On a Cisco router, the number of input queue drops should not exceed 50 in any hour.
No formula is available to determine this number; however, the output from the show interface ethernet port clearly provides this information, as the following code demonstrates:
Last input 0:00:00, output 0:00:00, output hang never Output queue 0/40, 0 drops; input queue 0/75, 2 drops Five minute input rate 61000 bits/sec, 4 packets/sec Five minute output rate 1000 bits/sec, 2 packets/sec
Input queue drops are registered if the incoming frame rate is faster than the outgoing frame rate; when the frame rate is faster, the queue is filled up. Incoming data from an interface gets into the input queue for further processing. Looking at an Ethernet interface, data is coming from the Ethernet segment, not going out to it. After the queue is full, all the subsequent incoming frames are dropped.
Drops indicate that the interface is overpowering the router. On a LAN, this would likely be a busy or congested LAN segment; most likely, frames are being flooded onto the wire into the router's interface.
The input queue can be modified using the interface subcommand hold-queue xx in, where xx is a value. Where xx in the previous example is configured for 75, the default for xx is 40. The command path to change this configuration is as follows:
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