How does the am37xx chip on the beaglebone uses Ethernet??
BeagleBone does not use the AM37xx procesor. That is the BeagleBoard-xM
http://www.elinux.org/Beagleboard:BeagleBoard-xM
BeagleBone uses the AM3358 and has a MII interface that uses the internal MAC. Check the schematics.
http://www.elinux.org/Beagleboard:BeagleBoneBlack
Gerald
Read the manual !