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My problem isn't just that a computer downloads/uploads at its maximum, saturating the LAN's bandwith by doing so.

The thing is it only uses 1-1.5 Mo/s when other computers on the LAN can download at 9 Mo/s (not from the Internet but between computers on the LAN).

And over this, when this computer hits its maximum of 1-1.5 Mo/sec, all computers using the same router lose access to Internet. Computers on an other router don't, though.

The network has about 20 computers, all plugged with cables, one main switch with computers and routers plugged in it.

Does anyone understand why/how this is happening?

Cable is fine, I tested it on my computer... Network card broken / too cheap maybe?

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Sounds like an auto-speed or auto-duplex mismatch error.

My understanding your your environment:

  • 1 Switch with 19 hosts on 100mbps (~10MB/s) links plus a 20th host on a 10mbps (~1MB/s) link
  • the Switch port to the 20th host is capable of 100mbps speeds and so is the cable
  • when the 20th host starts to send a lot of packets, all hosts on the switch lose their connection


  • Confirm that the NIC on the 20th host supports 100mbps speeds and not just 10mbps
  • Confirm that the NIC supports both Auto-duplex and Auto Speed
  • If the Switch is a managed switch, log into it and find out what speed (10mbps or 100mbps) it thinks the host is and what duplex (half or full duplex)
  • If the Switch is not managed, try to figure out the speed and duplex using indicator lights
  • Log onto the 20th host and determine what speed and duplex the host thinks it is using
  • The speed and duplex on the host and the switch should match! If they don't then auto detect is not working so you can try to manually configure the speed and duplex on each device to the fastest setting that is supported by both devices
  • Also check the error count on the interface for the NIC on host 20 and the switch as well if it is managed. A high error count will probably confirm the mismatched settings
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I will check thi out, it may have to be tomorrow morning though. I'll post what results I got (success / doesn't help) – Ludovic Migneault Jul 8 '13 at 20:44

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