I have an HP c7000 blade chassis which contains Cisco 3120X and Cisco 3120G switches running ios 12.2(58)SE1. The blades themselves are very lightly loaded yet many interfaces on different blade switches in the chassis show a fairly high number of output drops. If I check the number of output drops repeatedly I not only see the counter increasing but sometimes it decreases. The numbers don't correlate with the packets/s recorded on the interface. QoS settings are default for the platform.
The following samples were all taken within a 30 second period:
bc1019-3120-stack>sh int gi2/0/7 | i output drops Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 2255550 bc1019-3120-stack>sh int gi2/0/7 | i output drops Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 2255550 bc1019-3120-stack>sh int gi2/0/7 | i output drops Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 2255550 bc1019-3120-stack>sh int gi2/0/7 | i output drops Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 2255550 bc1019-3120-stack>sh int gi2/0/7 | i output drops Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 2255550 bc1019-3120-stack>sh int gi2/0/7 | i output drops Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 2255550 bc1019-3120-stack>sh int gi2/0/7 | i output drops Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 451110 bc1019-3120-stack>sh int gi2/0/7 | i output drops Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 451110 bc1019-3120-stack>sh int gi2/0/7 | i output drops Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 902220 bc1019-3120-stack>sh int gi2/0/7 | i output drops Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 1353330 bc1019-3120-stack>sh int gi2/0/7 | i output drops Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 1804440 bc1019-3120-stack>sh int gi2/0/7 | i output drops Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 1804440 bc1019-3120-stack>sh int gi2/0/7 | i output drops Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 1804440 bc1019-3120-stack>sh int gi2/0/7 | i output drops Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 451490 bc1019-3120-stack>sh int gi2/0/7 | i output rate 5 minute output rate 301000 bits/sec, 119 packets/sec
1) Is there anything else that can cause output drops besides the server nic not receiving the frames quickly enough?
2) What is the maximum number of output drops the interface counter can record? Does it rollover when it reaches the max?
3) What would be considered a healthy rate of output drops?
Best Answer
Unless someone is clearing counters you should never see any odometer-type counters (those that increment based on a packet action) decrease, they should always increase. That part sounds like a bug.
As far as what causes output drops in particular, there are so many different causes that it's very difficult to pinpoint it exactly. Sometimes there is congestion inside the switch's backplane and those might show up as output drops on the outgoing interface. In rare circumstances you can also get microbursts that don't show up when polled at 1 minute intervals that quickly overload the interface, but then drop back down very quickly. I would suggest grabbing the SNMP OID for output drops and then graphing that and see how it corresponds to the CLI counter.
Generally speaking, you don't want any output drops as they indicate a packet that did not make it to its destination. But, if you're running your links hot (which you say you're not) they're unavoidable to an extent, mostly due to interior switch buffering, etc.