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I'm working with a Cisco 2621 and NAT to map ip addresses to machines in my cluster. Starting yesterday, I noticed heavy packet loss (60% loss reported by ping) to one ip address mapped to three ports on the same machine. However, all of the other ip addresses were responding fine and I experienced no packet loss when pinging the same machine from within the cluster. This continued until today, when I reset the routes (deleted and re-added all three NAT entries). Now pings appear to report no packet loss.

I haven't modified my router configuration in months. Does anyone have any idea what may have happened? I'd like to prevent another occurrence of this in the future.

Thanks, -Harish

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I'm not sure I understand. You have 3 NICs configured with one IP address on the same machine? How are they configured, as a bridge? If so, how are you avoiding briding loops? Is Spanning-Tree Protocol being used? – Charles Hooper Oct 5 '09 at 18:59
What's the actual load on your 2621? – l0c0b0x Oct 7 '09 at 6:18

Have you checked the counters on the router? At the time this was occurring did you look at memory and CPU usage on the router? I've had a few cases of packet loss caused by increased memory usage (about 50% memory use in my case).

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How do I check 'counters'? What are they? I didn't look at either memory usage or CPU usage - if it happens again, I'll be sure to do so. – user20906 Sep 22 '09 at 21:11
You can type show interface counters errors or something along these lines. – Antoine Benkemoun Oct 7 '09 at 6:43

Hmm not sure how one can analyze such a behavior when it's not occuring anymore. Maybe there was some kind of ARP messup. Could you post the sanitized relevant parts of your config?

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I'd rather not post too many details. I had three nat entries which looked like: ip nat inside source static tcp [internal ip] [internal port] [external ip] [external port] extendable I admittedly am not a router expert so I'm not sure if I'm doing anything obviously wrong here - any advice would be appreciated. – user20906 Sep 22 '09 at 21:14

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