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Recently, I added two new computers to a LAN already running with two. This sounds simple, but due to wiring issues and the fact that I wanted the new PCs in a different area (and we had an extra router), I ended up making a network more complex than I can really troubleshoot myself. (I wish I could generalize the question, but since I can't isolate what exactly is causing the problem, the question can only really be answered after I give detailed information about it) So I have an SMC router, with 8 ports. This router has my modem connected into the WAN port, and two other cables connected, one to a PC, the other to my second router. This has always been the case, and I haven't had problems. The cord from my SMC's port is plugged into one of the standard ports on a Linksys wireless router (WRT54G) (not the WAN port, since it's acting as a Router and not a Gateway). Before, I had only one other port occupied on the Linksys router, running to my PC. Recently, I added two more computers practically adjacent to mine, filled up the non-WAN ports on the Linksys, and let it run. After some short time (I've never been able to time it, probably ~1 hour or so), the internet connection begins to slow to a crawl, and eventually stops entirely until I reset the SMC router.

What might be causing this?

The few networking classes I've taken make me think there's some kind of loop in the network, just because it's something that seems to gradually occur over time, but I wouldn't know how to find out whether or not that's the issue, and how to fix it. Are there possibly some routing options on both that I need to fiddle with to reduce the fail?

One important detail: Issues with the DHCP server had me turn it off for LAN for both routers, and I'm using static IP's of 192.168.2.(3-6), with the DNS server set as the IP for the router (192.168.2.1). I'm only bothering to mention this because it seems like it could be an issue with DNS servers rather than internet connectivity as a whole. Any advice on troubleshooting this?

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migrated from serverfault.com Jul 3 '10 at 19:40

This question came from our site for professional system and network administrators.

2 Answers 2

So is the text diagram below a reasonable accurate description of your network?

    (Internet )
       |
    ( ISP )
       |
       | ?.?.?.?  (WAN IP from ISP's DHCP)
     [SMC router] (NAT device, no DHCP]
       | 192.168.2.1 LAN IP        |
       | default route & DNS       |
       |                        [PC1]
       |                   192.168.2.3 static IP
       |    *
       |    | WAN not connected
       | [Linksys WRT54G wireless router, no DHCP]
       |  192.168.2.2 LAN IP  |       |     |
       |   |                  |       |     |
       |___|                  |       |   [PC4]
                              |       | 192.168.2.6 static IP
                              |       |
                              |     [PC3]
                              | 192.168.2.5 static IP
                              |
                           [PC2]
                       192.168.2.4 static IP

Some questions:

  1. Have you checked the logs for the SMC and WRT54G to see if there is anything unusual in them when this occurs? (If the router allows filtering what messages get logged, you would want to set it to the highest level of verbosity.)

  2. How do you detect the condition? How do you know it is a router problem as opposed to, say, a problem in the OS of the PC you are currently using?

  3. When the problem occurs is it only Internet traffic that is affected or is the performance of, say, file transfers on you local LAN also degraded?

  4. Are both routers using current firmware? (Probably doesn't matter, but figured I might as well rule it out. Perhaps this is a problem which was fixed in the firmware? Long shot, I know ...)

  5. If you are using either Vista or Windows 7, can you use the Network Mapping tool in the Network and Sharing Center? Just thinking it would be an easy way to rule out any configuration errors such as two systems using the same static IP. (One reason why I try to avoid using static IPs).

  6. What problem did you have which made you think you needed to disable DCHP on both routers to fix it? Typically all that is required is to ensure that DHCP is only active on one router and that the fixed IP of the non-DHCP router is not in the DHCP range. Usually the router with the WAN connection provides the DHCP function since it would also be the default route and either provide the addresses of the DNS servers or act as a "DNS relay".

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Try a traceroute from a PC connected to your WRT54G to a reliable IP:

tracert 163.1.2.1

If that stops completely at the Linksys or the SMC device, you know which is being silly (misconfigured). If it gets through but tracert doesn't give you hostnames for the hops, blame DNS.

Two SOHO router/gateway devices on the same network is always fun. Can you find a way to get rid of one or the other?

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