Input queue on Fast Ethernet and Gigabit Ethernet

Unanswered Question
Mar 3rd, 2008


I am facing a problem in input queue drops in my router. I am getting input drops in my Fast Ethernet interface. If I am replacing this with a Gigabit interface, I am not getting any drops. I can see that that max input queue length of both interfaces are 75. So why is it not dropping in gigabit interfaces?

I understand that Fast ethernet is 100 Mbps and that means it can process 100 Mb of traffic per second. Is it same for incoming traffic also.

Where does this input queue come into picture? Is it when the tx-ring is full?

Please help.



I have this problem too.
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Joseph W. Doherty Mon, 03/03/2008 - 21:33

As I understand it, input queue drops are when the router doesn't have enough processing time to pull them of the input queue to forward them.


Everything else being equal, I would expect a faster interface to be even worst. Perhaps, everything else isn't equal. Take note within the above document's mention of switching path being used. Perhaps the size of the input queue differ.


Although a fastEthernet interface handles frames whose bits are modulated at 100 Mbps, this doesn't also mean that the router can sustain that level of performance over lots of multiple frames at that rate. That's where the PPS (packets per second) performance of the router comes into play.

cisco_query Mon, 03/03/2008 - 22:35


The input queue depth is same for both fastethernet and gigabit ethernet interfaces. I think the processing speed is different in 2 interfaces.



cisco_query Mon, 03/03/2008 - 23:12


Can anyone give the link for understanding how exactly packets are processed on incoming and outgoing interfaces and also how interface speed affects this.How packects are processed in inputbuffers , rx rings and input queues.? Similary how packets processed in output buffers, tx ring and output queue?

Thank you,



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