Here is an explanation of underruns & output buffer failures:
Output buffer failures description: Cisco IOS sh interfaces counter. The number of failed buffers and the number of buffers swapped out.
Common Causes: As an example, consider a scenario where a 1gig multicast stream is being forwarded to 24 100 Mbps ports. If an egress interface is over-subscribed, it would be normal to see output buffer
failures incrementing along with Out-Discards.
Underruns Description: The number of times that the transmitter has been running faster than the switch can handle.
Common Causes: This can occur in a high throughput situation where an interface is being hit with a high volume of bursty traffic from many other interfaces all at once. Interface resets may be occurring along
with the underruns.
It basically means due to overwhelming amount of traffic sent to the switch port, it will use Tx buffer and when this buffer is full, we start to see dropping of the packets and counters for the underruns and output buffer failures will be increased.
**** It is normal to see a large number of frames being discarded when an egress
interface is over-subscribed .The "Discarded frames" count indicates how many packets are dropped because the egress notify queue is full due to over-subscription. This is also reflected in the "Output buffer failures"
statistics in "show interface", which has nothing to do with the CPU buffers in "show buffers".
You can check the bug CSCdy09531 :Output buffer failures on fastethernet ports.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...