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Currently, my Cisco 2600 router running CIOS 12.1 is experiencing packet drops on the interface that connects to the ISP.

#show int fa0/0
FastEthernet0/0 is up, line protocol is up
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 23/255
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 03:45:44
Queueing strategy: fifo
**Output queue 0/40, 0 drops; input queue 0/75, 145 drops**
5 minute input rate 9285000 bits/sec, 872 packets/sec
5 minute output rate 656000 bits/sec, 493 packets/sec
    10523044 packets input, 906814473 bytes
    Received 25631 broadcasts, 0 runts, 0 giants, 0 throttles
    22116 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
    0 watchdog
    0 input packets with dribble condition detected
    6061031 packets output, 1078369785 bytes, 0 underruns
    0 output errors, 0 collisions, 0 interface resets
    0 babbles, 0 late collision, 0 deferred
    0 lost carrier, 0 no carrier
    0 output buffer failures, 0 output buffers swapped out`

After checking Cisco's support pages, they suggested that this may be caused by CEF being disabled and too much traffic to the interface. However, neither issue would be the case as I've verified CEF to be enabled as well as utilizing only 20% of my bandwidth. Any idea of what other checks I may have missed that could help determine what is causing this issue?

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1 Answer

Input queue drops are one of these things that are very difficult to track down the true cause of. First, I'd look at your input error counter - you have 22116 input errors, but they are not categorized any further. So we know they're not CRC errors, etc. What is on the other side of this router? If the outgoing interface (internal to your network) is overloaded or busy or half-duplex or any one of a number of other things, the packets might get dropped on the incoming side because they can't be handed off to the outgoing side. I would also take a look at http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech_note09186a0080094791.shtml and read it top to bottom very carefully, the answers you want may be in there.

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