Input Queue Drops - please explain

ajsouthard · ‎06-29-2005

I am seeing input queue drops as follows...

Input queue: 0/2000/645675/0 (size/max/drops/flushes)

The drops counter is slowly incrementing but the size counter is always zero. Why are packets being dropped when the queue is empty. I thought the size counter would show the queue as being full.

Also the size of the queue has been automatically increased to 2000 (default 75), we did not configure this. Is this normal behaviour ?

We are running 12.2SXD2 on 7600 platform.

Georg Pauwen · ‎06-30-2005

Hello,

as I understand it, the size counter shows the number of packets in the queue when at the moment you issue the ' show interfaces' command, so it is very possible that it is 0.

The default is indeed 75...the only way to change this is, TMBK, manually with the 'hold-queue' command.

There are numerous possible reasons for input queue drops, you might want to check the document below:

Troubleshooting Input Queue Drops and Output Queue Drops

http://www.cisco.com/en/US/partner/products/hw/routers/ps133/products_tech_note09186a0080094791.shtml

HTH,

GP

ajsouthard · ‎06-30-2005

Thanks GP.

I understand what you're saying but I have entered the command hundreds of times as fast as I can and the size is always zero even though the drops are incrementing.

Also we have not entered a "hold queue" command to change the buffer size, the router seems to have done it automatically........triggered by these drops perhaps ??

daveo · ‎06-30-2005

Is this interface a dot1Q trunk? If so inbound packets with vlan tags that are not allowed via the switchport trunk allowed vlan command will be displayed as input drops. This is also true if untagged packets are received and the native vlan is not allowed as part of the trunk.

I have verified this by changing the native vlan to other than 1 and allowing that vlan on the trunk.

I have also seen input queue drops (not sure if on 7600) when the packet fails any configured access lists.

ajsouthard · ‎07-02-2005

yes, it is a trunk. This could explain a lot. Thats very good to know, thanks.