OUTPUT PACKETS in Catalyst 3850 12S with 16.6.2

[flielguera]

Good morning friends;
I have a Catalyst 3850 SW platform with IOS 16.6.2, the trunk interfaces via PORT CHANNEL that link these SWs, shows high OUTPUT PACKET DROPS, in only one of the PORT CHANNEL interfaces, do any of you have any idea what I can be happening.
I appreciate your help and comments.

Regards Fernando.

[balaji.bandi]

i would first physically check the cable and replace with a new cable and test it. (since you are seeing only 1 interface output drops)

https://www.cisco.com/c/en/us/support/docs/switches/catalyst-3850-series-switches/200594-Catalyst-3850-Troubleshooting-Output-dr.html

[Georg Pauwen]

Hello,

your first line of defense is typically to configure the below on your switch. Give that a try and check if the output drops decrease.

mls qos

qos queue-softmax-multiplier 1200

[Joseph W. Doherty]

". . . do any of you have any idea what I can be happening."

Likely, transient congestion on that port overruns either the logical and/or physical queue resources for that port.

As to why it's happening on only one port of a port channel, there are several possible reasons.  First, port channels do not dynamically load balance across their links and flows with same hashing attributes will always used the same link (assuming no change in number of links).  So, for example, if there's a pair of busy hosts, they can consistently overrun just one port of a port channel.

It's also possible, I believe, to have different logical and/or physical queue limitations per port as it's also possible, I believe, to have different physical loading on the ASICs and/or hardware buffers that support a particular port.  I.e. one port of an Etherchannel may not have the same available resources as another port.

If the switch has QoS enabled, Georg's recommendation is a good thing to try first.  NB: this change is suggested in the link provided by Balaji.

[balaji.bandi]

Sure that is one of the things need to consider and monitor (over utilization)  of the link (based on the config LB method defined).