Lukas Beeler's IT Blog

The System i has come a long way, and so have most of it’s administrators. Back when i started working with Networks, 10mbit Ethernet using 10BASE2 was the norm, but just a few months later i’ve switched to 10BASE-T.

But the System i has dealt with a lot more LAN standards than i ever did. From Twinax (which still sees some use for connecting legacy printers or consoles), over Token Ring and of course some more obscure variants. Even though IBM has moved on regarding to the hardware, and all current 5xx models ship with two Gigabit ports standard.

The problem is that many System i admins never moved away from their 10BASE-T Ethernet knowledge, and stuck with that. This leads to many ethernet lines which are not configured correctly, or not for optimal performance.

Today, Ethernet auto negotiation works perfectly. But many setups use fixed values on the System i side (like 10mbit full duplex), but leave the switch/network side on auto negotiation – this is prone to troubles which is usually called a “duplex mismatch”. This duplex mismatch will not cause your ethernet line to cease functioning, but it will work at extremely degraded speeds (usually just a few kilobyte per seconds). If you’re just using 5250 to connect to your i5/OS instance, you won’t notice this. But as soon as you start using data transfer to your System i (e.G. Image Catalogs), you will notice the extremely degraded performance.

Now, there are two ways to fix this problem:

Configure your System i correctly

Use DSPLIND to have a look at your Ethernet line. It should look like this:

Übertragungsgeschwindigkeit  . . . :   *AUTO
Aktuelle Übertragungsgeschw. . . . :   100M
Duplex . . . . . . . . . . . . . . :   *AUTO
Aktueller Duplexwert . . . . . . . :   *FULL

This means that you’re using autonegotiation, and the system currently has negotiated 100mbit full duplex (of course, it might’ve negotiated different things on your networks, depending on the capability of your network).

If it looks like this, your system is not configured correctly:

Übertragungsgeschwindigkeit  . . . :   10M
Aktuelle Übertragungsgeschw. . . . :   10M
Duplex . . . . . . . . . . . . . . :   *FULL
Aktueller Duplexwert . . . . . . . :   *FULL

This means that your System i is configured to use 10mbit, full duplex no matter what the other end thinks. This can lead to the aforementioned duplex mismatches.

Fixing is easy, but requires the Ethernet line to be varied off. So you’ll need to do this after hours, from a console:

CHGLINETH LIND(ETHLINEX)
          LINESPEED(*AUTO)
          DUPLEX(*AUTO)

After varying on the Ethernet line, you should be having full network performance. Please note that not all cards support autonegotiation. The 2838 that are used in the models 170, 800, 270 work perfectly though, but if my memory serves correctly the cards that usually ship with the model 150 did not. It might also depend on the OS level, and i don’t have a V4Rx machine around for testing. The integrated 5706 in the 5xx models also work perfectly (and also support Gigabit speeds, if your switch supports them).

Configure the switch to use fixed values

If your DSPLIND looks like this:

Übertragungsgeschwindigkeit  . . . :   10M
Aktuelle Übertragungsgeschw. . . . :   10M
Duplex . . . . . . . . . . . . . . :   *FULL
Aktueller Duplexwert . . . . . . . :   *FULL

And you can’t or don’t want to change your line description, you will need to reconfigure the switch (or hub) to use fixed values. This is only possible if you have a managed switch or hub, with a telnet/ssh/web/serial interface, and this differs from manufacturer to manufacturer. It’s important that you configure the switch to the exactly same values as your System i – this will make sure that you don’t have a duplex mismatch or much worse a speed mismatch. I don’t recommend this approach, but it will work just as fine.