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Input queue drops and throttles on an interface

Core Issue

The input queue drops counter is increasing on an interface. This is the sample output of the show interfaces command:

Router# show interfaces FastEthernet1/0/0
FastEthernet1/0/0 is up, line protocol is up 
  Hardware is cyBus FastEthernet Interface, address is 4000.0000.0003 (bia 0002.4a6b.6820)
  Description: *** IT marais ***
  Internet address is 45.81.131.32/20
  MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, 
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, 100BaseTX/FX
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters never
  Input queue: 61/75/3242/0 (size/max/drops/flushes); Total output drops: 0

The causes of the input queue drops indicate that excessive amounts of packets are being sent to the process switching path, instead of being handled in the interrupt switching path.

If the input queue is constantly overloaded, the Cisco IOS  Software throttles the interface. This prevents the input queue from accepting any new packets for a short period of time, allowing the router to empty the input queue.

Resolution

For information on input queue drops, refer to Troubleshooting Input Queue Drops and Output Queue Drops.

For a security advisory, refer to Cisco Security Advisory: Cisco IOS Interface Blocked by IPv4 Packets.


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