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Hunting down OutDiscards



My Setup:

I have a productive setup of two stacked Cisco WS-C3650-48TQ (core) and 14 pairs of stacked Cisco WX-C2960-XR (leaf).

They all have LACP trunks between them.


The issue:

I have tons ouf OutDiscards on every interface using VLAN A. Either on access vlan A or trunk vlan A or native vlan A. They occure in chunks of 100 up to 600.000 per second on every interface using this vlan. There are hours without discards and hours with discards every minute.

You have a smart idea to track them and find the source of the issue?

1 Reply 1

Joseph W. Doherty
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

Issue might be a "simple" as VLAN A has lots of microbursts hitting its interfaces (BTW, if all the interfaces, in just one VLAN, are having this issue concurrently, possibly some form of broadcast storm).

Catalyst 3K and 2K switches are a bit "infamous" having smallish buffer resources (worst on the 2K) compounded by default QoS buffer allocations (believe your 3650 has QoS on by default, and 2K QoS off by default).

"Tweaking" buffer allocations can sometimes provide a huge improvement (i.e. decrease in discards).

In your issue is related to some form of broadcast storm, you might "analyze" traffic on that VLAN (e.g. WireShark) and/or try a couple of storm control settings (to see if that decreases output discards).  (If you find, in fact, you have broadcast storms, than you can work to identify the cause and work or its elimination or mitigation.)

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