show interface question - Last input, output

Unanswered Question
Mar 5th, 2008

I am wondering how the "Last input, output" counters work on switch interfaces. I am very confused at the moment, and hope somebody can clear this up for me. According to the documentation I have read from Cisco, it simply is showing you the last time in hours:minutes:seconds since a "packet" was successfully transmitted or received on an interface. That seems to make sense, except that I am on a purely layer 2 switch (Cisco 2950), so it processes frames on it's switch interfaces not packets. Anyways, I don't understand how in many cases the "Last input" can be "never" , but the output can be something other than never.

For instance, here is a port that has a machine attached to it in a production network:

Switch#sh int status | i Fa0/14

Fa0/14 connected a-full a-100 100BaseTX

Switch#show interface fa0/14 | i Last input

Last input never, output 00:00:01, output hang never

If I ping the machine on this port, the last input stays at "never" and the last output continues to increment. I successfully pinged the machine, so I am thinking OK a frame came IN to the port I am on, was OUTPUT on the port of the machine I pinged, and when the ping reply packet was sent, a frame was INPUT on the interface the machine I pinged is on, and OUTPUT on the interface I am on. I don't get it, am I missing something here????

Thanks, any help appreciated.

I have this problem too.
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Jeroen Huysmans Tue, 03/06/2012 - 05:01

I have the same problem, but with different HW... I'm troubleshooting this on a 6509-E with WS-X6708-10GE.

I also decided to ping the device behind this TenGig interface and noticed the last input counter was increasing, hence I thought no traffic was passing the interface.

But then I found this:

Number of hours, minutes, and seconds since the last packet was  successfully transmitted by an interface. Useful for knowing when a dead  interface failed. This counter is updated only when packets are process  switched, not when packets are fast switched.


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