Packet forwarding from other network fails??

[Thorvald Natvig]

Ok, this one is very bizarre, and I have no idea what's causing it.. So
I'm asking you :)

I recently upgraded to a new machine based on the ASUS P2B-LS motherboard.
This MB has a 82557 100 Mbit onboard. At the same time, I got a 100 Mbit
fiber cable to my office, and a media converter to convert from FX to TX.

However, I also have 2 modems connected to this machine, and users dialing
in using PPP. And this is where the problem starts.

Once they have successfully connected with PPP, they are able to reach any
machine on the same local net as my machine, but not anything else.

Looking at tcpdump, I see that (when pinging from the PPP dialup) the
packets are sent back to this machine from our router, but they are never
sent to the PPP interface. It looks like the packets are being dropped in
the kernel somewhere. Again, packets from any other machine on the local
net can reach them just fine.

Anyway... I initially assumed this was a configuration problem on my part
and that I had done a foofoo.... However, after a LOT of messing around, I
finally ended up back on my old 10Mbit TX connection and a tulip card...
And then it suddenly worked. If I just move the cable over to the inbuilt
one and reboot, it again drops the incoming PPP packets, and when I move
the cable to the tulip card again it works like a charm.

What on earth is wrong here? As I can see the reply packets coming back on
the net, it seems to be physically working, but why are the packets being
dropped?

If anyone can help, I'd *really* appreciate it :) (And if you need any
more information, just yell).