Troubleshoot by looking at your LAN broadcast traffic

I administer a very small network consisting of a cable modem, a router, a switch, and five PCs. Two PCs are connected directly to the Netgear RP114 router, while the other three PCs are connected to a Linksys EZXS88W 8-port switch, which is connected to the router. PCs connected to the switch lose connectivity after about 100 minutes (ping fails after about 6297 packets), while the PCs connected to the router keep right on working. I can restore connectivity by cycling power on the switch, or by disconnecting/reconnecting the cable to the router.

What's happening here, and how can I fix this problem? I've tried replacing the switch with another make with the same results.

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In theory there should be absolutely no difference between the PCs connected to the external switch and those directly connected to the 4-port switch that's integrated into your Netgear RP114 router. But obviously something is going on; I'd guess something associated with LAN broadcast traffic.

I'd start by looking at DHCP lease expiration. Possibly these PCs lose their DHCP-assigned IP addresses after 100 minutes when they have trouble renewing their leases. You can find out whether this is the problem or not by temporarily assigning a static IP to one of the PCs on the switch to see if it can stay connected longer than 100 minutes. If so, then DHCP is the culprit. You don't have much control over this on the RP114, so see if you can live with static IPs on those PCs.

If you rule out DHCP, then I'd try sniffing LAN traffic around that 100 minute mark, both from the external switch and from the integrated switch. If you don't have an LAN analyzer, try using Ethereal - it's a great shareware sniffer that's fairly easy to use. Compare what you see on the two switch ports to figure out what isn't being propagated by the external switch to the PCs that lose connectivity. There may be lots of LAN traffic and that will make it hard to compare, but look specifically at LAN broadcast traffic and any traffic to/from the affected PCs, of course.

This was first published in August 2003

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