GARP (gratuitous ARP) on Routers

Hi,
I have a query on the Updation of ARP cache in Cisco 3745 router (3700 series). When ever I send an GARP to the router, it does not update its ARP cache with the GARP values but it updates with only the ARP Request. Is there any settings that needs to be enabled to make the router to take the values in GARP and update the ARP cache (or) how to make the router to update the ARP cache with GARP packets?

PS: GARP is sent to notify the IP address is being used in the network and for IP conflict detection (duplicate IP).

thanks
Parikshith