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New Member

Input que drops

Hi, I am getting input que drops on cisco 3845 router gig interface. I have verified the physical layer connectivity. It is fine. CEF is enabled on the other end of the switch. What needs to be done to minimize the errors

Cisco Employee

Re: Input que drops


There might be different reasons for input drops. Can you post your interface config?

In the meantime you could follow the guide "Troubleshooting Input Queue Drops and Output Queue Drops"

It is written for the Cisco 10k but for a good deal also applies to the 3845.

Usually following the guide at least points to the source of the problem. If you cannot solve your issues, let us know the changes you applied and the output from the different show commands entered when following the guide.

Hope this helps!

Regards, Martin

New Member

Re: Input que drops

Hi Matin, Thanks for your response. Please find the interface config.

dcnrtr1#sh int gi 0/0

GigabitEthernet0/0 is up, line protocol is up

Hardware is BCM1125 Internal MAC, address is xxxx.xxxx.xxxx.xxxx

Description: Native_VLAN

MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

Full-duplex, 1000Mb/s, media type is RJ45

output flow-control is XON, input flow-control is XON

ARP type: ARPA, ARP Timeout 04:00:00

Last input 00:00:00, output 00:00:01, output hang never

Last clearing of "show interface" counters never

Input queue: 0/75/202815/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: fifo

Output queue: 0/40 (size/max)

5 minute input rate 124000 bits/sec, 70 packets/sec

5 minute output rate 16000 bits/sec, 4 packets/sec

148401043 packets input, 1664019696 bytes, 86 no buffer

Received 111889932 broadcasts, 0 runts, 0 giants, 9499 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

0 watchdog, 229043484 multicast, 0 pause input

0 input packets with dribble condition detected

15461202 packets output, 406723229 bytes, 0 underruns

4 output errors, 0 collisions, 3 interface resets

0 babbles, 0 late collision, 0 deferred

4 lost carrier, 0 no carrier, 0 pause output

0 output buffer failures, 0 output buffers swapped out


dcnrtr1#sh int gigabitEthernet 0/0 switching

GigabitEthernet0/0 Native_VLAN

Throttle count 12920

Drops RP 276860 SP 0

SPD Flushes Fast 0 SSE 0

SPD Aggress Fast 0

SPD Priority Inputs 155283171 Drops 0

Protocol IP

Switching path Pkts In Chars In Pkts Out Chars Out

Process 667590 62004188 339975 34890737

Cache misses 0 - - -

Fast 0 0 0 0

Auton/SSE 0 0 0 0

I have verified the duplex settings and physical layer connectivities which looks fine. Please suggest how to overcome this problem.

Cisco Employee

Re: Input que drops


Troubleshooting Input queue drops after they happened is close to being impossible without historic information.

Nevertheless some comments: there are a large number of broadcast and multicast packets. What is the origin for those packets? A spanning tree issue resulting in a broadcast storm could very well explain the drops. The router then just cannot handle all the broadcasts received in CPU. Have you experienced such issues in the past?

The second question is about the multicast traffic: what could that be? There are simple explanations like OSPF, but it could also be improper configuration of multicast routing in your network - once again the switches need to be considered, as they will broadcast any multicast traffic, if not configured properly. This can also overrun the input queue.

Without further details about your topology, configuration and applications it is hard to pinpoint, what caused the drops.

I would setup network management to collect CPU load of the router and connected switch, packet counters and interface load to get more information than presented here. In addition check your logs for any spanning tree related issues. Finally try to understand the source of multicast and broadcast traffic entering the router interface. Setting up a SPAN port on the switch and using a packet analyzer (f.e. Wireshark) would be an option to consider.

Last not least make sure CEF switching is turned on on the interface.

Hope this helps!

Regards, Martin

New Member

Re: Input que drops


I ran across an interesting problem on a router here. I was seeing very high CPU load on it and a lot of input (overrun and ignored) errors on the Ethernet Interface. We swapped hardware, software, cleaned up configs, used MARS, Netflow etc looking for any sign of network attack.

When we globally turned off CEF on the router, we saw the CPU load immediately drop and the errors stopped climbing. The customers connected to that router also expressed a drastic improvement in their circuit performance.

This is odd as CEF is actually supposed to improve router performace. CIsco is still investigating it. It has been a week and so far everything works fine.

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