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
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.
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.
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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