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You can set a static arp entry, but I found that still doesn't work. Your best bet, if you aren't doing it already, would be to filter RFC1918 addresses on your outside interface with the exception of the modem's management IP and any other addresses...
Julio is incorrect in this instance.You need to issue the following command: arp permit-nonconnected This will allow you to use arp on the Outside interface for non-connected networks; this is turned off by default for the obvious security reasons. B...