Packet loss - Professional help, I needs it :'(

FUBAR · 2008-01-14 21:15:43

First off, this is probably not an Arch issue, but I desperately need some help on the issue. This problem is bugging the crap out of me.

Here's my setup:

ADSL PSTN line ---->---- Alcatel Speedtouch ---->---- WRT54GL v1.1 ---->---- 3com 100MBit switch ---->---- PC

The Alcatel Speedtouch is nothing more than an ADSL ethernet modem. The WRT54GL handles the pppoe connection and it does it well.

The WRT54GL is connected to the 3com switch by a 20m UTP cat 5e cable. The PC is connected with a shorter UTP cable to the 3com.

In this setup, I suffer 25% to 35% packet loss between the PC and the Linksys. This is enough to bring every download to a painfully slow 15KB/s. DHCP requests often fail, and it's even impossible to SSH into the PC from a different client on the LAN (connected with Wifi).

When I take the 3com out of the equation, *bam*, problem solved. So I figured the 3com was faulty. I replaced it with a different switch (D-Link, 100Mbit, 5 port). The same problem arose. Then I switched the NIC in the PC. It has two onboard NIC's, so it's easy to switch. The problem stayed.

I have tried all of the 4 ports on the Linksys switch aswell, it doesn't matter.

The most annoying part of it all is that sometimes I can use the 3com switch flawlessly on the network.

I'm also confident the 20m UTP is well made because I have no trouble when I connect my PC directly to the Linksys with it.

Can anyone please give me some tips on how to resolve this. Thanks in advance.

Cerebral · 2008-01-14 21:26:04

You're confident the 20m UTP is well made, but what about the other cable, connecting PC to switch? It seems to be the only part of the setup you haven't talked about. Have you tried replacing that?

cactus · 2008-01-14 23:53:20

1. replace the 3com to pc network cable.
2. Check the speed/duplex on the workstation when it is connected to the 3com. mii-tool
3. Use ifconfig to see if there are frame errors
4. use wireshark and capture some traffic. look for anomolies (like excessive tcp checksum errors)

FUBAR · 2008-01-15 07:33:45

Cerebral wrote:

You're confident the 20m UTP is well made, but what about the other cable, connecting PC to switch? It seems to be the only part of the setup you haven't talked about. Have you tried replacing that?

I've tried 3 different (short) cables to connect the PC to the switch. I'm very confident about at least one of those: I've been using it for years without any trouble. The second cable came with the new (D-Link) switch, so I'm pretty sure that one also works. The third seems OK, but I wouldn't bet my life on it.

cactus wrote:

1. replace the 3com to pc network cable.
2. Check the speed/duplex on the workstation when it is connected to the 3com. mii-tool
3. Use ifconfig to see if there are frame errors
4. use wireshark and capture some traffic. look for anomolies (like excessive tcp checksum errors)

1. Tried that already, but the problem stays. Even when I "switch" the NIC on the PC (i.e. use the other onboard NIC).
2. The PC is always connected at 100MBit FD, which is also the speed of the integrated switch of the Linksys. I've tried forcing 10MBit HD using mii-tool or ethtool, but I can't. mii-tool fails giving me an error that looks like incompatibility, ethtool doesn't give me any output but after using "ethtool -s eth0 speed 10 duplex half" the card is still stuck at 100MBit FD.
3. ifconfig reports no error or dropped packets.
4. I haven't tried that, I'll do that tonight.

Another piece of (interesting?) information: I put the second switch (D-Link) between the 3com and the PC which resulted in even more packet loss, 75% to be exact. I thought my original packet loss might have been caused by faulty autosensing ports, but I've also used a crossed link, and then I got no link at all (which seems logical).

FUBAR · 2008-01-16 09:59:33

I've found the solution: the 20m UTP cat5e was faulty. I couldn't believe it at first because it worked perfectly when it wasn't connecting two switches (Linksys and 3com or D-Link) to each other. I used a different cable and now everything is OK.

Arch Linux

#1 2008-01-14 21:15:43

Packet loss - Professional help, I needs it :'(

#2 2008-01-14 21:26:04

Re: Packet loss - Professional help, I needs it :'(

#3 2008-01-14 23:53:20

Re: Packet loss - Professional help, I needs it :'(

#4 2008-01-15 07:33:45

Re: Packet loss - Professional help, I needs it :'(

#5 2008-01-16 09:59:33

Re: Packet loss - Professional help, I needs it :'(

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