
input flow-control is on, output flow-control is on
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:45, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 7556661
Queueing strategy: Class-based queueing
Output queue: 0/40 (size/max)
5 minute input rate 5364000 bits/sec, 1098 packets/sec
5 minute output rate 22402000 bits/sec, 2980 packets/sec
 863690200 packets input, 401065551603 bytes, 0 no buffer
 Received 6568826 broadcasts (1428513 multicasts)
 0 runts, 0 giants, 0 throttles
 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
 0 input packets with dribble condition detected
 2787171938 packets output, 1962145408543 bytes, 0 underruns
 0 output errors, 0 collisions, 4 interface resets
 0 unknown protocol drops
 0 babbles, 0 late collision, 0 deferred
 0 lost carrier, 0 no carrier
 0 output buffer failures, 0 output buffers swapped out

Most probably the cause of these drops are due to the fact that the traffic is comming from a higher bandwitdh interface to a lower bandwith one. the buffer gets full quickly and the switch as to discard some packets.

Is there a command where i can see the packet drops at a buffer level? Are there more causes for output drops? how can i troubleshoot these output drops more in detail?

Thanks

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The most likely scenario is that multiple downstream interfaces are receiving bursts of traffic bound for this 10G uplink, and it is unable to cope for short periods of time, leading to these drops.

Your Interface reliability counter is showing 255/255, so it is unlikely that this condition is regular, however clearing down the interface stats each day (or even graphing them in a 3rd-party SNMP tool) would be worthwhile to correlate new output errors with current traffic volumes.

Another possibility is that you have a QoS policy applied to the interface that is limiting outbound traffic in some way - check this with show policy-map interface TenGigabitEthernet1/1.

  • Thank for the reply, my interface reliability is 255/255, i can rule out traffic congestion. i have been looking at the QoS i have applied on that interface and i found out that the class-default is discarding udp packets that fill up the buffer quick to prevent congestion. this is a technique used by the cisco 4500. – Gngogh Mar 13 '17 at 7:01

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