What is the difference between input errors, overruns, input queue drops and output drops?
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Input errors are frames that arrived damaged or malformed: CRC, runts, giants, framing. Overruns and ignored mean the frame was fine, but the receiving hardware or its buffers could not take it in time. Input queue drops on a router mean packets punted to the CPU arrived faster than it could process them. Output drops mean the egress queue was full: more traffic was aimed at the port than it could send, which is congestion, very often in short bursts.
Go deeper
| Counter | Where | Points to |
|---|---|---|
| CRC, frame, runts, giants | receive, the frame is bad | cabling, optics, duplex, MTU |
| overrun | receive, the hardware FIFO filled before the frame moved to memory | the receiving device cannot keep up with line-rate bursts |
| ignored, no buffer | receive, no free buffer | bursts, buffer shortage |
Input queue drops (third number in Input queue) | packets for the router's CPU | too much punted traffic: control plane, ARP, packets not switched in hardware |
| Total output drops | transmit, egress queue full | congestion: speed mismatch such as 10G into 1G, many-to-one, microbursts |
| output errors, underruns | transmit hardware | rare; hardware |
Fault or capacity?
Output drops with no input errors anywhere on the path are capacity, not cabling. The answers are QoS (decide what gets dropped), more bandwidth, spreading load across links, or more buffer where the platform allows it.
Where the drops really are
show interfaces gives a total. Per-queue drops live in show policy-map interface on routers with a
QoS policy, and in platform-specific queue commands on switches.
They will ask next
A 1G server port shows output drops, but its 5-minute rate is 150 Mb/s. How?
Microbursts. Several 10G uplinks can send to the same 1G port within a few milliseconds and overflow its buffer, while the 5-minute average stays low.
How do you reduce output drops without buying bandwidth?
Apply QoS so the drops fall on bulk traffic instead of voice and critical apps, give the queue more buffer where the platform supports it (on Catalyst 9000,
qos queue-softmax-multiplier 1200is a common change), or spread the load over more links.
Traps
- Sending someone to replace a cable because of output drops. Output drops are a capacity or burst problem.