High input/output discard on interface C3850

[AndreyPokorskiy]

Hello Cisco Community!
I would to confirm if I'm right in my thoughts
This is the notification from Cisco DNAC center:
Assurance Issue Details
High Input/Output discard Rx 0.0% Tx 21.55% on interface GigabitEthernet2/0/48 (Interface description: xxx_switch ), Device xxx_switch

From the CLI, I can see:
xxx_switch#show interfaces GigabitEthernet2/0/48
GigabitEthernet2/0/48 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is 
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, 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 on, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:17, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 19234
Queueing strategy: Class-based queueing
Output queue: 0/40 (size/max)
5 minute input rate 6000 bits/sec, 3 packets/sec
5 minute output rate 69000 bits/sec, 46 packets/sec
8418533 packets input, 4476794580 bytes, 0 no buffer
Received 22722 broadcasts (16456 multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 16456 multicast, 0 pause input
0 input packets with dribble condition detected
33704516 packets output, 15380880236 bytes, 0 underruns
0 output errors, 0 collisions, 2 interface resets
725 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

I discovered Cisco article that explains how to calculate the percentage of drops on an interface
https://www.cisco.com/c/en/us/support/docs/dial-access/integrated-services-digital-networks-isdn-channel-associated-signaling-cas/10374-debug.html
(Total output drops : packets output ) x 100 = %
or
(19234:33704516)x100=0.057066%

I would appreciate it if you could confirm whether I am correct or not.
Sincerely,
Andrey P.

[MHM Cisco World · Accepted solution]

Total output drop is little high but we can sure that this happened in short time or long time, clear counter and check the drop if it increases rapidly then there is issue if not then it normal.

MHM

[AndreyPokorskiy · Accepted solution]

Thank you Sir MHM!

[Joseph W. Doherty · Accepted solution]

"I would appreciate it if you could confirm whether I am correct or not."

Possible not - try:

(Total output drops / (Total output drops + packets output)) x 100 = %

Although the (very) old rule of thumb, that TCP could have up to a 1% drop rate (total end-to-end [not just one interface]), without a problem, but such a rule goes back to FTP kind of data transfers.  Lots of modern application traffic can be particular sensitive to drops.  Further, a microburst for a few milliseconds might cause a large drop rate, which over a longer time interval looks benign.

Most Cisco 3K (and 2K) Catalyst switches default buffer allocations often lead to premature high drop rates when there are microbursts.  Such drops can often be much mitigated with revised buffer settings.

[AndreyPokorskiy]

Thank you Joseph W. Doherty!
This formula is definitely going in my pocket for safekeeping and use!