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New Member

Input Drops on 3660

We are experiencing occasionally high levels of input drops/receive discards on the 3660 router that connects our central site (300+ nodes) to the internet via a 100MB link. The prevailing consensus has been - it's an old, verging-on-obsolete router that is simply unable to keep up with the demands that an 100MB link (and the user community addicted to it) makes. However, due to the current economic situation, we are stuck with the device for at least one more year so we are trying to make the best of it.

On to the meat of the matter ...

As inferred above, we have been using this 3660 as our main Internet router for quite a few years. This router used to have dual T1 lines from providers going into it. Now it has the single 100MB internet feed from an ISP going into it. The router is running a rather aged IOS of 12.2(23). We have been investigating loading a newer 12.2 mainline IOS - 12.2(46a), any recommendations to do this to help resolve the high amount of input drops/receive discards we get from time to time?

In researching the issue, we came across a document on Cisco's website "Troubleshooting Input Queue Drops and Output Drops."

Following the steps recommended in troubleshooting our specific issue (input drops), we came across the mention of CEF (Cisco Express Forwarding.)

We issued a SHOW IP INT FA 1/0 command and discovered that while fast switching is indeed enable, both IP Flow (Netflow?) and Cisco Express Forwarding are disabled.

Would enabled either one or both these be of any assistance in reducing the input drops?

If so, while we understand CEF would have to be configured both globally and on the specific interface with the input drop issue (obviously this is the ethernet interface that has the 100MB link coming into it), we are confused somewhat by the apparent restriction of "logging disables CEF." Does this logging restriction only apply to the application of the log keyword to an access-list on this router? Or does this apply to any sort of logging (console, syslog, etc.?) We don't think so, but want to make sure.

Lots of questions here - hopefully they have been presented clearly enough for all of you.

Hall of Fame Super Gold

Re: Input Drops on 3660

Yes, you should upgrade the IOS.

Yes, enabling CEF should be done, and should not interact with netflow.

After that, you can try some buffer tuning to alleviate the problem.

However, the most important thing is, quantify "high levels", comparing the number of drops to the total number of packets received.

Super Bronze

Re: Input Drops on 3660

I think it's unlikely you'll see much, if any, of an improvement upgrading from 12.2(23) to 12.2(46a) (although not that I'm against such an upgrade).

Input drops, I believe, are often caused by the router being unable to quickly enough process an inbound burst. Using either/both flow cache or/and CEF may help. (Often a good idea to use in any case.)

You can also consider increasing the input queue depth; perhaps 2x to 4x the default.

If actual provided bandwidth doesn't exceed 10 Mbps (or average usage doesn't generally exceed 10 Mbps), you might also consider running the interface at 10 Mbps (if supported by your ISP).

If average rate and/or provided rate above 10 but not 100, you could also ask the ISP to shape slightly above average rate. If they can't or won't, you could place a small 2960 switch between your 3660 and ISP and "shape" using it.

Long term, the 3660's forwarding rate isn't really up to sustained 100 Mbps. For that bandwidth, you'll likely want to upgrade to a 3845, or depending on required IOS features, a 3560 (NB: the 8 port 3560 is relatively inexpensive) or one of the Metro Ethernet switches might be an option too.

New Member

Re: Input Drops on 3660

I am in process of determining the percentage of drops now - should have thought of that, it makes sense to find out whether the problem is that much of a problem.

Regardless, it sounds like CEF is a good idea. But I still have the question about logging and CEF - is it only the LOG switch applied on an access list that disables CEF, or logging in general?

Thanks to all for your time and kind assistance!

Btw, in response to the last reply, we did have specked out a dual 3750 switch setup - adding a second link for redundancy and load purposes. However that went bye-bye when the economy went sour. But your post got me to thinking - maybe we can sell a single 3560, espcially the 8-port model in the next budget.

Hall of Fame Super Gold

Re: Input Drops on 3660

Yes the 3560 8 port is great bang for the buck, on the other hand when facing the internet or a wan in most cases you want a true router and not a L3 switch.

If nothing else helps another thing you can do is set full duplex on the 100 mbps link (both ISP and yuor side). This will give the router a bit more breath as there will be no more trains of back-to-back packets.