Input Queue drops

[fast5658659]

Hi guys I am having lot of input queue drops on one of our remote router which has got an ipsec protected gre tunnel towards our main branch

 Input queue: 0/75/168173/8 (size/max/drops/flushes); Total output drops: 0

 Throttle count         37
 Drops         RP    1770332         SP          0

This is the output of an interface connecting to ISP

The other thing I would like to mention here is we have some users who connects via cisco vpn client to other sites over an already established ipsec/gre tunnel so would that be a reason of having drops?

Do I need to alter the mtu for an additional gre overhead of second tunnel

Currently tunnel interface settings are

ip mtu 1476

ip tcp adjust-mss 1380

Many thanks

[Vasilii Mikhailovskii]

Hello.

Input queue drops might be a result of CPU overutilization and/or excessive traffic spikes.

Do you see the counter continuously increasing?

[paul driver]

hello

i would say go ahead and apply the mtu settings also do you see any crc errors on the physical interface?

try setting the tunnel bw to that of the actual physical interface cir and also if you have qos running apply qos pre qualify 

res

paul

[fast5658659]

Hi Paul thanks for your reply I don't see any crc errors on the physical interface nor I see any drops on the tunnel interface. drops are there only on the actual physical interface connecting to ISP. We are not running any QoS

I will try to lower the mtu value and see if it makes any difference

thanks

[fast5658659]

Hi Vasilii thanks for your comments CPU utilization is normal and there isn't any significant spikes in traffic as well when I reset the counters they start incrementing straight away. 

[Joseph W. Doherty]

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Posting

Input queue drops can be an indication of process switching (which is unable to keep up with what's offered).

If it's just short bursts, increasing the input queue value might mitigate.

Have you reviewed Cisco's documents for troubleshooting this?

BTW, if your GRE tunnel is "protected", the MTU is likely too large as might also be your MSS adjust.

If you're just doing GRE, your MSS adjust is too small.