udp multicast packet leakage out to wrong network

i am pulling my hair out with this one. here is the problem.

I have dell blade setup; each blade has 2 network cards in it. We have
configured it such that [eth0=world] [eth1=internal/192.168.0.xxx].
One of the blades is configured to have both network cards active at
the same time.

Now here is the rub.

There is lots of internal traffic flowing around

192.168.0.a -> 192.168.0.b
192.168.0.b -> 192.168.0.c

With no problems whatsoever. This traffic is kept within the network
with no packets 'leaking' out onto the [world] network.

However, we have mod_backhand installed on the internal machines,
pulsing out a small multicast UDP packet on 192.168.0.255 every 1
second. All multicast packets are leaking out onto the [world]
network, with the MAC address of those packets originating from the
actual internal machine.

The internal machines cannot ping/access any machines on the outside
world, so i can only conclude that the machine i have setup that has
both network cards in it, is some how rebroadcasting the UDP Multicast
packet out onto the [world] network.

Where should I be looking? The route table for the gateway machine is:

-------------------------
# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.0.0 * 255.255.255.0 U 0 0 0 eth1
81.a.b.0 * 255.255.255.0 U 0 0 0 eth0
169.254.0.0 * 255.255.0.0 U 0 0 0 eth1
127.0.0.0 * 255.0.0.0 U 0 0 0 lo
default 81.a.b.c 0.0.0.0 UG 0 0 0 eth0
-------------------------

How can i say that for 192.168.0.x traffic there is no gateway; it
should remain within the network? Or am i barking up the wrong tree
here?

I appreciate any light anyone could shed on this.

thanks

alan