tcpdump + IFS = no sense?

Ok, perhaps one of the experts here can help. I'm just trying to write
a small script that will start a tcpdump. The dump will be started
with a variable for tcpdump's parameters. This works fine:

cmd="-i eth0"
tcpdump $cmd

BUT, if I then add an IFS command before it (to eventually allow me to
read the parameters from a config file) it doesn't work. This fails:

IFS="
"
cmd="-i eth0"
tcpdump $cmd

with a "tcpdump: ioctl: No such device"

I've tried it with Redhat EL3 and FC5 with the same results. Can
someone explain what the heck's going on?

Thanks!
Tony

.

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