Re: TCP/IP config question - using one i5 as a "gateway"/"router" foranother?

Mostly I agree with Lukas. Get yerself a GbE switch and be done with it.

While Lukas is correct that you can't do bridging in i5/OS you can do the next best thing which is Proxy ARP. This only works if the addresses used on the link between the two machines (Your crossover cable) are consecutive and the address above and below are available as well. This is because with Proxy Arp you would use for example address x.x.x.49 and x.x.x.50 with a subnet mask of 255.255.255.252 on that link. You need IP Forwarding enabled on the i5 which is connected to the LAN and you also need Proxy Arp enabled on the interface which is connected to the local network. One last thing, you need to set the default route on the back end system to be the IP address of the connected system. If you do all that the back end system looks like it's on the local LAN so long as the connected system is up and running.

Will it work, yep. Good idea? Nope.

- Larry

Lukas Beeler wrote:

First, let me say that this is a really stupid idea. If you have two
System i5, you're working in a fairly big shop which should be able to
afford a working Gigabit backbone for their servers. If you're not a
fairly big shop, you can get 8 Port Gigabit switches for around 100US$
at Best Buy. That would solve your problem fast, efficient, and without
nasty hacks.

Okay, regarding your approach, the best one doesn't work because i5/OS
does not support Ethernet bridging. You would have to give the second
System i5 an IP Address in a different subnet (which might or might not
be a problem).

The rest is fairly normal TCP/IP Configuration. Create a new line
description for the second i5/OS Ethernet card (ETHLINE2), give A an IP
Address in the new subnet, enable IP Forwarding (CHGTCPA, IPDTGFWD). On
B, create a line description (ETHLINE1), give it an appropriate IP
Address, and then configure the IP Address of ETHLINE2 on System A as
the default gateway (by adding an appropriate default route).

Another approach would be to add a second card to System B, and create a
dedicated LAN for inter-system communication. This would not affect the
IP Address used on the LAN side.

This should work fine, but performance could be an issue. The 5706 LAN
Controllers are IOPless, and I doubt that i5/OS has something like CEF,
so all packets going to System B would hit the CPU. I have no idea how
much performance this consumes.

-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx
[mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of Peter Clifford
Sent: Sunday, May 27, 2007 2:43 PM
To: midrange-l@xxxxxxxxxxxx
Subject: TCP/IP config question - using one i5 as a "gateway"/"router"
foranother?

I have two i5s, physically located next to one another in the same room.

They both need to communicate with printers, PCs etc. physically located

elsewhere in the building.

One of the i5s only has the 2 embedded ethernet ports and no other
ethernet port. The other i5 has a third ethernet port. The first ethernet port on

each system is reserved for the LAN console. At present the second
ethernet port is used to connect each system separately to the LAN so they can communicate with the PCs, printers etc.

My problem is that the LAN generally is slow (for reasons I won't go
into) so sending a large files between the two i5s can take quite a while.
I've tried linking the two i5s directly so the data doesn't have to flow
around the slow LAN just to get from the one machine to the other and file transfers are dramatically quicker. The problem is that then means the
one i5 is not connected directly to the wider network and cannot communicate

with the rest of the LAN.

Question: is it possible to configure things so the i5 B physically connected just to i5 A, and not to the rest of the LAN, and have i5 B communicate with the rest of the LAN through i5 A, using it as a gateway
or router?

I'm guessing this should be possible but haven't worked out the TCP/IP interface and route settings to do it.

Can anyone help?

Pete