High Output drops on Interface

Network administrators at times face the challenge of output drops which causes data traffic loss. Output drops are generally caused by congested interfaces and best possible option to address this situation is to look into increasing the speed of that link. Other factors may also be responsible for causing output drops on interfaces.

Some of common causes of output drops on interface are enlisted below –

  • Bad cable
  • Oversubscription on the port or module
  • Speed/Duplex mismatch.

Sometime overall traffic utilization may be showing low, however traffic bursts under “show interface” may not be giving clear picture. The default output rate is 5 minutes which may miss the bursty traffic. One option is to shorten the load interval from the default 5 minutes to 30 seconds using the interface-level command of “load-interval 30”.

Some of other Troubleshooting methods include –

  • Using wireshark for detailed understanding of output drops.
  • Increasing queue-limit
  • Increasing hold queue

Infact following list of command can be very useful in troubleshooting output drops on interface –

  • show interfaces switching
  • show interfaces stats
  • ip accounting mac-address
  • show interfaces mac-accountin
  • sh int <int no> | include drop

Below is the output of “show interface” command to show output drops as under –

GigabitEthernet0/1 is up, line protocol is up (connected)

Hardware is Gigabit Ethernet, address is 00aa.bbbb.cccc (bia 00aa.bbbb.cccc)

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 00:00:24, output never, output hang never

Last clearing of “show interface” counters 02:55:56

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 413678

Queueing strategy: fifo

Output queue: 0/40 (size/max)

5 minute input rate 0 bits/sec, 0 packets/sec

5 minute output rate 64000 bits/sec, 32 packets/sec

765432 packets input, 84505586 bytes, 0 no buffer

Received 4508 broadcasts (4009 multicasts)

0 runts, 0 giants, 0 throttles

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

0 watchdog, 4009 multicast, 0 pause input

0 input packets with dribble condition detected

765432 packets output, 84505586 bytes, 0 underruns

527779 output errors, 0 collisions, 1 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

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