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Alexander T
Asked: 2009-12-10 12:31:29 +0800 CST

Troubleshoot slow LAN on ISP routers

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This is sort of a vague question, but its really been bugging me for a long time. I cannot seem to get a connection between two computers to go above 300 kbit/s in my LAN. This seems to be a magical limit, since it has been present with two different switches, one windows box, one ubuntu box, one debian NAS and others. I'm starting to suspect that the settings of ISP routers is to prioritize internet traffic and cap the LAN. I've tried to cable computers together without the switches (for instance ubuntu<->debian NAS 30 MB/s, windows<->ubuntu similar), so that's not the problem. I also get the same cap for both wired and wireless communication, so encryption does not seem to be the bottleneck. Futhermore, the internet connection is excellent (10 Mbit/s without a problem), so the routers can apparently handle the load. There is also no defect cable or mysterious connected device, since I've tried every possible and simple configuration but still cannot get past the mysterious limit.

The routers I've tried are a Davolink DV-2020 and a Netgear WGT 624.

My question is if anyone else has seen this type of behaviour and can give some advice. Thanks in advance.

UPDATE: The NAS says full duplex: 

eth0: link up<5>, full duplex<5>, speed 100 Mbps<5>

eth0      Link encap:Ethernet  HWaddr 00:16:XX:XX:XX:XX
          inet addr:192.168.1.12  Bcast:192.168.1.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:258 errors:0 dropped:0 overruns:0 frame:0
          TX packets:380 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:512
          RX bytes:23632 (23.0 KiB)  TX bytes:51782 (50.5 KiB)
          Interrupt:21
router

1 Answers

  1. Best Answer

    Is everything hooked up to the router/gateway/edge device or do you have separate switches in-between the router/gateway/edge device and the internal network?

    Here are few ideas for you to look into

    • Check to see if any of the devices involved show any errors on their network interfaces.
    • Check the MTU settings as most small consumer/soho routers barely have a cpu so they will get overwhelmed quite easily if they are having to fragment packets.
    • Traceroute to see what the data path looks like on the network to make sure it's not doing anything crazy.
    • Do a quick packet capture on your network to make sure there's not a client that is effectively causing a DOS with broadcast packets or the like. This may indicate that a machine is infected.
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