Router disconnects when the PC is connected to the switch

RESULTS FROM "ifconfig"

eth0 Link encap:Ethernet HWaddr 00:0B:DB:B1:D2:9A
          inet addr:192.168.1.35 Bcast:192.168.1.255 Mask:255.255.255.0
          inet6 addr: fe80::20b:dbff:feb1:d29a/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
          RX packets:7 errors:0 dropped:0 overruns:0 frame:0
          TX packets:40 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:1038 (1.0 KB) TX bytes:6148 (6.0 KB)
          Interrupt:20

lo Link encap:Local Loopback
          inet addr:127.0.0.1 Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING MTU:16436 Metric:1
          RX packets:11 errors:0 dropped:0 overruns:0 frame:0
          TX packets:11 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:624 (624.0 b) TX bytes:624 (624.0 b)

ppp0 Link encap:Point-to-Point Protocol
          inet addr:77.210.113.72 P-t-P:10.64.64.64 Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
          RX packets:11662 errors:0 dropped:0 overruns:0 frame:0
          TX packets:11812 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3
          RX bytes:4513062 (4.3 MB) TX bytes:1426963 (1.3 MB)

(Before you ask, of course I also tried with ppp0 -a 3G UTMS/GPRS modem that I'm using right now to post this- disconnected, it doesn't make any difference.)

(Not solved yet, wrong button :P. Opening the question again.)

CURRENT ERROR LOG FROM THE ROUTER (via telnet)

0 Fri Nov 02 21:07:29 2007 PP0f INFO Call Failed
   2 Fri Nov 02 21:08:02 2007 PP0f INFO Call Failed
   4 Fri Nov 02 21:08:35 2007 PP0f INFO Call Failed
   6 Fri Nov 02 21:09:08 2007 PP0f INFO Call Failed
   8 Fri Nov 02 21:09:41 2007 PP0f INFO Call Failed
  10 Fri Nov 02 21:10:14 2007 PP0f INFO Call Failed
  12 Fri Nov 02 21:10:47 2007 PP0f INFO Call Failed
  14 Fri Nov 02 21:11:20 2007 PP0f INFO Call Failed
  17 Fri Nov 02 21:11:49 2007 PP10 -WARN SNMP TRAP 2: link down
  18 Fri Nov 02 21:11:49 2007 PP0b WARN MPOA Link Down
  19 Fri Nov 02 21:11:49 2007 PP10 INFO vc closed, vc=1, vpi=8, vci=32
  20 Fri Nov 02 21:11:49 2007 PP10 WARN mpoaChannDown: ch<0> null iface
  21 Fri Nov 02 21:11:53 2007 PP0f INFO Call Failed
  23 Fri Nov 02 21:12:06 2007 PP10 WARN netMakeChannDial: err=-3001 rn_p=9444c6c0
  24 Fri Nov 02 21:12:32 2007 PP0b WARN Last errorlog repeat 67 Times
  25 Fri Nov 02 21:12:32 2007 PP0b WARN MPOA Link Up
  26 Fri Nov 02 21:12:32 2007 PP10 INFO vc opened,vc=1,vpi=8,vci=32,qos=2
  27 Fri Nov 02 21:12:32 2007 PP10 INFO vc opened,vc=2,vpi=8,vci=36,qos=2
  28 Fri Nov 02 21:12:45 2007 PP0f INFO Call Failed

With a dhcp server, (your router) i always set my Ubuntu pc network card into DHCP mode... and i always disable ipv6.
https://answers.launchpad.net/ubuntu/+question/16335

HTH

Doesn't work neither. If I set the card to DHCP it can't even ping the router nor the other computers.

- Router ZyXEL P660R-D1 monoport connected to a 8 port switch together with 3 other computers:
- Computer A: Ubuntu Gutsy, two network cards (one in roaming mode, one with static IP 192.168.1.10)
- Computer B: Win XP, one network card (DHCP)
- Computer C: Ubuntu Gutsy, also in roaming mode

Please connect all your pc to the switch-hub be sure your network is in fail state:
open a terminal on the pc A and C and type:

sudo ifconfig -a

copy and paste the result here.
Thank you.

eth0 Link encap:Ethernet HWaddr 00:0B:DB:B1:D2:9A
          inet addr:192.168.1.35 Bcast:192.168.1.255 Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
          RX packets:1398 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1421 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:106706 (104.2 KB) TX bytes:100385 (98.0 KB)
          Interrupt:20

lo Link encap:Local Loopback
          inet addr:127.0.0.1 Mask:255.0.0.0
          UP LOOPBACK RUNNING MTU:16436 Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)

ppp0 Link encap:Point-to-Point Protocol
          inet addr:77.210.46.126 P-t-P:10.64.64.64 Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
          RX packets:27149 errors:0 dropped:0 overruns:0 frame:0
          TX packets:27773 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3
          RX bytes:21025435 (20.0 MB) TX bytes:3606975 (3.4 MB)

Some more information:

 - I said I can access 192.168.1.1 by web, but sometimes it doesn't accept the password (I've no idea why). Telnet works always.

 - Computer C is on a different room than the others, so there are 2 cables from the PC to the switch: Cable A, that goes from the computer to the wall, and Cable B (inside the wall) that goes to the switch in the other room. I've tried bringing the router to the room where Computer C is and connecting them directly, and this way it worked, but connecting them directly being in the different rooms (so also using cable B) it has the same behavior as if I'd connect it to the switch. My dad will try to get a tester this weekend to check if cable B might be the problem.

Thanks for your help!

I can confirm now that the problem was the cable. The ethernet tester marked some problems with it, and after getting a new one it works perfectly (at least the network is way more faster now, I couldn't try with the router because we changes the provider for a faster, wifi based one, and I'm still awaiting their new router).

Thanks anyways for your help marcobra, I really appreciate it :).

Router is working :).

(It seems the connection with the old provider isn't canceled yet.)